RISK FACTORS

Investing in our ordinary shares involves a high degree of risk. You should carefully consider the risks and uncertainties described below together with all of the other information contained in this prospectus, including our financial statements and the related notes appearing at the end of this prospectus, before deciding to invest in our ordinary shares. If any of the following risks actually occurs, our business, prospects, operating results and financial condition could suffer materially. In such event, the trading price of our ordinary shares could decline and you could lose part or all of your investment.

Risks Related to Our Financial Position and Need for Additional Capital

We have incurred significant losses to date, expect to incur losses over the next several years and may never achieve or maintain profitability.

We have incurred significant losses to date. We had a net loss of €20.4 million in the first nine months of 2013, €14.7 million in 2012 and €17.3 million in 2011. As of September 30, 2013, we had an accumulated deficit of €138.0 million. To date, we have financed our operations primarily through the sale of equity securities and convertible debt and, to a lesser extent, through milestone payments, subsidies and grants from governmental agencies and fees for services. We have devoted substantially all of our financial resources and efforts to research and development, including preclinical studies and clinical trials. Our product, Glybera, received marketing approval under exceptional circumstances from the European Commission in October 2012. We plan in the future to apply for marketing approval for Glybera in the United States and other countries and expect that we will be required to conduct one or more additional clinical trials of Glybera. We are still in the early stages of development of the other product candidates in our pipeline. We expect to continue to incur significant expenses and losses over the next several years. Our net losses may fluctuate significantly from quarter to quarter and year to year. We anticipate that our expenses will increase substantially as we:

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complete the EMA-mandated post-approval clinical trial of Glybera and implementation of an LPLD patient registry;



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conduct a confirmatory clinical trial of Glybera, either as part of the EMA-mandated post-approval clinical trial or separately, to obtain data needed to file a BLA for Glybera with the FDA;



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seek marketing approval of Glybera in the United States and other countries;



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initiate a Phase I/II clinical trial of AMT-060 for hemophilia B in collaboration with Chiesi;



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advance the clinical development of our other product candidates, most of which are at early stages of development, and seek to discover and develop additional product candidates;



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seek marketing approval for any product candidates that successfully complete clinical trials;



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establish a sales, marketing and medical affairs infrastructure in the United States;



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continue the build-out of our manufacturing facility in Lexington, Massachusetts to expand our manufacturing capabilities for Glybera and our pipeline of product candidates;



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maintain, expand and protect our intellectual property portfolio, including in-licensing additional intellectual property rights from third parties;



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hire additional personnel, particularly in our manufacturing, research, clinical development, medical affairs, commercial and quality control groups; and



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add operational, financial and management information systems and related finance and compliance personnel necessary to operate as a public company.

We are only in the preliminary stages of most of these activities. We and our collaborators may never succeed in these activities and, even if we do, may never generate revenues that are significant enough to achieve profitability.

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Even if we do achieve profitability, we may not be able to sustain or increase profitability. Our failure to become and remain profitable would depress the value of our company and could impair our ability to expand our business, maintain our research and development efforts, diversify our product offerings or even continue our operations. A decline in the value of our company could also cause you to lose part or all of your investment.

Our financial results will substantially depend on the commercial success of sales of Glybera.

We anticipate that our collaborator Chiesi will commercially launch Glybera in the European Union in the first half of 2014 and that revenues from sales of Glybera will be one of the principal sources of funds for our business for at least the next several years. Because our business is currently dependent on Glybera, failure to achieve anticipated revenues from this product would have an adverse effect on our results of operations and cause the value of our ordinary shares to decline. A number of factors, some of which are out of our control, may adversely affect the commercial success of Glybera, including the following:

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our collaborator Chiesi may not successfully commercialize Glybera in the European Union and other specified countries in the Chiesi territory;



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the post-approval requirements imposed by the EMA in connection with Glybera's approval under exceptional circumstances may be costly or may eventually lead to withdrawal of approval;



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we may never be able to obtain marketing approval for Glybera in the United States or other countries;



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Glybera may fail to achieve market acceptance by physicians, patients, third party payors and others in the medical community;



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other alternative treatments for LPLD may be developed and gain commercial acceptance, eroding Glybera's market share;



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the limited label we have received for Glybera in the European Union may limit our addressable market, and other regulatory agencies may approve Glybera only with a similarly limited label;



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we may be unable to establish or maintain sales, marketing and medical affairs capabilities for the commercialization of Glybera in the United States, even if we receive FDA approval; and



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coverage, pricing and reimbursement levels may be lower than we expect.

Even if our commercialization of Glybera or other product candidates for which we obtain marketing approval is successful, we may not be financially successful due to our obligations to third parties.

We have obtained exclusive or non-exclusive rights from third parties under a range of patents and other technology that we are exploiting in Glybera and our development programs. Our agreements with these third parties generally grant us a license to make, use, sell, offer to sell and import products covered by the licensed patent rights in exchange for our payment of some combination of an upfront amount, annual fees, royalties, a portion of amounts we receive from our sublicensees and payments upon the achievement of specified development, regulatory or commercial milestones. For example, we are contractually obligated to pay royalties and other obligations to third parties on net sales of Glybera by us, Chiesi or other sublicensees or on other amounts we receive, including from Chiesi or other sublicensees for their sales of Glybera. We also received a technical development loan from the Dutch government, which requires repayment based on the timing and amount of revenues we receive from the sale of Glybera. These financial obligations to third parties are an expense to us, which could adversely affect our financial position.

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We will likely need to raise additional funding, particularly if we experience delays in implementing our development programs or commercialization efforts. Additional funding may not be available on acceptable terms, or at all, and any failure to obtain capital when needed may force us to delay, limit or terminate our product development efforts or other operations.

We expect to incur significant expenses in connection with our ongoing activities and expect that we will likely need to obtain substantial additional funding in connection with our continuing operations. We have based our estimate of our financing requirements on assumptions that may prove to be wrong, and we could use our capital resources sooner than we currently expect. Our future capital requirements will depend on many factors, including:

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the commercial success of Glybera, including the timing and amount of revenues generated, as well as our cost of goods sold;



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our collaboration agreements remaining in effect and our ability to obtain research and development funding and achieve milestones under these agreements;



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the progress and results of our current and planned clinical trials and those of our collaborators;



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the scope, progress, results and costs of preclinical development, laboratory testing and clinical trials for our additional product candidates;



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the number and development requirements of other product candidates that we pursue;



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the cost, timing and outcome of regulatory review of our product candidates, particularly for approval of Glybera in the United States;



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the cost and timing of future commercialization activities by us or our collaborators, including product manufacturing, marketing, sales and distribution, for Glybera and any of our product candidates for which we receive marketing approval in the future;



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the amount and timing of revenues, if any, we receive from commercial sales of any product candidates for which we receive marketing approval in the future;



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the costs and timing of preparing, filing and prosecuting patent applications, maintaining and enforcing our intellectual property rights and defending any intellectual property-related claims;



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the extent to which we acquire or in-license other products or technologies; and



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the cost and progress of the build-out of our Lexington, Massachusetts manufacturing facility.

Adequate capital may not be available to us when needed or may not be available on acceptable terms. Further, our ability to obtain debt financing may be limited by covenants we have made under our Loan and Security Agreement with Hercules Technology Growth Capital, Inc., or Hercules, and our pledge to Hercules of substantially all of our assets as collateral. These covenants, with certain exceptions, limit the ability of the Company to incur additional indebtedness and liens, pay dividends, make acquisitions, or sell or dispose of property and assets. Additionally, they require us to maintain cash equivalents on deposit in the United States of at least the lesser of (1) 100% of the then outstanding principal amount or (2) 50% of all the worldwide cash and cash equivalents. If we are unable to raise capital when needed or on attractive terms, we could be forced to delay, reduce or eliminate our research and development programs or any future commercialization efforts and would have a negative impact on our financial condition.

In addition, we may wish to seek additional capital due to favorable market conditions or strategic considerations, even if we believe we have sufficient funds for our current or future operating plans.

Raising additional capital may cause dilution to our shareholders, including purchasers in this offering, restrict our operations or require us to relinquish rights to our technologies or product candidates.

To the extent that we raise additional capital through the sale of equity or convertible debt securities, your ownership interest will be diluted, and the terms of these securities may include liquidation or other preferences that adversely affect your rights as an ordinary shareholder. Debt financing and preferred equity financing, if available, may involve agreements that include covenants limiting or restricting our ability to take specific actions, such as incurring additional debt, making capital expenditures or declaring dividends.

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If we raise additional funds through collaborations, strategic alliances or marketing, distribution or licensing arrangements with third parties, we may have to relinquish valuable rights to our technologies, future revenue streams, products or product candidates or grant licenses on terms that may not be favorable to us. If we are unable to raise additional funds through equity or debt financings when needed, we may be required to delay, limit, reduce or terminate our product development or future commercialization efforts or grant rights to develop and market products or product candidates that we would otherwise prefer to develop and market ourselves.

Our existing and any future indebtedness could adversely affect our ability to operate our business.

As of September 30, 2013, we had recognized a liability of €7.3 million ($9.8 million) of outstanding borrowings under our Loan and Security Agreement with Hercules, which we are required to repay in monthly installments through October 1, 2016. We do not intend to use the net proceeds of this offering to prepay these obligations. We could in the future incur additional debt obligations beyond our borrowings from Hercules. Our existing loan obligations, together with other similar obligations that we may incur in the future, could have significant adverse consequences, including:

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requiring us to dedicate a portion of our cash resources to the payment of interest and principal, reducing money available to fund working capital, capital expenditures, product development and other general corporate purposes;



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increasing our vulnerability to adverse changes in general economic, industry and market conditions;



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subjecting us to restrictive covenants that may reduce our ability to take certain corporate actions or obtain further debt or equity financing;



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limiting our flexibility in planning for, or reacting to, changes in our business and the industry in which we compete; and



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placing us at a competitive disadvantage compared to our competitors that have less debt or better debt servicing options.

We may not have sufficient funds, and may be unable to arrange for additional financing, to pay the amounts due under our existing loan obligations. Failure to make payments or comply with other covenants under our existing debt could result in an event of default and acceleration of amounts due. Under our agreement with Hercules, the occurrence of an event that would reasonably be expected to have a material adverse effect on our business, operations, assets or condition is an event of default. If an event of default occurs and the lender accelerates the amounts due, we may not be able to make accelerated payments, and the lender could seek to enforce security interests in the collateral securing such indebtedness, which includes substantially all of our assets. In addition, the covenants under our existing debt, and the pledge of our assets as collateral, could limit our ability to obtain additional debt financing.

Our business operations may be negatively affected by the strategic restructuring we undertook in 2012.

At the end of 2011, following the initial rejection of our application for marketing approval for Glybera in the European Union, our predecessor entity, Amsterdam Molecular Therapeutics, or AMT, initiated a strategic restructuring in order to conserve resources and improve its financial position. As part of this effort, AMT significantly reduced personnel, programs and spending. As a result, we lost many talented employees, including employees with an extensive understanding of our clinical programs as well as our regulatory and financial affairs. In the fourth quarter of 2011, total staff was reduced from 92 to 49. Since that time, we have hired a number of new staff, and total headcount as of September 30, 2013 was 76. In addition, we have engaged 33 consultants and contract workers. Nevertheless, this loss of talent and institutional knowledge has adversely affected our operations during the past year and may result in delays in preparing regulatory filings, completing clinical trials and other related activities, and could negatively impact our future business operations.

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Risks Related to the Development of Our Product Candidates

We may not be successful in our efforts to use our gene therapy technology platform to build a pipeline of additional product candidates.

A key element of our strategy is to use our gene therapy technology platform to expand our pipeline of gene therapies and to progress these product candidates through clinical development together with our collaborators. Although we currently have a pipeline of programs at various stages of development, we may not be able to identify or develop product candidates that are safe and effective. Even if we are successful in continuing to build our pipeline, the potential product candidates that we identify may not be suitable for clinical development, including as a result of being shown to have harmful side effects or other characteristics that indicate that they are unlikely to receive marketing approval and achieve market acceptance. Research programs to identify new product candidates require substantial technical, financial and human resources. We or our collaborators may focus our efforts and resources on potential programs or product candidates that ultimately prove to be unsuccessful. If we do not continue to successfully develop and commercialize product candidates based upon our technology, we may face difficulty in obtaining product revenues in future periods, which could result in significant harm to our financial position and adversely affect our share price.

Our strategy of obtaining rights to key technologies through in-licenses may not be successful.

We seek to expand our product pipeline in part by in-licensing the rights to key technologies, including those related to gene delivery and gene cassettes. The future growth of our business will depend in significant part on our ability to in-license or otherwise acquire the rights to additional product candidates or technologies, particularly through our collaborations with academic research institutions. However, we may be unable to in-license or acquire the rights to any such product candidates or technologies from third parties on acceptable terms or at all. The in-licensing and acquisition of these technologies is a competitive area, and a number of more established companies are also pursuing strategies to license or acquire product candidates or technologies that we may consider attractive. These established companies may have a competitive advantage over us due to their size, cash resources and greater clinical development and commercialization capabilities. In addition, companies that perceive us to be a competitor may be unwilling to license rights to us. Furthermore, we may be unable to identify suitable product candidates or technologies within our area of focus. If we are unable to successfully obtain rights to suitable product candidates or technologies, our business, financial condition and prospects could suffer.

We may encounter substantial delays in our clinical trials or fail to demonstrate the safety and efficacy of our product candidates.

Clinical development is expensive, time-consuming and uncertain as to outcome. Our product candidates are in early clinical or preclinical development, and there is a significant risk of failure or delay in each of these programs. In several of our programs, we intend to transition a collaborator's program to a different viral vector or to our insect-cell based manufacturing process, which could result in additional development challenges and delays. We cannot guarantee that any clinical trials will be conducted as planned or completed on schedule, if at all. A failure of one or more clinical trials can occur at any stage of testing. Events that may prevent successful or timely completion of clinical development include:

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delays in reaching a consensus with regulatory agencies on study design;



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delays in reaching agreement on acceptable terms with prospective clinical research organizations, or CROs, and clinical trial sites;



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delays in obtaining required Institutional Review Board, or IRB, approval at each clinical trial site;



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delays in recruiting suitable patients to participate in our clinical trials, particularly due to the rare nature of many of our target orphan monogenic diseases;



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imposition of a clinical hold by regulatory agencies after an inspection of our clinical trial operations or trial sites;

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failure by CROs, other third parties or us to adhere to clinical trial requirements;



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failure to perform in accordance with the FDA's good clinical practices, or GCP, or applicable regulatory guidelines in other countries;



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delays in the testing, validation, manufacturing and delivery of our product candidates to the clinical sites;



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delays in having patients complete participation in a study or return for post-treatment follow-up;



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clinical trial sites or patients dropping out of a study;



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occurrence of serious adverse events associated with a product candidate that are viewed to outweigh its potential benefits; or



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changes in regulatory requirements and guidance that require amending or submitting new clinical protocols.

Any inability to successfully complete preclinical and clinical development could result in additional costs to us or impair our ability to generate revenues from product sales, regulatory and commercialization milestones and royalties. In addition, if we make manufacturing or formulation changes to our product candidates, including changes in the vector or manufacturing process used, we may need to conduct additional studies to bridge our modified product candidates to earlier versions. Clinical trial delays could also shorten any periods during which we may have the exclusive right to commercialize our product candidates or allow our competitors to bring products to market before we do, which could impair our ability to successfully commercialize our product candidates and may harm our business and results of operations.

If clinical trials of our product candidates fail to satisfactorily demonstrate safety and efficacy to regulatory authorities, we may incur additional costs or experience delays in completing, or ultimately be unable to complete, the development and commercialization of our product candidates.

Before obtaining marketing approval from regulatory authorities for the sale of our product candidates, we must conduct extensive clinical trials to demonstrate the safety and efficacy of the product candidates in humans. It is impossible to predict when or if any of our product candidates will prove effective or safe in humans or will receive marketing approval. If the results of our clinical trials are inconclusive or if there are safety concerns or adverse events associated with our product candidates, we may:

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be delayed in or altogether prevented from obtaining marketing approval for our product candidates;



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obtain approval for indications or patient populations that are not as broad as intended or desired;



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obtain approval with labeling that includes significant use or distribution restrictions or safety warnings;



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be subject to changes with the way the product is administered;



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be required to perform additional clinical trials to support approval or be subject to additional post-marketing testing requirements;



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have regulatory authorities withdraw their approval of the product or impose restrictions on its distribution in the form of a modified risk evaluation and mitigation strategy;



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be subject to the addition of labeling statements, such as warnings or contraindications;



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be sued; or



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experience damage to our reputation.

Any of these events could prevent us from achieving or maintaining market acceptance of our product candidates and impair our ability to commercialize our products.

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We may experience delays or difficulties in the enrollment of patients in clinical trials, particularly for orphan indications, which may delay or prevent our receipt of necessary regulatory approvals.

Identifying and qualifying patients to participate in clinical trials of our product candidates is critical to our success. The timing of our clinical trials and those of our collaborators depends on the speed at which we or they can recruit patients to participate in such trials. We or our collaborators may not be able to locate and enroll a sufficient number of eligible patients to participate in these trials as required by the FDA, the EMA or similar regulatory authorities outside the United States and the European Union. In particular, because several of our programs are focused on the treatment of patients with orphan diseases, our ability to enroll eligible patients in these trials may be limited or slower than we anticipate in light of the small patient populations involved. For example, we reduced the number of patients enrolled in our second Phase II/III clinical trial of Glybera from the 16 patients originally planned to five patients due to slow recruitment. In addition, our potential competitors, including major pharmaceutical, specialty pharmaceutical and biotechnology companies, academic institutions and governmental agencies and public and private research institutions, may seek to develop competing therapies, which would further limit the small patient pool available for our studies. Patient enrollment may be affected by other factors including:

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the severity of the disease under investigation;



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design of the study protocol;



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the eligibility criteria for the study;



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the perceived risks, benefits and convenience of administration of the product candidate being studied;



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our efforts to facilitate timely enrollment in clinical trials;



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the patient referral practices of physicians; and



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the proximity and availability of clinical trial sites to prospective patients.

An inability by us or our collaborators to locate and enroll a sufficient number of patients for clinical trials may result in our failure to initiate or continue clinical trials for our product candidates, or may cause significant delays in such trials and could require us or our collaborators to abandon one or more clinical trials altogether. Enrollment delays may also result in increased development costs for our product candidates, which could cause the value of our company to decline.

Our progress in early-stage clinical trials may not be indicative of long-term efficacy in late-stage clinical trials, and our progress in trials for one product candidate may not be indicative of progress in trials for other product candidates.

With the exception of Glybera, the product candidates in our pipeline are at early-stages of development. Study designs and results from previous studies are not necessarily predictive of our future clinical study designs or results, and initial results may not be confirmed upon full analysis of the complete study data. Our product candidates may fail to show the desired safety and efficacy in later stages of clinical development despite having successfully advanced through initial clinical studies.

A number of companies in the pharmaceutical and biotechnology industries have suffered significant setbacks in late-stage clinical trials even after achieving promising results in early-stage clinical trials. If a larger population of patients does not experience positive results, if these results are not reproducible, or if our products show diminishing activity over time, our products may not receive approval from the EMA or FDA. Data obtained from preclinical and clinical activities are subject to varying interpretations, which may delay, limit or prevent regulatory approval. In addition, we may encounter regulatory delays or rejections as a result of many factors, including changes in regulatory policy during the period of product development. Failure to confirm favorable results from earlier trials by demonstrating the safety and effectiveness of our products in late stage clinical trials with larger patient populations could have a material adverse effect on our business that would cause our share price to decline.

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Progress in trials of Glybera and its approval in the European Union do not indicate that we will make similar progress in trials for our other product candidates. While Glybera uses an AAV1 vector for gene delivery, the rest of the product candidates in our pipeline use other AAV vector variants, such as AAV5 or AAV2. Also, while Glybera is injected directly into the muscles of the leg, the rest of the products in our pipeline target other tissues. Due to these variations, trials for our other product candidates may be less successful than the trials for Glybera.

If serious adverse events occur or unacceptable side effects are identified in any gene therapy products or product candidates, whether ours or those of our competitors, we may need to abandon or limit the sale or development of Glybera or our product candidates.

Glybera or our product candidates may prove to have undesirable or unintended side effects, toxicities or other characteristics that may preclude our obtaining additional marketing approval or prevent or limit commercial use. In our clinical development program for Glybera, there were a total of 48 serious adverse events, two of which were determined to be related to Glybera, a pulmonary embolism and fever. In our partner's clinical development program for AIP, there was one serious adverse event that was determined by the investigator not to be treatment-related. If Glybera or any of our product candidates or those of other parties are associated with undesirable side effects in clinical trials or have characteristics that are unexpected, we may need to abandon their development or limit development to more narrow uses or subpopulations in which the undesirable side effects or other characteristics are less prevalent, less severe or more acceptable from a risk-benefit perspective.

Negative public opinion and increased regulatory scrutiny of gene therapy and genetic research may damage public perception of Glybera and our product candidates or adversely affect our ability to conduct our business or obtain further marketing approvals for Glybera and marketing approvals for our product candidates.

Public perception may be influenced by claims that gene therapy is unsafe, and gene therapy may not gain the acceptance of the public or the medical community. In particular, our success will depend upon physicians specializing in the treatment of those diseases that our product candidates target prescribing treatments that involve the use of our product candidates in lieu of, or in addition to, existing symptomatic treatments they are already familiar with and for which greater clinical data may be available.

A generalized public backlash developed against gene therapy following the death in September 1999 of an 18-year-old who had volunteered for a gene therapy experiment at the University of Pennsylvania. Researchers at the university had infused the volunteer's liver with a gene aimed at reversing a rare metabolic disease of the liver. The procedure triggered an extreme immune-system reaction that caused multiple-organ failure in a very short time, leading to the first death to occur as a direct result of a gene therapy experiment. In addition, in two gene therapy studies in 2003, 20 subjects treated for X-linked severe combined immunodeficiency using a murine gamma-retroviral vector showed correction of the disease. However, the studies were terminated after five subjects developed leukemia.

Although none of our current product candidates utilize the gamma-retroviruses used in the 2003 studies, our product candidates do use a viral vector delivery system. The risk of cancer remains a concern for gene therapy and we cannot assure that it will not occur in any of our planned or future clinical studies. In addition, there is the potential risk of delayed adverse events following exposure to gene therapy products due to persistent biological activity of the genetic material or other components of products used to carry the genetic material.

Adverse events in our clinical trials or those conducted by other parties, even if not ultimately attributable to our product candidates, and the resulting publicity could result in increased governmental regulation, unfavorable public perception, potential regulatory delays in the testing or approval of our potential product candidates, stricter labeling requirements for those product candidates that are approved and a decrease in

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demand for any such product candidates. If any such adverse events occur, commercialization of Glybera or further advancement of our clinical trials could be halted or delayed, which would have a material adverse effect on our business and operations.

Risks Related to the Regulatory Approval of Our Product Candidates

Even if we complete the necessary preclinical tests and clinical trials, we cannot predict when or if we will obtain marketing approval to commercialize a product candidate, and any approval we receive may be for a more narrow indication than we expect.

We cannot commercialize a product until the appropriate regulatory authorities have reviewed and approved the product candidate. The development and commercialization of our product candidates, including their design, testing, manufacture, safety, efficacy, purity, recordkeeping, labeling, storage, approval, advertising, promotion, sale and distribution, are subject to comprehensive regulation by the EMA and other regulatory agencies of the member states of the European Union, by the FDA and other regulatory agencies in the United States, and similar regulatory authorities outside the European Union and the United States. Failure to obtain marketing approval for a product candidate in a specific jurisdiction will prevent us from commercializing the product candidate in that jurisdiction.

We have not received approval to market any of our products or product candidates from regulatory authorities in the United States. We received marketing authorization for Glybera from the European Commission in October 2012 under exceptional circumstances for a subset of LPLD patients, after our initial application was rejected in June 2011. We plan to file an IND with the FDA for Glybera in the first half of 2014. The FDA may not allow us to rely on the results of our prior clinical trials of Glybera, all of which were conducted outside the United States, or ultimately approve Glybera for marketing in the United States. The FDA may, for example, reject or discount the results of prior trials, or consider them to be inadequate or not well-controlled. Based on our meetings with the FDA in August and December 2013, we believe that to obtain marketing approval for Glybera in the United States, we will need to successfully conduct an adequate and appropriately controlled confirmatory clinical trial. We have not yet completed the design of this trial or prepared or submitted a protocol for this trial to the FDA. We may seek to amend the protocol for our European Union post-approval trial of Glybera so that such trial also could serve as such a confirmatory trial. The FDA may require preclinical testing or clinical trials beyond this confirmatory clinical trial as a basis for marketing approval of Glybera, which would be expensive and time consuming. If we fail to obtain marketing approval of Glybera in the United States on our anticipated timeframe, or obtain only limited approval for a specific patient population, our business could be materially adversely affected.

The process of obtaining marketing approval for our product candidates in the European Union, the United States and other countries is expensive and may take many years, if approval is obtained at all. Additional clinical trials may be required and can vary substantially based upon a variety of factors, including the type, complexity and novelty of the product candidates involved. Changes in marketing approval policies during the development period, changes in or the enactment of additional statutes or regulations, or changes in regulatory review for each submitted product application, may cause delays in the approval or rejection of an application. Regulatory authorities have substantial discretion in the approval process and may refuse to accept any application, may decide that our data are insufficient for approval, may require additional preclinical, clinical or other studies and may not complete their review in a timely manner. The FDA does not maintain a regulatory approval process similar to the EMA's marketing authorization under exceptional circumstances, which may make it more difficult to obtain marketing authorization for Glybera or other product candidates in the United States. Further, any marketing approval we ultimately obtain may be for only limited indications, or be subject to stringent labeling or other restrictions or post-approval commitments that render the approved product not commercially viable. For example, we received marketing authorization for Glybera in the European Union only for a restricted patient population and other regulatory agencies may approve Glybera only with a similarly limited label, which limits our addressable

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market. Further, Glybera received marketing approval subject to post-approval restrictions including the requirement to conduct a post-approval clinical study, and if we fail to adequately satisfy these post-approval requirements the EMA may withdraw its approval.

If we experience delays in obtaining approval or if we fail to maintain approval of Glybera in the European Union or obtain approval of Glybera in the United States or elsewhere or of any of our product candidates in the United States or other countries, the commercial prospects for Glybera or our other product candidates may be harmed and our ability to generate revenues will be materially impaired.

The European Commission authorized marketing of Glybera under exceptional circumstances, and only after its subsidiary committees had made negative decisions involving the use of Glybera for the treatment of all patients with LPLD.

The process for obtaining approval of Glybera in the European Union was protracted and complicated by initial decisions against approval by the committees charged with review of our marketing authorization application. In their initial decision in June 2011, both the CAT and the Committee for Human Medicinal Products, or CHMP, determined that the benefit-risk balance for Glybera was negative for the treatment of all patients with LPLD.

In June 2012, the CAT gave a positive opinion and the CHMP then reassessed Glybera and recommended approval for adult patients diagnosed with familial LPLD and suffering from severe or multiple pancreatitis attacks despite dietary fat restrictions. This was a more restricted patient population than we had sought in our original application. The European Commission granted this approval in October 2012, subject to certain conditions including additional post-marketing studies for efficacy.

Our receipt of marketing authorization under exceptional circumstances in the European Union does not provide any assurance that we will be able to obtain marketing authorization for Glybera elsewhere, including in the United States, or for our other gene therapies in any country.

Our product, Glybera, was approved in Europe through a special regulatory scheme allowing for marketing of certain products under exceptional circumstances. A similar pathway to approval does not exist in the United States. As a result, approval of Glybera in Europe does not guarantee or increase the likelihood of approval of Glybera by the FDA in the United States.

In October 2012, the European Commission authorized marketing of Glybera under exceptional circumstances. The FDA does not maintain a regulatory approval process similar to the EMA's marketing authorization under exceptional circumstances, which may make it more difficult to obtain approval for Glybera in the United States. In the United States, the FDA will generally only approve a product on the basis of two full clinical studies that contain substantial evidence of the safety and efficacy of the proposed new product. Clinical trials in the United States must be conducted in accordance with GCP requirements. A protocol for each clinical trial and any subsequent protocol amendments must be submitted to the FDA as part of an IND. Each institution participating in the clinical trial is subject to the initial approval, continuing review, and at least annual reapproval, by an IRB.

Given the differences between the regulatory schemes for approval of new products in Europe and the United States, approval of Glybera in the European Union does not assure or increase the likelihood of approval of the product in the United States. In addition, the FDA may not allow us to rely on the results of our prior clinical trials of Glybera, all of which were conducted outside the United States. The FDA may, for example, reject or discount the results of prior trials, or consider them to be inadequate or not well-controlled. In addition, the FDA may not conclude that the results of the trials demonstrate that Glybera is safe or effective, or may otherwise require additional clinical trials as a basis for marketing approval, which would be expensive and time consuming. If we fail to obtain marketing approval in the United States on our anticipated timeframe, or obtain only limited approval for a specific patient population, our business could be materially adversely affected.

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The FDA will require us to conduct comparability studies evaluating the products manufactured at our Amsterdam facility with those to be manufactured at our Lexington, Massachusetts facility, which is currently under construction. Those studies and their results could substantially delay or preclude our ability to commercialize Glybera and our product candidates in the United States.

The FDA maintains strict requirements governing the manufacturing process for biologics. When a manufacturer seeks to modify or change that process, the FDA typically requires the applicant to conduct non clinical and, depending on the magnitude of the changes, potentially clinical comparability studies that evaluate the potential differences in the product resulting from the change in the manufacturing process. In connection with any application we may file with the FDA seeking marketing approval for Glybera or any of our other product candidates in the United States, we will be required to conduct comparability studies assessing product manufactured at our facility in Amsterdam with product to be manufactured at our facility in Lexington, Massachusetts, which we are currently building out and equipping. The FDA may be especially concerned about the need for such a comparability study for Glybera if the clinical studies on which we rely for approval of our application only involved product manufactured at our facility in the Netherlands and if we intend to market only product manufactured in Lexington in the United States.

Delays in designing and completing a comparability study to the satisfaction of FDA could delay or preclude our development and commercialization plans and, thereby, limit our revenues and growth. For example, for Glybera, we may attempt to show comparability of the product manufactured at the different facilities through the use of non-clinical data, such as potency assays and animal studies. There is a risk that such data may not show acceptable comparability of the product manufactured at the different sites to the satisfaction of FDA. In addition, in the event that the FDA does not accept such non-clinical comparability data, we may need to conduct a study involving dosing of patients with product from our Lexington facility. That study may result in a delay of the approval or launch of Glybera in the United States.

We are subject to potentially costly post-approval requirements in the European Union that may restrict or eliminate the commercial success of Glybera.

As part of our marketing approval under exceptional circumstances in the European Union, the EMA has approved Glybera for the treatment of a subset of adult LPLD patients with familial LPLD diagnosed by genetic testing and suffering from severe or multiple pancreatitis attacks despite dietary fat restrictions, thereby imposing significant restrictions on the indicated uses and marketing of Glybera. Further, the EMA has imposed ongoing requirements for a potentially costly post-approval study and market surveillance. Specifically, as a condition to approval we are required to complete a post-approval clinical trial and implement a disease registry for long term surveillance of patients, as well as implement risk management procedures, distribute educational materials to healthcare professionals and patients, comply with certain notification obligations and undergo annual reassessment, the outcome of which could eventually lead to a withdrawal of the approval. The expense and uncertain result of these post-approval requirements may delay, limit or terminate our commercialization plan for Glybera and adversely affect our financial position.

The risks associated with the marketing approval process are heightened by our products' status as gene therapies.

Glybera has been evaluated as a gene therapy by the EMA. We believe that all of our current product candidates, including Glybera, will be viewed as gene therapy products by the EMA, FDA and other regulatory authorities. Gene therapies are relatively new treatments and regulators do not have extensive experience or standard review and approval processes for gene therapies. The FDA has never approved a gene therapy product as safe and effective and, unlike the EMA, does not have an exceptional circumstances approval pathway. The EMA has approved only one gene therapy, Glybera, for a subset of LPLD patients, under exceptional circumstances, and only did so by a vote of 17 to 15 and after twice denying approval. Given the novelty and complexity of our technology, we intend to discuss with the EMA, the FDA and other regulatory authorities the appropriate scientific analysis and evaluation of our methods to

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support applications for marketing approval for our product candidates. The application process will take time and resources, may require independent third-party analysis and may still not be accepted by the EMA, FDA or other regulatory authorities.

The EMA and FDA have demonstrated caution in their regulation of gene therapy treatments, and ethical and legal concerns about gene therapy and genetic testing may result in additional regulations or restrictions on the development and commercialization of our product candidates that are difficult to predict. For example, the EMA advocates a risk-based approach to the development of a gene therapy product. Agencies at both the United States federal and state level, as well as congressional committees and foreign governments, have sometimes expressed interest in further regulating biotechnology. Such action may delay or prevent commercialization of some or all of our product candidates. For example, in 2003, the FDA suspended 27 gene therapy trials involving several hundred patients after learning that a child treated in France had developed a condition resembling leukemia. Although the FDA was not aware that any of the patients treated in the 27 American trials had suffered illnesses similar to that of the infant in France, it nevertheless took precautions. This temporary halt, the largest such action involving gene therapy trials, was a setback for the field.

Regulatory requirements affecting gene therapy have changed frequently and may continue to change. For example, the European Commission conducted a public consultation in early 2013 on the application of European Union legislation that governs advanced therapy medicinal products, including gene therapy products, that could result in changes in the data we need to submit to the EMA in order for our product candidates to gain regulatory approval. In addition, divergent scientific opinions among the various bodies involved in the review process may result in delays and require additional resources and may ultimately result in rejection. For further discussion about the regulation we face in Europe and the United States, please see "Business—Government Regulation and Reimbursement."

In the United States, the FDA has established the Office of Cellular, Tissue and Gene Therapies within the Agency's Center for Biologics Evaluation and Research, or CBER, to consolidate the review of gene therapy and related products, and the Cellular, Tissue and Gene Therapies Advisory Committee to advise CBER on its review. Gene therapy clinical trials conducted at institutions that receive funding for recombinant DNA research from the NIH are also subject to review by the NIH Office of Biotechnology Activities' Recombinant DNA Advisory Committee, or the RAC. Although the FDA decides whether individual protocols may proceed, the RAC review process can impede the initiation of a clinical trial, even if the FDA has reviewed the study and approved its initiation. In addition, the FDA can put an IND on clinical hold even if the RAC has provided a favorable review. Also, before a clinical trial can begin at an NIH-funded institution, that institution's institutional review board, or IRB, and its Institutional Biosafety Committee will review the proposed clinical trial to assess the safety of the study. Adverse developments in clinical trials of gene therapy products conducted by others may cause the FDA or other regulatory bodies to change the requirements for approval of any of our product candidates.

The FDA and the EMA have issued various guidance documents pertaining to gene therapy products, with which we likely must comply to gain regulatory approval of any of our product candidates in the United States or European Union, respectively. Those guidance documents may require us, for example, to observe the subjects of our clinical trials for a longer period than normal following completion of a trial, or to undertake more extensive preclinical assessments, in particular concerning our vector and transgene expression cassette system, before beginning clinical trials at all. We also may need to conduct environmental risk assessments and special long term studies to monitor the safety and efficacy of our products. The close regulatory scrutiny of gene therapy products may result in delays and increased costs, and may ultimately lead to any gene therapy product not being approved.

These regulatory review committees and advisory groups and the new guidelines they promulgate may lengthen the regulatory review process, require us to perform additional studies, increase our development costs, lead to changes in regulatory positions and interpretations, delay or prevent approval and

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commercialization of these product candidates or lead to significant post-approval limitations or restrictions. As we advance our product candidates, we will be required to consult with these regulatory and advisory groups, and comply with applicable guidelines. If we fail to do so, we may be required to delay or discontinue development of our product candidates. Delay or failure to obtain, or unexpected costs in obtaining, the regulatory approval necessary to bring a potential product to market could decrease our ability to generate sufficient product revenues to maintain our business.

If we are not able to obtain or maintain orphan product exclusivity for any of our product candidates for which we seek this status, or if our competitors are able to obtain orphan product exclusivity before we do, we may not be able to obtain approval for our competing products for a significant period of time.

Regulatory authorities in some jurisdictions, including the European Union and the United States, may designate drugs for relatively small patient populations as orphan drugs. Under the Orphan Drug Act, the FDA may designate a product as an orphan drug if it is a drug intended to treat a rare disease or condition, which is generally defined as a patient population of fewer than 200,000 individuals annually in the United States or not more than five in 10,000 people in the European Union. Generally, if a product with an orphan drug designation subsequently receives the first marketing approval for the indication for which it has such designation, the product is entitled to a period of market exclusivity, which precludes the EMA or FDA from approving another marketing application for the same drug for the same indication for that time period. The applicable period is seven years in the United States and ten years in the European Union following marketing approval. The European Union exclusivity period can be reduced to six years if a drug no longer meets the criteria for orphan drug designation, for example if the drug is sufficiently profitable so that market exclusivity is no longer justified.

Orphan drug exclusivity may be lost if the EMA or FDA determines that the request for designation was materially defective, or if the manufacturer is unable to assure sufficient quantity of the drug to meet the needs of patients with the rare disease or condition or if the incidence and prevalence of patients who are eligible to receive the drug in these markets materially increase. Although we have obtained orphan designation for Glybera in the European Union and the United States, even after an orphan drug is approved, the same drug can subsequently be approved for the same condition if the competent regulatory agency concludes that the later drug is clinically superior in that it is shown to be safer, more effective or makes a major contribution to patient care.

If we lose orphan drug exclusivity or if our competitors obtain orphan drug exclusivity before we do, we may be precluded from obtaining marketing authorization or we may lose out on the potential benefits of market exclusivity.

Fast track designation by the FDA may not lead to a faster development or regulatory review or approval process, and it does not increase the likelihood that any of our product candidates will receive marketing approval in the United States.

We may in the future seek fast track designation for Glybera or other product candidates as appropriate in the United States. If a drug is intended for the treatment of a serious or life-threatening condition or disease and the drug demonstrates the potential to address unmet medical needs for this condition, the drug sponsor may apply for FDA fast track designation. The FDA has broad discretion whether or not to grant this designation, so even if we believe a particular product candidate is eligible for this designation, we cannot assure you that the FDA would decide to grant it. Even if we do apply for and receive fast track designation, we may not experience a faster development, review or approval process compared to conventional FDA procedures. The FDA may withdraw fast track designation if it believes that the designation is no longer supported by data from our clinical development program.

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Breakthrough therapy designation by the FDA may not lead to a faster development or regulatory review or approval process, and it does not increase the likelihood that any of our product candidates will receive marketing approval in the United States.

We may in the future apply for breakthrough therapy designation for Glybera or other product candidates in the United States. A breakthrough therapy is defined as a product that is intended, alone or in combination with one or more other drugs, to treat a serious or life-threatening disease or condition, and for which preliminary clinical evidence indicates substantial improvement over existing therapies on one or more clinically significant endpoints, such as substantial treatment effects observed early in clinical development. For drugs and biologics that have been designated as breakthrough therapies, interaction and communication between the FDA and the sponsor of the trial can help to identify the most efficient path for clinical development while minimizing the number of patients placed in ineffective control regimens. Products designated as breakthrough therapies by the FDA are also eligible for accelerated approval.

Designation as a breakthrough therapy is within the discretion of the FDA. Accordingly, even if we believe one of our products or product candidates meets the criteria for designation as a breakthrough therapy, the FDA may disagree. In any event, the receipt of a breakthrough therapy designation for a product or product candidate may not result in a faster development process, review or approval compared to products considered for approval under conventional FDA procedures and, in any event, does not assure ultimate approval by the FDA. In addition, even if one or more of our products or product candidates qualify as breakthrough therapies, the FDA may later decide that the products no longer meet the conditions for qualification or decide that the time period for FDA review or approval will not be shortened.

Any of our product candidates for which we obtain marketing approval in the future could be subject to post-marketing restrictions or other regulatory requirements.

Glybera and any of our product candidates for which we obtain marketing approval in the future, as well as the manufacturing process, post-approval studies and measures, labeling, advertising and promotional activities for such products, will be subject to continued requirements of and review by the FDA, EMA and other regulatory authorities. These requirements include submissions of safety and other post-marketing information and reports, registration and listing requirements, requirements relating to manufacturing, quality control, quality assurance and corresponding maintenance of records and documents and recordkeeping. Even if marketing approval of a product candidate is granted, the approval may be subject to limitations on the indicated uses for which the product may be marketed or to the conditions of approval, including the requirement to develop and implement a risk evaluation and mitigation strategy. For example, the EMA's approval of Glybera was contingent upon our agreeing to post-approval obligations described elsewhere in this prospectus.

The FDA may also impose requirements for costly post-marketing studies or clinical trials and surveillance to monitor the safety or efficacy of a product. The FDA and other agencies, including the Department of Justice, closely regulate and monitor the post-approval marketing and promotion of products to ensure that they are manufactured, marketed and distributed only for the approved indications and in accordance with the provisions of the approved label. The FDA imposes stringent restrictions on manufacturers' communications regarding off-label use and if we do not market our products for their approved indications, we may be subject to an enforcement action for off-label marketing. Violations of the Federal Food, Drug, and Cosmetic Act and other statutes, including the False Claims Act, relating to the promotion of prescription drugs may lead to investigations alleging violations of federal and state health care fraud and abuse laws, as well as state consumer protection laws. Similar regulations apply in many other foreign jurisdictions.

In addition, later discovery of previously unknown adverse events or other problems with our products, or failure to comply with regulatory requirements, may yield various adverse results, including:

•

restrictions on such products or manufacturing processes;

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restrictions on the labeling or marketing of a product;



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restrictions on product distribution or use;



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requirements to conduct post-marketing studies or clinical trials;



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warning letters;



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withdrawal of the products from the market;



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refusal to approve pending applications or supplements to approved applications that we submit;



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recall of products;



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fines, restitution or disgorgement of profits or revenues;



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suspension or withdrawal of marketing approvals;



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refusal to permit the import or export of our products;



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product seizure; and



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injunctions or the imposition of civil or criminal penalties.

Non-compliance with European Union requirements regarding safety monitoring or pharmacovigilance can also result in significant financial penalties. Similarly, failure to comply with the European Union's requirements regarding the protection of personal information can also lead to significant penalties and sanctions.

Any government investigation of alleged violations of law could require us to expend significant time and resources and could generate negative publicity. The occurrence of any event or penalty may inhibit our ability or that of our collaborators to commercialize Glybera and any other products and generate revenues, and may also adversely affect our ability to obtain FDA approval. Failure to maintain marketing approval for Glybera in the European Union or to obtain regulatory approval for Glybera in other jurisdictions may also adversely affect our ability to develop other product candidates, given the general applicability of our technology platform to the development of our current and future product candidates.

Risks Related to the Commercialization of Glybera and Our Product Candidates

If we or our collaborators are unable to commercialize Glybera or our other product candidates or experience significant delays in doing so, our business will be materially harmed.

Our ability to generate product revenues will depend heavily on the successful commercialization of Glybera and development and eventual commercialization of other product candidates. The success of our product candidates will depend on several factors, including the following:

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successful completion of preclinical studies and clinical trials;



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receipt of marketing approvals from applicable regulatory authorities;



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obtaining and maintaining patent and trade secret protection and non-patent, orphan drug exclusivity for our product candidates;



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completing the build-out of, and obtaining regulatory approval for, our new manufacturing facility in Lexington, Massachusetts;



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launching commercial sales of our products, if and when approved, whether alone or in collaboration with others;



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identifying and engaging effective distributors or other third party resellers on acceptable terms in certain jurisdiction where we plan to utilize third parties for the marketing and sale of Glybera or other candidate products;



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acceptance of our products, if and when approved, by patients, the medical community and third party payors;



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effectively competing with other therapies;



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obtaining and maintaining healthcare coverage and adequate reimbursement; and

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complying with post-approval requirements of the EMA and maintaining a continued acceptable overall safety profile based on the EMA's risk-benefit analysis.

Failure to achieve or implement any of these elements could result in significant delays or an inability to successfully commercialize Glybera or our product candidates, which could materially harm our business.

The affected populations for Glybera and product candidates may be smaller than we or third parties currently project, which may affect the addressable markets for Glybera and product candidates.

Our projections of the number of people who have the diseases we are seeking to treat, as well as the subset of people with these diseases who have the potential to benefit from treatment with Glybera or our product candidates, are estimates based on our knowledge and understanding of these diseases. The total addressable market opportunity for Glybera and our product candidates will ultimately depend upon a number of factors including the diagnosis and treatment criteria included in the final label, if approved for sale in specified indications, acceptance by the medical community, patient access and product pricing and reimbursement. Prevalence estimates are frequently based on information and assumptions that are not exact and may not be appropriate, and the methodology is forward-looking and speculative. The process we have used in developing an estimated prevalence range for the indications we are targeting has involved collating limited data from multiple sources. Accordingly, the prevalence estimates included in this prospectus should be viewed with caution. Further, the data and statistical information used in this prospectus, including estimates derived from them, may differ from information and estimates made by our competitors or from current or future studies conducted by independent sources.

The use of such data involves risks and uncertainties and is subject to change based on various factors. Our estimates may prove to be incorrect and new studies may change the estimated incidence or prevalence of the diseases we seek to address. The number of patients with the diseases we are targeting in the European Union, the United States and elsewhere may turn out to be lower than expected or may not be otherwise amenable to treatment with our products, or new patients may become increasingly difficult to identify or access, all of which would adversely affect our results of operations and our business.

Glybera, and any other product candidate that receives marketing approval in the future, may fail to achieve the degree of market acceptance by physicians, patients, third-party payors and others in the medical community necessary for commercial success.

Doctors may be reluctant to accept a gene therapy as a treatment option or, where available, choose to continue to rely on existing symptomatic treatments. If Glybera does not achieve an adequate level of acceptance, we may not generate significant revenues from this product and we may never achieve profitability. The degree of market acceptance of Glybera, as well as of any of our product candidates that receive marketing approval in the future, will depend on a number of factors, including:

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the efficacy and potential advantages of our therapies compared with alternative treatments;



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our ability to convince payors of the long-term cost-effectiveness of our therapies and, consequently, the availability of third-party coverage and adequate reimbursement;



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the limitations on use and label requirements imposed by regulators;



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the convenience and ease of administration of our gene therapies, which in the case of Glybera requires spinal anaesthesia and multiple intramuscular injections, compared to alternative treatments;



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the willingness of the target patient population to try new therapies, especially a gene therapy, and of physicians to administer these therapies;



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the strength of marketing and distribution support;



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the prevalence and severity of any side effects; and



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any restrictions on the use of our products.

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In the case of Glybera in the European Union, we are required to put in place a restricted access program to ensure that the product is used appropriately when the diagnosis is confirmed, mandating that the product only be supplied to doctors who have received the appropriate educational materials and only be used to treat patients participating in a registry to monitor the outcome of patients treated with Glybera. These requirements may further limit our ability to gain sufficient market acceptance.

If our collaboration with Chiesi is not successful, we may not effectively commercialize Glybera in the European Union and other countries covered by our partnership with Chiesi.

We have entered into a collaboration with Chiesi for the commercialization of Glybera in the European Union, China, Russia and other specified countries. As a result, we are dependent on the efforts of Chiesi to successfully commercialize Glybera in these countries. There is a risk that Chiesi:

•

may not perform its obligations as expected;



•

may have difficulties gaining acceptance of the use of Glybera in the clinical community and achieving satisfactory pricing and reimbursement of Glybera;



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may terminate, or may elect not to continue or renew, our commercialization arrangements based on changes in its strategic focus or available funding, or external factors, such as an acquisition, that divert resources or create competing priorities; and



•

may not commit sufficient resources to the marketing and distribution of Glybera.

In addition, we are required to manufacture Glybera for sale by Chiesi. Should we encounter manufacturing problems, we may fail to adequately supply Glybera to Chiesi. If any of these circumstances related to our collaboration with Chiesi are realized, they may adversely affect the commercial success of Glybera in the European Union and other countries covered by our partnership with Chiesi.

We face substantial competition, which may result in others discovering, developing or commercializing products before or more successfully than we do.

The development and commercialization of new biotechnology and biopharmaceutical products, including gene therapies, is highly competitive. We may face competition with respect to Glybera and our current product candidates, as well as with respect to any product candidates that we may seek to develop or commercialize in the future, from large and specialty pharmaceutical companies and biotechnology companies worldwide, who currently market and sell products or are pursuing the development of products for the treatment of many of the disease indications for which we are developing our product candidates. Potential competitors also include academic institutions, government agencies and other public and private research organizations that conduct research, seek patent protection and establish collaborative arrangements for research, development, manufacturing and commercialization. In recent years, there has been a significant increase in commercial and scientific interest and financial investment in gene therapy as a therapeutic approach, which has intensified the competition in this area.

We are aware of several companies focused on developing gene therapies in various indications, including AGTC, Asklepios, Audentes Therapeutics, BioMarin, bluebird bio, Dimension/Regen X, Oxford BioSciences, Sangamo BioScience, and Spark Therapeutics, as well as several companies addressing other methods for modifying genes and regulating gene expression. We may also face competition with respect to the treatment of some of the diseases that we are seeking to target with our gene therapies from protein pharmaceuticals under development at pharmaceutical and biotechnology companies, including Pfizer, Baxter, Bayer, Novo Nordisk, Genzyme, Shire, BioMarin and Biogen Idec. We must also compete with existing standards of care, therapies and symptomatic treatments, as well as any new therapies that may become available in the future for the indications we are targeting.

Our commercial opportunity could be reduced or eliminated if our competitors develop and commercialize products that are safer, more effective, have fewer or less severe side effects, are more convenient or are less expensive than the products that we develop. Our competitors also may obtain EMA, FDA or other

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regulatory approval for their products more rapidly than we do, which could result in our competitors establishing a strong market position before we are able to enter the market. Because we expect that gene therapy patients may generally require only a single administration, we believe that the first gene therapy product to enter the market for a particular indication will likely enjoy a significant commercial advantage, and may also obtain market exclusivity under applicable orphan drug regimes.

Many of the companies with which we are competing or may compete in the future have significantly greater financial resources and expertise than we do in research and development, manufacturing, preclinical testing, conducting clinical trials, obtaining regulatory approvals and marketing approved products. Mergers and acquisitions in the pharmaceutical and biotechnology industries may result in more resources being concentrated among a smaller number of our competitors. Smaller and other early-stage companies may also prove to be significant competitors, particularly through collaborative arrangements with large and established companies. These third parties compete with us in recruiting and retaining qualified scientific and management personnel, establishing clinical trial sites and patient registration for clinical trials, as well as in acquiring technologies complementary to, or necessary for, our programs.

Even if we are successful in achieving regulatory approval to commercialize a product candidate faster than our competitors, it is conceivable that we may face competition from biosimilars. In the United States, the Biologics Price Competition and Innovation Act of 2009 authorized the FDA to approve products that are "highly similar," or biosimilar, to or "interchangeable" with an FDA-approved biological product following expiration of a 12 year period of exclusivity. In the European Union, a competitor may reference data from biological products already approved, but will not be able to get on the market until ten years after the time of approval, although that may be extended to 11 years under specified circumstances. If competitors are able to obtain marketing approval for biosimilars after the applicable period of non-patent exclusivity expires, the attendant competitive pressure and consequences. Expiration or successful challenge of our applicable patent rights could also trigger competition from other products.

Risks Related to Our Dependence on Third Parties for Glybera and our Product Pipeline

We rely on third parties for important aspects of our development programs. If these parties do not perform successfully or if we are unable to maintain any of our collaboration arrangements, our business could be adversely affected.

We have entered into collaborations with other companies and academic research institutions with respect to important elements of our commercial and development programs. For example, we have collaboration agreements with Chiesi, for both commercialization of Glybera in the European Union and certain other countries and co-development and commercialization of our hemophilia B program, and development programs with Digna Biotech, Institut Pasteur and UCSF. We believe that these arrangements provide us with access to important technologies and capabilities, and in many cases to data from our collaborators' preclinical and clinical development programs. Our collaboration with Chiesi has also provided us with important funding for our Glybera and hemophilia B development programs.

Our existing collaborations, and any future collaborations we enter into, may pose a number of risks, including the following:

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collaborators have significant discretion in determining the efforts and resources that they will apply to these collaborations;



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in our current collaborations, we generally have limited or no control over the design or conduct of clinical trials sponsored by our collaborators;



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if our collaborators do not conduct the clinical trials they sponsor in accordance with regulatory requirements or stated protocols, we will not be able to rely on the data produced in such trials in our further development efforts;

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collaborators may not perform their obligations as expected;



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collaborators may also have relationships with other entities, some of which may be our competitors;



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collaborators may not pursue development and commercialization of any product candidates or may elect not to continue or renew development or commercialization programs based on clinical trial results, changes in the collaborators' strategic focus or available funding, or external factors, such as an acquisition, that divert resources or create competing priorities;



•

collaborators may delay clinical trials, provide insufficient funding for a clinical trial program, stop a clinical trial or abandon a product candidate, repeat or conduct new clinical trials or require a new formulation of a product candidate for clinical testing;



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collaborators could independently develop, or develop with third parties, products that compete directly or indirectly with our products or product candidates if the collaborators believe that competitive products are more likely to be successfully developed or can be commercialized under terms that are more economically attractive than ours;



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our collaboration arrangements may impose restrictions on our ability to undertake other development efforts that may appear to be attractive to us;



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product candidates discovered in collaboration with us may be viewed by our collaborators as competitive with their own product candidates or products, which may cause collaborators to cease to devote resources to the commercialization of our product candidates;



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a collaborator with marketing and distribution rights to Glybera or one or more of our product candidates that achieve regulatory approval may not commit sufficient resources to the marketing and distribution of such product or products;



•

disagreements with collaborators, including disagreements over proprietary rights, contract interpretation or the preferred course of development, could cause delays or termination of the research, development or commercialization of product candidates, lead to additional responsibilities for us, or result in litigation or arbitration, any of which would be time-consuming and expensive;



•

collaborators may not properly maintain or defend our intellectual property rights or may use our proprietary information in such a way as to invite litigation that could jeopardize or invalidate our intellectual property or proprietary information or expose us to potential litigation;



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collaborators may infringe the intellectual property rights of third parties, which may expose us to litigation and potential liability; and



•

collaborations may in some cases be terminated for the convenience of the collaborator and, if terminated, we could be required to expend additional funds to pursue further development or commercialization of the applicable product or product candidates.

If our collaborations do not result in the successful development and commercialization of products or if one of our collaborators terminates its agreement with us, we may not receive any future research funding or milestone or royalty payments under the collaboration. If we do not receive the funding we expect under these agreements, our development of our product candidates could be delayed and we may need additional resources to develop our product candidates. All of the risks relating to product development, regulatory approval and commercialization described in this prospectus also apply to the activities of our development collaborators.

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If we are unable to enter into additional collaborations in the future, or if our new collaborations are not successful, we may not be able to develop or market our product candidates or obtain a strategic position in the development of new gene therapies.

We believe collaborations enable us to gain access to early-stage clinical programs and related data, as well as to promising transgenes and other intellectual property, with limited financial investment by us. Part of our strategy is to leverage our experience and expertise in gene therapy research and development, as well as our proprietary manufacturing capabilities, to be an attractive collaborator for academic research institutions and biotechnology and pharmaceutical companies seeking to advance their programs into larger, late-stage clinical trials that require commercial-scale manufacturing. We face significant competition from other gene therapy, biotechnology and pharmaceutical companies, and we may be unable to attract suitable collaborators or reach agreements with them on acceptable terms, which could limit our access to attractive development programs.

For some of our product candidates, particularly for chronic and degenerative diseases that will require large clinical trials and a retail sales force to address the market, we may in the future collaborate with pharmaceutical and biotechnology companies for development and potential commercialization. We face significant competition in seeking appropriate collaborators. Our ability to reach a definitive agreement for a collaboration will depend, among other things, upon our assessment of the collaborator's resources and expertise, the terms and conditions of the proposed collaboration and the proposed collaborator's evaluation of a number of factors. Many of our agreements with our licensors, including our agreements with the NIH, require us to obtain consent from the licensor before we can enter into arrangements involving the sublicensing of technology we have licensed from such licensors. Our licensors may withhold such consent, or may provide such consent only if we agree to reduce our rights or increase our financial or other obligations to them. Obtaining such consent may also hamper our ability to enter into collaboration arrangements on a timely basis.

If we are unable to reach agreements with suitable collaborators on a timely basis, on acceptable terms, or at all, we may have to curtail the development of a product candidate, reduce or delay its development or one or more of our other development programs, delay its potential commercialization or reduce the scope of any sales or marketing activities, or increase our expenditures and undertake development or commercialization activities at our own expense. If we elect to fund and undertake development or commercialization activities on our own, we may need to obtain additional expertise and additional capital, which may not be available to us on acceptable terms or at all. If we fail to enter into collaborations and do not have sufficient funds or expertise to undertake the necessary development and commercialization activities, we may not be able to further develop our product candidates or bring them to market or continue to develop our product platform, and our business may be materially and adversely affected.

We do not currently have a sales or marketing infrastructure and have no experience in the sale, marketing or distribution of pharmaceutical products. To achieve commercial success for Glybera in the European Union and certain other countries, we are dependent on Chiesi's sales and marketing infrastructure. We may enter into similar arrangements with other parties in respect of the commercialization of products in other jurisdictions. Entering into arrangements with other third parties to perform these services may result in lower product revenues and profitability, if any, than if we were to market, sell and distribute Glybera or other products ourselves. In addition, we may not be successful in entering into arrangements with third parties in the future to sell, market and distribute our product candidates, including Glybera in other territories, or may be unable to do so on terms that are favorable to us. We likely will have little control over such third parties, and any of them may fail to devote the necessary resources and attention to sell and market our products effectively.

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Risks Related to Our Manufacturing

Gene therapies, including Glybera, are complex and difficult to manufacture. We could experience production problems that result in delays in our development or commercialization schedules or otherwise adversely affect our business.

We manufacture Glybera and clinical supplies of our product candidates ourselves in our facility in Amsterdam and plan to commence production in our Lexington, Massachusetts facility, which is currently under construction. The insect-cell based manufacturing process we use to produce Glybera and our other product candidates is highly complex and in the normal course is subject to production difficulties. A number of factors could cause production interruptions, including equipment malfunctions, facility contamination, labor problems, raw material shortages or contamination, natural disasters, disruption in utility services, terrorist activities, human error or disruptions in the operations of our suppliers.

Our viral vectors require processing steps that are more complex than those required for most chemical pharmaceuticals. Moreover, unlike chemical pharmaceuticals, the physical and chemical properties of a biologic such as ours generally cannot be adequately characterized prior to manufacturing the final product. As a result, an assay of the finished product is not sufficient to ensure that the product will perform in the intended manner. Accordingly, we employ multiple steps to attempt to control our manufacturing process to assure that the process works and the product or product candidate is made strictly and consistently in compliance with the process. Problems with the manufacturing process, even minor deviations from the normal process, could result in product defects or manufacturing failures that result in lot failures, product recalls, product liability claims and insufficient inventory. We may encounter problems achieving adequate or clinical-grade materials that meet EMA, FDA or other applicable standards or specifications with consistent and acceptable production yields and costs.

In addition, the EMA, FDA and other regulatory bodies may require us to submit samples of any lot of any approved product together with the protocols showing the results of applicable tests at any time. Under some circumstances, the EMA, FDA or other regulatory bodies may require that we not distribute a lot until the agency authorizes its release. Slight deviations anywhere in the manufacturing process, including stability and quality control, may result in unacceptable changes in the products that could result in lot failures or product recalls. Lot failures or product recalls could cause us to delay product launches or clinical trials, which could be costly to us and otherwise harm our business.

We also may encounter problems hiring and retaining the experienced specialist personal needed to operating our manufacturing process, which could result in delays in our production or difficulties in maintaining compliance with applicable regulatory requirements.

Any problems in our manufacturing process or facilities could make us a less attractive collaborator for academic research institutions and other parties, which could limit our access to additional attractive development programs.

Any contamination in our manufacturing process, shortages of raw materials or failure of any of our key suppliers to deliver necessary components of our platform could result in delays in our clinical development or marketing schedules.

Given the nature of biologics manufacturing, there is a risk of contamination. Any contamination could materially adversely affect our ability to produce our gene therapies, including Glybera, on schedule and could therefore harm our results of operations and cause reputational damage.

Some of the raw materials required in our manufacturing process are derived from biological sources. Such raw materials are difficult to procure and may also be subject to contamination or recall. A material shortage, contamination, recall, or restriction on the use of biologically derived substances in the manufacture of Glybera or our product candidates could adversely impact or disrupt the commercial

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manufacturing of Glybera or the production of clinical material, which could materially and adversely affect our operating results and development timelines.

We rely on single suppliers for the supply and manufacture of certain components of our technology. Specifically, we have only one source of supply for some of the materials used in the chromatography step of our manufacturing process. We are not currently seeking to establish secondary suppliers for these materials. We may not be successful in establishing secondary suppliers on acceptable terms, if at all, should our suppliers discontinue supply of these materials. Further, these suppliers are not required to give us advance notice in the event they discontinue supply of the relevant materials. Should our ability to procure these material components from our sole suppliers be compromised, our ability to continuously operate would be impaired until an alternative supplier is sourced, qualified and tested, which could limit our ability to produce a commercial supply of Glybera, delay the development programs of Glybera and our other product candidates and harm our business.

Delays in completing and receiving regulatory approvals for our new U.S. manufacturing facility could delay our development and commercialization plans and thereby limit our revenues and growth.

We are expending significant funds for the build-out of our leased 53,000 square foot manufacturing facility in Lexington, Massachusetts. This project may result in unanticipated delays and cost more than expected due to a number of factors, including regulatory requirements. If construction or regulatory approval of our new facility is delayed, we may not be able to manufacture sufficient quantities of Glybera or our product candidates, which would limit our commercialization and development activities and our opportunities for growth. Cost overruns associated with this facility could also require us to raise additional funds from external sources, which may be unavailable on favorable terms or at all.

Our manufacturing facility in Amsterdam is, and our facility in Lexington that is under construction will be, subject to significant government regulations and approvals, which are often costly. If we fail to comply with these regulations or maintain these approvals, our business will be materially harmed.

Our manufacturing facility in Amsterdam is, and our new facility in Lexington will be, subject to ongoing regulation and periodic inspection by the EMA, FDA and other regulatory bodies to ensure compliance with cGMP. Any failure to follow and document our adherence to such cGMP regulations or other regulatory requirements may lead to significant delays in the availability of products for commercial use or clinical study, may result in the termination of or a hold on a clinical study, or may delay or prevent filing or approval of marketing applications for our products. In addition, if we alter our manufacturing process for gene therapies, including Glybera, or during the development of a product candidate, the EMA, FDA or other regulatory authorities may require additional testing and clinical studies to ensure adequate safety and efficacy.

To monitor our compliance with applicable regulations, the EMA, FDA and comparable agencies in other jurisdictions routinely conduct inspections of our facilities and may identify potential deficiencies for us to address. For example, the FDA issues what are referred to as "FDA Form 483s" that set forth observations and concerns that are identified during its inspections. We may receive Form 483s in the future. Failure to satisfactorily address the concerns or potential deficiencies identified in a Form 483 could result in us being issued a warning letter, a notice of what the FDA believes to be significant regulatory violations requiring prompt corrective actions. If we fail to adequately respond to a warning letter, or otherwise fail to comply with applicable regulatory requirements, we could be subject to enforcement, remedial and/or punitive actions by the FDA or other regulatory authorities.

Failure to comply with applicable regulations could also result in the EMA, FDA or other applicable authorities taking various actions, including:

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levying fines and other civil penalties;



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imposing consent decrees or injunctions;

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a requirement to suspend or put on hold one or more of our clinical trials;



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suspending or withdrawing regulatory approvals;



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delaying or refusing to approve pending applications or supplements to approved applications;



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suspending manufacturing activities or product sales, imports or exports;



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requiring us to communicate with physicians and other customers about concerns related to actual or potential safety, efficacy, and other issues involving our products;



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mandating product recalls or seizing products;



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imposing operating restrictions; and



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seeking criminal prosecutions.

Any of the foregoing could materially harm our business.

Our use of viruses, chemicals and other hazardous materials requires us to comply with regulatory requirements and exposes us to significant potential liabilities.

Our development and manufacturing processes involve the use of viruses, chemicals and other hazardous materials, and produce waste products. Accordingly, we will be subject to federal, state and local laws and regulations in the United States, and are subject to comparable regulations in the Netherlands, governing the use, manufacture, distribution, storage, handling, treatment and disposal of these materials. In addition to ensuring the safe handling of these materials, applicable requirements require increased safeguards and security measures for many of these agents, including controlling access and screening of entities and personnel who have access to them, and establishing a comprehensive national database of registered entities. In the event of an accident or failure to comply with environmental, occupational health and safety and export control laws and regulations, we could be held liable for damages that result, and any such liability could exceed our assets and resources.

Risks Related to Our Intellectual Property

We license intellectual property from third parties, and such licenses may not provide adequate rights, may not be available in the future on commercially reasonable terms or at all, or our licensors may be unable to obtain and maintain patent protection for the technology or products that we license from them.

We currently are heavily reliant upon licenses of proprietary technology from third parties that is important or necessary to the development of our technology and products, including technology related to our manufacturing process, our vector platform, our gene cassettes and the therapeutic genes of interest we are using. These and other licenses may not provide adequate rights to use such technology in all relevant fields of use. Licenses to additional third-party technology that may be required for our development programs may not be available in the future or may not be available on commercially reasonable terms, which could have a material adverse effect on our business and financial condition.

In some circumstances, we may not have the right to control the preparation, filing and prosecution of patent applications, or to maintain the patents, covering technology that we license from third parties. In addition, some of our agreements with our licensors require us to obtain consent from the licensor before we can enforce patent rights, and our licensor may withhold such consent or may not provide it on a timely basis. Therefore, we cannot be certain that these patents and applications will be prosecuted and enforced in a manner consistent with the best interests of our business. In addition, if third parties who license patents to us fail to maintain such patents, or lose rights to those patents, the rights we have licensed may be reduced or eliminated.

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Our intellectual property licenses with third parties may be subject to disagreements over contract interpretation, which could narrow the scope of our rights to the relevant intellectual property or technology or increase our financial or other obligations to our licensors.

The agreements under which we currently license intellectual property or technology from third parties are complex, and certain provisions in such agreements may be susceptible to multiple interpretations. The resolution of any contract interpretation disagreement that may arise could narrow what we believe to be the scope of our rights to the relevant intellectual property or technology, or increase what we believe to be our financial or other obligations under the relevant agreement, either of which could have a material adverse effect on our business and financial condition. For example, we have an exclusive license from the NIH for "the development and sale of AAV5 based therapeutic products to be delivered to the brain or liver for treatment of human diseases originating in the brain or liver," other than arthritis-related diseases. We also have a non-exclusive license from the NIH for the development and sale of AAV5 based therapeutic products to treat human diseases other than those covered by our exclusive license.

We believe that our exclusive license from the NIH includes the systemic administration of AAV5-based therapeutic products so long as such therapeutic products are "to be delivered to the brain or liver for treatment of human diseases originating in the brain or liver." However, Sangamo BioSciences, Inc., or Sangamo, has announced that it has broad worldwide licenses to use AAV vectors, including AAV5 and AAV6, for research, development and commercialization of therapies for hemophilia A and B, Huntington's disease and other targets. We believe Sangamo's view may be that our exclusive license excludes systemic administration because Sangamo interprets the phrase "to be delivered to" to require direct administration into the brain or liver. Our view is that the phrase "to be delivered to" indicates the ultimate destination of the therapy and not the location where it is first introduced into the body. Although we think our interpretation is correct, there can be no assurance that a court would agree with our interpretation regarding the meaning of this phrase. If our interpretation of the phrase "to be delivered to" is incorrect, then others may obtain licenses from the NIH that may enable them to compete with us in the systemic administration of AAV5-based therapeutics for treatment of human diseases originating in the brain or liver, which could harm our business.

If we fail to comply with our obligations in our intellectual property licenses with third parties, we could lose rights that are important to our business.

We in-license intellectual property from third parties that is material to Glybera and all of our product candidates, including technology related to our manufacturing process, our vector platform, and the therapeutic gene cassettes we are using. Our licensing arrangements with third parties impose diligence, development and commercialization timelines, milestone payment, royalty, insurance and other obligations on us. If we fail to comply with these obligations, our counterparties may have the right to terminate these agreements, in which case we might not be able to develop, manufacture or market any product that is covered by these agreements or may face other penalties under the agreements. Such an occurrence could materially adversely affect the value of the product candidate being developed under any such agreement. Termination of these agreements or reduction or elimination of our rights under these agreements may result in our having to negotiate new or amended agreements with less favorable terms, or cause us to lose our rights under these agreements, including our rights to important intellectual property or technology.

If we are unable to obtain and maintain patent protection for our technology and products, our ability to successfully commercialize our products may be impaired.

We rely upon a combination of in-licensed and owned patents, trade secret protection and confidentiality agreements to protect our intellectual property. Our success depends in large part on our ability to obtain and maintain this protection in the European Union, the United States and other countries, in part by filing patent applications related to our novel technologies and product candidates. Our patents may not provide us with any meaningful commercial protection, prevent competitors from competing with us or otherwise

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provide us with any competitive advantage. Our competitors may be able to circumvent our owned or licensed patents by developing similar or alternative technologies or products in a non-infringing manner.

The patent prosecution process is expensive and time-consuming, and we may not be able to file and prosecute all necessary or desirable patent applications at a reasonable cost or in a timely manner. It is also possible that we will fail to identify patentable aspects of our research and development output before it is too late to obtain patent protection.

The patent position of biotechnology and pharmaceutical companies generally is highly uncertain, involves complex legal and factual questions and has in recent years been the subject of much litigation. In addition, the laws of foreign countries may not protect our rights to the same extent as the laws of the United States. For example, European Union patent law with respect to the patentability of methods of treatment of the human body is more limited than United States law. Publications of discoveries in the scientific literature often lag behind the actual discoveries, and patent applications in the United States and other jurisdictions are typically not published until 18 months after their priority date, or in some cases at all. Therefore, we cannot know with certainty whether we were the first to make the inventions or that we were the first to file for patent protection of the inventions claimed in our owned or licensed patents or pending patent applications. As a result, the issuance, scope, validity, enforceability and commercial value of our patent rights are highly uncertain. Our pending and future patent applications may not result in patents being issued that protect our technology or products, in whole or in part, or which effectively prevent others from commercializing competitive technologies and products. Changes in either the patent laws or interpretation of the patent laws in the European Union, the United States or other countries may diminish the value of our patents or narrow the scope of our patent protection.

If the scope of the patent protection we obtain is not sufficiently broad, our ability to successfully commercialize our technology and products may be impaired.

The issuance of a patent is not conclusive as to its inventorship, scope, validity or enforceability, and our owned and licensed patents may be challenged in the courts or patent offices in the European Union, the United States or elsewhere. Such challenges may result in loss of exclusivity or freedom to operate or in patent claims being narrowed, invalidated or held unenforceable, in whole or in part, which could limit our ability to stop others from using or commercializing similar or identical technology and products, or limit the duration of the patent protection of our technology and products. Given the amount of time required for the development, testing and regulatory review of new product candidates, patents protecting such candidates might expire before or shortly after such candidates are commercialized. As a result, our owned and licensed patent portfolio may not provide us with sufficient rights to exclude others from commercializing products similar or identical to ours.

Recent patent reform legislation could increase the uncertainties and costs surrounding the prosecution of our patent applications and the enforcement or defense of our issued patents. In September 2011, the Leahy-Smith America Invents Act, or the Leahy-Smith Act, was signed into law. The Leahy-Smith Act includes a number of significant changes to United States patent law. These include provisions that affect the way patent applications are prosecuted and may also affect patent litigation. The United States Patent Office recently developed new regulations and procedures to govern administration of the Leahy-Smith Act, and many of the substantive changes to patent law associated with the Leahy-Smith Act and, in particular, the first to file provisions, only became effective in March 2013. Accordingly, it is not clear what, if any, impact the Leahy-Smith Act will have on the operation of our business. However, the Leahy-Smith Act and its implementation could increase the uncertainties and costs surrounding the prosecution of our patent applications and the enforcement or defense of our issued patents, all of which could have a material adverse effect on our business and financial condition.

Moreover, since September 2012, third parties now have standing to submit documents to the U.S. Patent and Trademark Office that relate to pending patent applications, known as pre-issuance submissions, for

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consideration during examination of a patent application. In addition, third parties may now challenge issued patents through inter partes reviews and post-grant reviews which include trials before a panel of administrative patent judges. We may be subject to such pre-issuance submissions, inter partes reviews or post-grant reviews, or become involved in opposition, re-examination, interference or derivation proceedings challenging our patent rights or the patent rights of others. An adverse determination in any such examination, proceeding or litigation could reduce the scope of, or invalidate, our patent rights, allow third parties to commercialize our technology or products and compete directly with us, without payment to us, or result in our inability to manufacture or commercialize products without infringing third party patent rights. In addition, if the breadth or strength of protection provided by our patents and patent applications is threatened, it could dissuade companies from collaborating with us to license, develop or commercialize current or future product candidates.

We may become involved in lawsuits to protect or enforce our patents or other intellectual property, which could be expensive, time consuming and unsuccessful.

Competitors may infringe our owned or licensed patents or other intellectual property. To counter infringement or unauthorized use, we may be required to file infringement claims, which can be expensive and time consuming. Any claims we assert against perceived infringers could provoke these parties to assert counterclaims against us alleging that we infringe their patents. In addition, in a patent infringement proceeding, a court may decide that a patent of ours is invalid or unenforceable, in whole or in part, construe the patent's claims narrowly or refuse to stop the other party from using the technology at issue on the grounds that our patents do not cover the technology in question. An adverse result in any litigation proceeding could put one or more of our patents at risk of being invalidated or interpreted narrowly.

Even if resolved in our favor, litigation or other legal proceedings relating to intellectual property claims may cause us to incur significant expenses, and could distract our technical and management personnel from their normal responsibilities. In addition, there could be public announcements of the results of hearings, motions or other interim proceedings or developments and if securities analysts or investors perceive these results to be negative, it could have a substantial adverse effect on the price of our ordinary shares. Such litigation or proceedings could substantially increase our operating losses and reduce the resources available for development activities or any future sales, marketing or distribution activities. We may not have sufficient financial or other resources to conduct such litigation or proceedings adequately. Some of our competitors may be able to sustain the costs of such litigation or proceedings more effectively than we can because of their greater financial resources. Uncertainties resulting from the initiation and continuation of patent litigation or other proceedings could compromise our ability to compete in the marketplace. Third parties may initiate legal proceedings alleging that we are infringing their intellectual property rights, the outcome of which would be uncertain and could have a material adverse effect on the success of our business.

Our commercial success depends upon our ability, and the ability of our collaborators, to develop, manufacture, market and sell our product candidates and use our proprietary technologies without infringing the proprietary rights of third parties. There is considerable intellectual property litigation in the biotechnology and pharmaceutical industries. We may become party to, or threatened with, future adversarial proceedings or litigation regarding intellectual property rights with respect to our products and technology, including interference or derivation proceedings before the U.S. Patent and Trademark Office. Third parties may assert infringement claims against us based on existing patents or patents that may be granted in the future.

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Third parties may assert their intellectual property rights against us, which could require us to defend lawsuits, obtain licenses, and cease or delay commercializing certain product candidates.

If we are found to infringe a third party's intellectual property rights, we could be required to obtain a license from such third party to continue developing and marketing our products and technology. However, we may not be able to obtain any required license on commercially reasonable terms or at all. Even if we were able to obtain a license, it could be non-exclusive, thereby giving our competitors access to the same technologies licensed to us. We could be forced, including by court order, to cease commercializing the infringing technology or product. In addition, we could be found liable for monetary damages, including treble damages and attorneys' fees if we are found to have wilfully infringed a patent. A finding of infringement could prevent us from commercializing our product candidates or force us to cease some of our business operations, which could materially harm our business. Claims that we have misappropriated the confidential information or trade secrets of third parties could have a similar negative impact on our business.

For example, we are aware of patents owned by third parties that relate to some aspects of our programs that are still in development. In some cases, because we have not determined the final methods of manufacture, the method of administration or the therapeutic compositions for these programs, we cannot determine whether rights under such third party patents will be needed. In addition, in some cases, we believe that the claims of these patents are invalid or not infringed, or will expire before commercialization. However, if such patents are needed and found to be valid and infringed, we could be required to obtain licenses, which might not be available on commercially reasonable terms, or to cease or delay commercializing certain product candidates, or to change our programs to avoid infringement.

Our reliance on third parties may require us to share our trade secrets, which could increase the possibility that a competitor will discover them or that our trade secrets will be misappropriated or disclosed.

Because we collaborate with various organizations and academic research institutions on the advancement of our gene therapy platform, we must, at times, share trade secrets with them. We seek to protect our proprietary technology in part by entering into confidentiality agreements and, if applicable, materials transfer agreements, collaborative research agreements, consulting agreements or other similar agreements with our collaborators, advisors and consultants prior to beginning research or disclosing proprietary information. These agreements typically limit the rights of the third parties to use or disclose our confidential information, such as trade secrets. Despite the contractual provisions employed when working with third parties, the need to share trade secrets and other confidential information increases the risk that such trade secrets become known by our competitors, are inadvertently incorporated into the technology of others, or are disclosed or used in violation of these agreements. Given that our proprietary position is based, in part, on our know-how and trade secrets, a competitor's discovery of our trade secrets or other unauthorized use or disclosure would impair our competitive position and may have a material adverse effect on our business.

In addition, these agreements typically restrict the ability of our collaborators, advisors and consultants to publish data potentially relating to our trade secrets. Our academic collaborators typically have rights to publish data, provided that we are notified in advance and may delay publication for a specified time in order to secure our intellectual property rights arising from the collaboration. In other cases, publication rights are controlled exclusively by us, although in some cases we may share these rights with other parties. We also conduct joint research and development programs that may require us to share trade secrets under the terms of our research and development partnerships or similar agreements.

Some courts inside and outside the United States are less willing or unwilling to protect trade secrets. If any of our trade secrets were to be lawfully obtained or independently developed by a competitor, we would have no right to prevent them, or those to whom they communicate it, from using that technology or information to compete with us.

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Risks Related to Pricing and Reimbursement

We face uncertainty related to insurance coverage of and pricing and reimbursement for Glybera and any product candidates for which we may receive marketing approval.

We anticipate that the cost of treatment using Glybera or our other product candidates will be significant. We expect that most patients and their families will not be capable of paying for our products themselves. There will be no commercially viable market for Glybera or our other product candidates without reimbursement from third-party payors, such as government health administration authorities, private health insurers and other organizations. Even if there is a commercially viable market, if the level of third-party reimbursement is below our expectations, our revenues and gross margins will be adversely affected and our business will be harmed.

Government authorities and other third party payors, such as private health insurers and health maintenance organizations, decide which medications they will pay for and establish reimbursement levels. Reimbursement systems vary significantly by country and by region, and reimbursement approvals must be obtained on a country-by-country basis. Government authorities and third party payors have attempted to control costs by limiting coverage and the amount of reimbursement for particular medications and procedures. Increasingly, third party payors require drug companies to provide them with predetermined discounts from list prices, are exerting influence on decisions regarding the use of particular treatments and are limiting covered indications. If coverage and adequate reimbursement are not available or reimbursement is available only at limited levels, we may not be able to successfully commercialize Glybera or any product candidate for which we obtain marketing approval.

The pricing review period and pricing negotiations for new medicines take considerable time and have uncertain results. Pricing review and negotiation often begins only after the receipt of regulatory marketing approval, and some authorities require approval of the sale price of a product before it can be marketed. In some markets, particularly the countries of the European Union, prescription pharmaceutical pricing remains subject to continuing direct governmental control and to drug reimbursement programs even after initial approval is granted, and price reductions may be imposed. Prices of medical products may also be subject to varying price control mechanisms or limitations as part of national health systems if products may be considered not to be cost-effective or where the drug company's profits are deemed excessive. In addition, pricing and reimbursement decisions in certain countries can lead to mandatory price reductions or additional reimbursement restrictions in other countries. As a result of these restrictions, Glybera, as well as any product candidates for which we may obtain marketing approval in the future, may be subject to price regulations that delay or prohibit our or our partners' commercial launch of the product. In addition, we or our collaborators may elect to reduce the price of our products in order to increase the likelihood of obtaining reimbursement approvals. In the event that countries impose prices which are not sufficient to allow us or our collaborators to generate a profit, we or our collaborators may refuse to launch the product in such countries or withdraw the product from the market. If pricing is set at unsatisfactory levels, or if the price decreases, our business could be harmed, possibly materially. If we fail to obtain and sustain an adequate level of coverage and reimbursement for our products by third party payors, our ability to market and sell our products would be adversely affected and our business would be harmed.

Due to the generally limited addressable market for our target orphan indications and the potential for Glybera and our product candidates to offer therapeutic benefit in a single administration, we face uncertainty related to pricing and reimbursement for these product candidates.

The relatively small market size for orphan indications and the potential for long-term therapeutic benefit from a single administration present particular challenges to pricing review and negotiation for Glybera and our product candidates for which we may obtain marketing authorization. The patient populations for Glybera and our product candidates targeted at orphan disease are relatively small. If we are unable to obtain adequate levels of reimbursement relative to the small market size in our target orphan indications,

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our ability to support our development and commercial infrastructure and to successfully market and sell Glybera and other product candidates for which we may obtain marketing approval will be adversely affected.

We also anticipate that Glybera and many or all of our gene therapy product candidates may provide long-term, and potentially curative benefit with a single administration. This is a different paradigm than that of other pharmaceutical therapies, which often require an extended course of treatment or frequent administration. As a result, governments and other payors may be reluctant to provide the significant level of reimbursement that we seek at the time of administration of our gene therapies or may seek to tie reimbursement to clinical evidence of continuing therapeutic benefit over time. Although we anticipate that Glybera will need to be administered only once, there may be situations in which we may need to readminister Glybera, which may further complicate the pricing and reimbursement for Glybera. In addition, in light of the anticipated cost of these therapies, governments and other payors may be particularly restrictive in making coverage decisions. These factors could limit our commercial success and harm our business.

Recently enacted and future legislation and related market pricing pressures may affect the coverage and pricing and reimbursement we may obtain for our products.

A primary trend in the U.S. healthcare industry and elsewhere is cost containment. In the United States and some foreign jurisdictions, there have been a number of legislative and regulatory changes and proposed changes regarding the healthcare system that could prevent or delay marketing approval of our product candidates, restrict or regulate post-approval activities and affect our ability to profitably sell any products for which we obtain marketing approval. We expect that existing, as well as other healthcare reform measures that may be adopted in the future, may result in more rigorous coverage criteria and in additional downward pressure on the price that we receive for any approved product. Any reduction in reimbursement from government programs may result in a similar reduction in payments from private payors. The implementation of cost containment measures or other healthcare reforms may prevent us from being able to generate revenues, attain profitability, or commercialize our products.

Risks Related to Other Legal Compliance Matters

Our relationships with customers and third-party payors will be subject to applicable anti-kickback, fraud and abuse and other healthcare laws and regulations, which could expose us to criminal sanctions, civil penalties, contractual damages, reputational harm and diminished profits and future earnings.

Healthcare providers, physicians and third-party payors will play a primary role in the recommendation and prescription of any products for which we obtain marketing approval. Our future arrangements with third-party payors and customers may expose us to broadly applicable fraud and abuse and other healthcare laws and regulations that may constrain the business or financial arrangements and relationships through which we market, sell and distribute any products for which we obtain marketing approval.

Within the European Union, the control of unlawful marketing activities is a matter of national law in each of the member states. We could face civil, criminal and administrative sanctions if any member state determines that we have breached our obligations under its national laws. Industry associations also closely monitor the activities of member companies. If these organizations or authorities name us as having breached our obligations under their regulations, rules or standards, our reputation would suffer and our business and financial condition could be adversely affected. Similarly, failure to comply with the European Union's requirements regarding the protection of personal information can also lead to significant penalties and sanctions.

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If we market a product in the United States in the future, we will be subject to various federal and state laws and regulations including:

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the federal Anti-Kickback Statute, which prohibits, among other things, persons from knowingly and wilfully soliciting, offering, receiving or providing remuneration, directly or indirectly, in cash or in kind, to induce or reward, or in return for, either the referral of an individual for, or the purchase, order or recommendation of, any good or service, for which payment may be made under a federal healthcare program such as Medicare and Medicaid;



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the federal False Claims Act, which imposes criminal and civil penalties, including civil whistleblower or qui tam actions, against individuals or entities for knowingly presenting, or causing to be presented, to the federal government, claims for payment that are false or fraudulent or making a false statement to avoid, decrease or conceal an obligation to pay money to the federal government;



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the federal Health Insurance Portability and Accountability Act of 1996, or HIPAA, which imposes criminal and civil liability for executing a scheme to defraud any healthcare benefit program or making false statements relating to healthcare matters and also imposes obligations, including mandatory contractual terms, with respect to safeguarding the privacy, security and transmission of individually identifiable health information;



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federal law that requires applicable manufacturers of covered drugs to report payments and other transfers of value to physicians and teaching hospitals;



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the U.S. Foreign Corrupt Practices Act, which prohibits the offering to pay, paying or promising to pay or authorizing the payment of money or anything of value to a foreign official in order to influence any act or decision of the foreign official or secure any improper advantage in order to obtain or retain business; and



•

certain state laws that require pharmaceutical companies to comply with the pharmaceutical industry's voluntary compliance guidelines and the relevant compliance guidance promulgated by the federal government and require drug manufacturers to report information related to payments and other transfers of value to physicians and other healthcare providers or marketing expenditures. State and foreign laws also govern the privacy and security of health information in some circumstances, many of which differ from each other in significant ways and often are not pre-empted by HIPAA, thus complicating compliance efforts.

We will also be subject to similar local laws in jurisdictions in which we may seek or obtain marketing authorization, or in which we may have operations or sales. Efforts to ensure that our business arrangements with third parties will comply with applicable laws and regulations will involve substantial costs. It is possible that governmental authorities will conclude that our business practices may not comply with current or future statutes, regulations or case law involving applicable fraud and abuse or other healthcare laws and regulations. If our operations, or the activities of our collaborators, distributors or other third-party agents are found to be in violation of any of these laws or any other governmental regulations that may apply to us, we may be subject to significant civil, criminal and administrative penalties, damages, fines, imprisonment, exclusion of products from government funded healthcare programs and the curtailment or restructuring of our operations. If any of the physicians or other healthcare providers or entities with whom we expect to do business is found to be not in compliance with applicable laws, they may be subject to criminal, civil or administrative sanctions, including exclusions from government funded healthcare programs.

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If we fail to comply with environmental, health and safety laws and regulations, we could become subject to fines or penalties or incur costs that could harm our business.

We are subject to numerous environmental, health and safety laws and regulations, including those governing laboratory procedures and the handling, use, storage, treatment and disposal of hazardous materials and wastes. Our operations involve the use of hazardous and flammable materials, including chemicals and biological materials. Our operations also produce hazardous waste products. We cannot eliminate the risk of contamination or injury from these materials. In the event of contamination or injury resulting from our use of hazardous materials, we could be held liable for any resulting damages, and any liability could exceed our resources. We also could incur significant costs associated with civil or criminal fines and penalties for failure to comply with such laws and regulations.

Although we maintain employer's liability insurance to cover us for costs and expenses we may incur due to injuries to our employees resulting from the use of hazardous materials, this insurance may not provide adequate coverage against potential liabilities. We do not maintain insurance for environmental liability or toxic tort claims that may be asserted against us in connection with our storage or disposal of biological, hazardous or radioactive materials.

In addition, we may incur substantial costs in order to comply with current or future environmental, health and safety laws and regulations. These current or future laws and regulations may impair our research, development or production efforts. Our failure to comply with these laws and regulations also may result in substantial fines, penalties or other sanctions.

Product liability lawsuits against us could cause us to incur substantial liabilities and to limit commercialization of Glybera and any products that we may develop in the future.

We face an inherent risk of product liability related to the testing of our product candidates in human clinical trials and will face an even greater risk when we commercially sell Glybera and any other products that we may develop. If we cannot successfully defend ourselves against claims that our product candidates or products caused injuries, we will incur substantial liabilities. Regardless of merit or eventual outcome, liability claims may result in:

•

decreased demand for any product candidates or products that we develop or sell;



•

injury to our reputation and significant negative media attention;



•

negative publicity or public opinion surrounding gene therapy;



•

withdrawal of clinical trial participants;



•

significant costs to defend the related litigation;



•

substantial monetary awards to trial participants or patients;



•

loss of revenue;



•

reduced resources of our management to pursue our business strategy; and



•

the inability to further develop or commercialize any products that we develop.

We currently hold €6,000,000 in clinical trial insurance coverage in the aggregate, with a per incident limit of €400,000 to €450,000, with respect to the clinical studies we conduct. Such coverage may not be adequate to cover all liabilities that we may incur. We may need to increase our insurance coverage as we expand our clinical trials and commercialize Glybera. In addition, insurance coverage is increasingly expensive. We may not be able to maintain insurance coverage at a reasonable cost or in an amount adequate to satisfy any liability that may arise.

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Risks Related to Employee Matters and Managing Growth

Our future success depends on our ability to retain key executives and technical staff and to attract, retain and motivate qualified personnel.

We are highly dependent on the research and development, clinical and business development expertise of our Chief Executive Officer, Jörn Aldag, our Chief Medical Officer, Christian Meyer, M.D., and our Vice President, Research and Development, Harald Petry, as well as the other principal members of our management, scientific and clinical team. Although we have entered into employment agreements with our senior managment, each of them may terminate their employment on relatively short notice. We do not maintain "key person" insurance for any of our senior management or employees.

Substantially all share options outstanding under our 2012 plan will vest in full upon the closing of this offering, including those held by our senior management and key employees. Following this offering, therefore, our outstanding options will no longer have retention value, and we may not have appropriate adequate equity incentives in place to retain or motivate these key personnel.

The loss of the services of our senior management or other key employees could impede the achievement of our research, development and commercialization objectives and seriously harm our ability to successfully implement our business strategy. Furthermore, replacing senior management and key employees may be difficult and may take an extended period of time because of the limited number of individuals in our industry with the breadth and depth of skills and experience required to successfully develop, gain regulatory approval of and commercialize gene therapy products. Competition to hire from this limited pool is intense, and we may be unable to hire, train, retain or motivate these key personnel on acceptable terms.

In connection with our corporate restructuring in 2012, we lost many talented employees, including employees with an extensive understanding of our clinical programs as well as our regulatory and financial affairs. This reduction in headcount may adversely affect our ability in the future to attract and retain other qualified staff.

We also experience competition for the hiring of scientific and clinical personnel from universities and research institutions. In addition, we rely on consultants and advisors, including scientific and clinical advisors, to assist us in formulating our research and development and commercialization strategy. Our consultants and advisors may have commitments under employment, consulting or advisory contracts with other entities that may limit their availability to us. If we are unable to continue to attract and retain high quality personnel, our ability to pursue our growth strategy will be limited.

We plan to expand our key capabilities and, as a result, may encounter difficulties in managing our growth, which could disrupt our operations. If we are unable to establish such capabilities we may not be successful in commercializing Glybera or our other product candidates in the United States or other countries, even if we receive marketing approval.

If we receive marketing approval, we intend to build a sales, marketing and medical affairs infrastructure to market Glybera and potentially other product candidates in the United States and other countries. We currently have no experience building and training an internal sales force. We expect in the future to experience significant growth in the number of our employees and the scope of our operations, particularly in the areas of manufacturing, clinical development, regulatory affairs and sales, marketing and distribution. To manage our anticipated future growth, we will be required to implement and improve our managerial, operational and financial systems, expand our facilities and recruit and train additional qualified personnel. Recruiting and training a sales force is expensive and time-consuming and could delay any ultimate launch of Glybera or other product candidates for which we are able to obtain marketing approval in the United States and other markets. Due to our limited financial resources and the limited experience of our management team in running a company with this level of anticipated growth, we may not be able to effectively manage the expansion of our operations or recruit and train additional qualified personnel.

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If the commercial launch of Glybera or any other product candidate for which we recruit a sales force and establish marketing capabilities is delayed or does not occur for any reason, we would have prematurely or unnecessarily incurred these commercialization expenses. This may be costly, and our investment would be lost if we cannot retain or reposition our sales and marketing personnel. Factors that may inhibit our efforts to commercialize Glybera or other product candidates on our own include:

•

our inability to recruit, train and retain adequate numbers of effective sales, marketing and medical affairs personnel;



•

the inability of sales, marketing and medical affairs personnel to obtain access to physicians or persuade adequate numbers of physicians to prescribe our gene therapies;



•

the lack of complementary products to be offered by our sales personnel, which may put us at a competitive disadvantage relative to companies with more extensive product lines; and



•

unforeseen costs and expenses associated with creating an independent sales and marketing organization.

If we do not successfully establish sales, marketing and medical affairs capabilities, either on our own or in collaboration with third parties, we will not be successful in commercializing Glybera or other product candidates in the United States.

Risks Related to this Offering and Ownership of our Ordinary Shares

There has been no public market for our ordinary shares prior to this offering, and you may not be able to resell our ordinary shares at or above the price you paid, or at all.

Prior to this initial public offering, there was no established public market for our ordinary shares. Our predecessor entity, Amsterdam Molecular Therapeutics, was previously listed on EuroNext Amsterdam. Since our acquisition of the business of AMT in 2012 as part of a corporate reorganization, there has been very limited liquidity for our ordinary shares through an electronic trading platform in the Netherlands. We have applied for listing of our ordinary shares on the NASDAQ Global Market. If an active trading market for our ordinary shares does not develop after this offering, the market price and liquidity of our ordinary shares will be materially and adversely affected.

The initial public offering price for our ordinary shares will be determined by negotiations between us and the underwriters and may bear no relationship to the market price for our ordinary shares after the initial public offering. We cannot assure you that an active trading market for our ordinary shares will develop or that the market price of our ordinary shares will not decline below the initial public offering price.

The price of our ordinary shares may be volatile and fluctuate substantially, which could result in substantial losses for purchasers of our ordinary shares in this offering.

Our share price is likely to be volatile. The stock market in general and the market for smaller biopharmaceutical companies in particular have experienced extreme volatility that has often been unrelated to the operating performance of particular companies. As a result of this volatility, you may not be able to sell your ordinary shares at or above the initial public offering price. The market price for our ordinary shares may be influenced by many factors, including:

•

the success of competitive products or technologies;



•

results of clinical trials of our product candidates or those of our competitors;



•

public perception of gene therapy;



•

regulatory delays and greater government regulation of potential products due to adverse events;



•

regulatory or legal developments in the European Union, the United States and other countries;



•

developments or disputes concerning patent applications, issued patents or other proprietary rights;



•

the recruitment or departure of key personnel;

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•

the level of expenses related to any of our product candidates or clinical development programs;



•

the results of our efforts to discover, develop, acquire or in-license additional product candidates or products;



•

actual or anticipated changes in estimates as to financial results, development timelines or recommendations by securities analysts;



•

variations in our financial results or those of companies that are perceived to be similar to us;



•

changes in the structure of healthcare payment systems;



•

market conditions in the pharmaceutical and biotechnology sectors; and



•

general economic, industry and market conditions.

After this offering, our senior managers, directors and principal shareholders, if they choose to act together, will continue to have the ability to control all matters submitted to shareholders for approval.

Upon the closing of this offering, our senior managers, and directors, combined with our shareholders who owned more than 5% of our outstanding ordinary shares before this offering, will, in the aggregate, beneficially own approximately          % of our share capital. As a result, if these shareholders were to choose to act together, they would be able to control all matters submitted to our shareholders for approval, as well as our management and affairs. For example, these persons, if they choose to act together, would control the election of supervisory board directors and approval of any merger, consolidation or sale of all or substantially all of our assets.

Provisions of our articles of association or Dutch corporate law might deter acquisition bids for us that might be considered favorable and prevent or frustrate any attempt to replace or remove the management board and supervisory board .

Certain provisions of our articles of association may make it more difficult for a third party to acquire control of us or effect a change in our management board or supervisory board. These provisions include:

•

the authorization of a class of preference shares that may be issued to a friendly party;



•

staggered three-year terms of our supervisory directors;



•

a provision that our managing directors and supervisory directors may only be removed at a general meeting of shareholders by a two-thirds majority of votes cast representing more than half of the issued share capital of the company (unless the removal was proposed by the supervisory board); and



•

a requirement that certain matters, including an amendment of our articles of association, may only be brought to our shareholders for a vote upon a proposal by our management board that has been approved by our supervisory board.

Our anti-takeover provision may prevent a beneficial change of control.

We have adopted an anti-takeover measure pursuant to which our management board may, subject to supervisory board approval but without shareholder approval, issue (or grant the right to acquire) preference shares. We may issue an amount of preference shares up to 100% of our issued capital as per the moment immediately prior to the issuance of such preference shares.

In such event, the preference shares (or right to acquire preference shares) will be issued to a separate, newly established foundation. The preference shares will be issued to the foundation for their nominal value, of which only 25% will be due upon issuance. The voting rights of our shares are based on nominal value and, if our shares trade substantially in excess of nominal value, preference shares issued at nominal value can obtain significant voting power for a substantially reduced price and thus be used as a defensive measure. These preference shares will have both a liquidation and dividend preference over our ordinary shares and will accrue cash dividends at a fixed rate.

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Our management board may issue these preference shares to protect us from influences that do not serve our best interests and threaten to undermine our continuity, independence and identity. These influences may include a third-party acquiring a significant percentage of our ordinary shares, the announcement of a public offer for our ordinary shares, other concentration of control over our ordinary shares or any other form of pressure on us to alter our strategic policies.

If the management board determines to issue the preference shares to such a foundation, the purpose clause in this foundation's articles of association will provide that it will act to serve the best interests of us, our associated business and all parties connected to us, by opposing any influences that conflict with these interests and threaten to undermine our continuity, independence and identity. This foundation will be structured to operate independently of us and we will not have direct control over or be able to influence the actions of the foundation.

We do not expect to pay dividends in the foreseeable future.

We have not paid any dividends since our incorporation. Even if future operations lead to significant levels of distributable profits, we currently intend that earnings, if any, will be reinvested in our business and that dividends will not be paid until we have an established revenue stream to support continuing dividends. Payment of future dividends to shareholders will, in addition, be at the discretion of the management board, subject to the approval of the supervisory board after taking into account various factors including our business prospects, cash requirements, financial performance, debt covenant limitations and new product development. Our current loan agreement with Hercules restricts our ability to pay dividends. In addition, payment of future dividends may be made only if our shareholders' equity exceeds the sum of our paid-in and called-up share capital plus the reserves required to be maintained by Dutch law or by our articles of association. Accordingly, investors cannot rely on dividend income from our ordinary shares and any returns on an investment in our ordinary shares will likely depend entirely upon any future appreciation in the price of our ordinary shares.

We expect that only a relatively small percentage of our ordinary shares will be publicly traded following this offering, which may limit the liquidity of your investment and may have a material adverse effect on the price of our ordinary shares.

After this offering,          % of our ordinary shares will be held by parties other than our directors, senior management, existing shareholders holding 5% or more of our ordinary shares, and their respective affiliates. As a result, we expect that only a relatively small number of our ordinary shares will be actively traded in the public market following this offering. Reduced liquidity may have a material adverse effect on the price of our ordinary shares.

You will not be able to trade our ordinary shares on any exchange outside the United States.

Our ordinary shares will be listed only in the United States on the NASDAQ Global Market and we have no plans to list our ordinary shares in any other jurisdiction. As a result, a holder of our ordinary shares outside the United States may not be able to effect transactions in our ordinary shares as readily as the holder may if our securities were listed on an exchange in that holder's home jurisdiction.

The sale of a substantial number of our ordinary shares following this offering may cause the market price of our ordinary shares to decline.

Sales of a substantial number of shares in the public market may occur at any time after the expiration of the lock-up agreements described in the "Underwriting" section of this prospectus. Our sale or the resale by our shareholders of shares, or a market expectation of such sales, after this offering may cause the market price of our ordinary shares to decline. After this offering, we will have outstanding               ordinary shares. Of these, the               ordinary shares sold in this offering will be freely transferable without restriction. The remaining               ordinary shares, or          % of our outstanding shares after this offering, are currently

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restricted as a result of securities laws or lock-up agreements but will be able to be sold, subject to any applicable volume limitations under U.S. federal securities laws with respect to affiliate sales, in the future as set forth in "Shares Eligible for Future Sale" and "Underwriting" below.

We are an "emerging growth company," and the reduced disclosure requirements applicable to emerging growth companies may make our ordinary shares less attractive to investors.

We are an "emerging growth company," as defined in the Jumpstart Our Business Startups Act of 2012, or the JOBS Act, and may remain an emerging growth company for up to five years. For so long as we remain an emerging growth company, we are permitted and intend to rely on exemptions from certain disclosure requirements that are applicable to other public companies that are not emerging growth companies. These exemptions include:

•

being permitted to provide only two years of audited financial statements, in addition to any required unaudited interim financial statements, with correspondingly reduced "Management's Discussion and Analysis of Financial Condition and Results of Operations" disclosure;



•

not being required to comply with the auditor attestation requirements in the assessment of our internal control over financial reporting; and



•

not being required to comply with any requirement that may be adopted by the Public Company Accounting Oversight Board regarding mandatory audit firm rotation or a supplement to the auditor's report providing additional information about the audit and the financial statements;

We have taken advantage of reduced reporting burdens in this prospectus. In particular, in this prospectus, we have provided only two years of audited financial statements. We cannot predict whether investors will find our ordinary shares less attractive if we rely on these exemptions. If some investors find our ordinary shares less attractive as a result, there may be a less active trading market for our ordinary shares and our share price may be more volatile.

We will be a foreign private issuer and, as a result, we will not be subject to U.S. proxy rules and will be subject to Exchange Act reporting obligations that, to some extent, are more lenient and less frequent than those of a U.S. domestic public company.

Upon consummation of this offering, we will report under the Securities Exchange Act of 1934, as amended, or the Exchange Act, as a non-U.S. company with foreign private issuer status. Because we qualify as a foreign private issuer under the Exchange Act and although we are subject to Dutch laws and regulations with regard to such matters and intend to furnish quarterly financial information to the SEC, we are exempt from certain provisions of the Exchange Act that are applicable to U.S. domestic public companies, including:

•

the sections of the Exchange Act regulating the solicitation of proxies, consents or authorizations in respect of a security registered under the Exchange Act;



•

the sections of the Exchange Act requiring insiders to file public reports of their stock ownership and trading activities and liability for insiders who profit from trades made in a short period of time; and



•

the rules under the Exchange Act requiring the filing with the Securities and Exchange Commission of quarterly reports on Form 10-Q containing unaudited financial and other specified information, or current reports on Form 8-K, upon the occurrence of specified significant events.

In addition, foreign private issuers are not be required to file their annual report on Form 20-F until 120 days after the end of each fiscal year, while U.S. domestic issuers that are accelerated filers are required to file their annual report on Form 10-K within 75 days after the end of each fiscal year. Foreign private issuers are also exempt from the Regulation Fair Disclosure, aimed at preventing issuers from making selective disclosures of material information. As a result of the above, you may not have the same protections afforded to shareholders of companies that are not foreign private issuers.

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We may lose our foreign private issuer status which would then require us to comply with the Exchange Act's domestic reporting regime and cause us to incur significant legal, accounting and other expenses.

We are a foreign private issuer and therefore we are not required to comply with all of the periodic disclosure and current reporting requirements of the Exchange Act applicable to U.S. domestic issuers. In order to maintain our current status as a foreign private issuer, either:

•

a majority of our ordinary shares must be either directly or indirectly owned of record by non-residents of the United States; or



•

a majority of our executive officers or directors may not be United States citizens or residents, more than 50 percent of our assets cannot be located in the United States and our business must be administered principally outside the United States.

If we lost this status, we would be required to comply with the Exchange Act reporting and other requirements applicable to U.S. domestic issuers, which are more detailed and extensive than the requirements for foreign private issuers.

We may also be required to make changes in our corporate governance practices in accordance with various SEC and NASDAQ rules. The regulatory and compliance costs to us under U.S. securities laws if we are required to comply with the reporting requirements applicable to a U.S. domestic issuer may be significantly higher than the cost we would incur as a foreign private issuer. As a result, we expect that a loss of foreign private issuer status would increase our legal and financial compliance costs and would make some activities highly time consuming and costly. We also expect that if we were required to comply with the rules and regulations applicable to U.S. domestic issuers, it would make it more difficult and expensive for us to obtain director and officer liability insurance, and we may be required to accept reduced coverage or incur substantially higher costs to obtain coverage. These rules and regulations could also make it more difficult for us to attract and retain qualified members of our supervisory board.

We have broad discretion in the use of the net proceeds from this offering and may not use them effectively.

Our management will have broad discretion in the application of the net proceeds from this offering and could spend the proceeds in ways that do not improve our results of operations or enhance the value of our ordinary shares. The failure by our management to apply these funds effectively could result in financial losses that could have a material adverse effect on our business, cause the price of our ordinary shares to decline and delay the development of our product candidates. Pending their use, we may invest the net proceeds from this offering in a manner that does not produce income or that loses value.

We will incur increased costs as a result of operating as a public company, and our management will be required to devote substantial time to new compliance initiatives and corporate governance practices.

As a public company, and particularly after we are no longer an emerging growth company, we will incur significant legal, accounting and other expenses that we did not incur as a private company. We currently estimate that we will incur incremental annual costs of approximately $1.2 million associated with operating as a public company, although it is possible that our actual incremental annual costs will be higher than we currently estimate. The Sarbanes-Oxley Act of 2002, the Dodd-Frank Wall Street Reform and Consumer Protection Act, the listing requirements of The NASDAQ Global Market and other applicable securities rules and regulations impose various requirements on public companies, including establishment and maintenance of effective disclosure and financial controls and corporate governance practices. Our management and other personnel will need to devote a substantial amount of time to these compliance initiatives. Moreover, these rules and regulations will increase our legal and financial compliance costs and will make some activities more time-consuming and costly. For example, we expect that these rules and regulations may make it more difficult and more expensive for us to obtain director and officer liability

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insurance, which in turn could make it more difficult for us to attract and retain qualified members of our supervisory board.

These rules and regulations are often subject to varying interpretations, in many cases due to their lack of specificity, and, as a result, their application in practice may evolve over time as new guidance is provided by regulatory and governing bodies. This could result in continuing uncertainty regarding compliance matters and higher costs necessitated by ongoing revisions to disclosure and governance practices and control environment process improvements.

If we fail to implement and maintain an effective system of internal controls, we may be unable to accurately report our results of operations or prevent fraud or fail to meet our reporting obligations, and investor confidence and the market price of our ordinary shares may be materially and adversely affected.

Prior to this offering, we were a private company with limited accounting personnel and other resources with which to address our internal controls and procedures. Our independent registered public accounting firm has not conducted an audit of our internal control over financial reporting. However, in connection with the audit of our consolidated financial statements as of and for year ended December 31, 2012 and the review of our consolidated financial statements as of and for the nine months ended September 30, 2013, we and our independent registered public accounting firm identified three material weaknesses in our internal control over financial reporting. A significant deficiency is a control deficiency, or a combination of control deficiencies, that adversely affects our ability to initiate, authorize, record, process, or report external financial data reliably in accordance with IFRS such that there is more than a remote likelihood that a misstatement of our annual or interim financial statements that is more than inconsequential will not be prevented or detected by our employees. A material weakness is a significant deficiency, or combination of significant deficiencies, that results in more than a remote likelihood that a material misstatement of our annual or interim financial statement will not be presented or detected by our employees. In response, we have begun the process of evaluating our internal control over financial reporting, although we may not complete our review until after this offering is completed. We have also taken several remedial actions to address these material weaknesses. For details, see "Management's Discussion and Analysis of Financial Condition and Results of Operations—Internal Control Over Financial Reporting."

Furthermore, it is possible that, had our independent registered public accounting firm conducted an audit of our internal control over financial reporting, such firm might have identified additional material weaknesses and deficiencies. Upon the completion of this offering, we will become a public company in the United States subject to the Sarbanes-Oxley Act of 2002. Section 404 of the Sarbanes-Oxley Act of 2002, or Section 404, will require that we include a report of management on our internal control over financial reporting in our annual report on Form 20-F beginning with our annual report for the fiscal year ending December 31, 2015. In addition, once we cease to be an "emerging growth company" as such term is defined in the JOBS Act, our independent registered public accounting firm must attest to and report on the effectiveness of our internal control over financial reporting. Our management may conclude that our internal control over financial reporting is not effective. Moreover, even if our management concludes that our internal control over financial reporting is effective, our independent registered public accounting firm, after conducting its own independent testing, may issue a report that is qualified if it is not satisfied with our internal controls or the level at which our controls are documented, designed, operated or reviewed, or if it interprets the relevant requirements differently from us. In addition, after we become a public company, our reporting obligations may place a significant strain on our management, operational and financial resources and systems for the foreseeable future. We may be unable to timely complete our evaluation testing and any required remediation.

During the course of documenting and testing our internal control procedures, in order to satisfy the requirements of Section 404, we may identify other weaknesses and deficiencies in our internal control over financial reporting. In addition, if we fail to maintain the adequacy of our internal control over financial reporting, as these standards are modified, supplemented or amended from time to time, we may not be

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able to conclude on an ongoing basis that we have effective internal control over financial reporting in accordance with Section 404. If we fail to achieve and maintain an effective internal control environment, we could suffer material misstatements in our financial statements and fail to meet our reporting obligations, which would likely cause investors to lose confidence in our reported financial information. This could in turn limit our access to capital markets, harm our results of operations, and lead to a decline in the trading price of our common shares. Additionally, ineffective internal control over financial reporting could expose us to increased risk of fraud or misuse of corporate assets and subject us to potential delisting from the stock exchange on which we list, regulatory investigations and civil or criminal sanctions. We may also be required to restate our financial statements for prior periods.

You will experience immediate and substantial dilution in the net tangible book value of ordinary shares purchased.

The initial public offering price per ordinary share will be substantially higher than the net tangible book value per ordinary share prior to the offering. Consequently, when you purchase ordinary shares in the offering at the assumed initial public offering price, you will incur immediate dilution of $               per ordinary share. See "Dilution."

We intend to rely on NASDAQ Stock Market rules that permit us to comply with applicable Dutch corporate governance practices, rather than the corresponding domestic U.S. corporate governance practices, and therefore your rights as a shareholder will differ from the rights you would have as a shareholder of a domestic U.S. issuer.

As a foreign private issuer whose ordinary shares are listed on the NASDAQ Global Market, we are permitted in certain cases to follow Dutch corporate governance practices instead of the corresponding requirements of the NASDAQ Marketplace Rules. A foreign private issuer that elects to follow a home country practice instead of NASDAQ requirements must submit to NASDAQ in advance a written statement from an independent counsel in such issuer's home country certifying that the issuer's practices are not prohibited by the home country's laws. In addition, a foreign private issuer must disclose in its annual reports filed with the Securities and Exchange Commission each such requirement that it does not follow and describe the home country practice followed instead of any such requirement. We intend to follow Dutch corporate governance practices with regard to the quorum requirements applicable to meetings of shareholders and the provision of proxy statements for general meetings of shareholders, rather than the corresponding domestic U.S. corporate governance practices. In accordance with Dutch law and generally accepted business practices, our articles of association do not provide quorum requirements generally applicable to general meetings of shareholders. Although we do intend to provide shareholders with an agenda and other relevant documents for the general meeting of shareholders, Dutch law does not have a regulatory regime for the solicitation of proxies and the solicitation of proxies is not a generally accepted business practice in the Netherlands. Accordingly, our shareholders may not be afforded the same protection as provided under NASDAQ's corporate governance rules.

We do not comply with all the provisions of the Dutch Corporate Governance Code. This may affect your rights as a shareholder.

As a Dutch company we are subject to the Dutch Corporate Governance Code, or DCGC. The DCGC contains both principles and best practice provisions for management boards, supervisory boards, shareholders and general meetings of shareholders, financial reporting, auditors, disclosure, compliance and enforcement standards. The DCGC applies to all Dutch companies listed on a government-recognized stock exchange, whether in the Netherlands or elsewhere, including the NASDAQ Global Market. The principles and best practice provisions apply to our management board and supervisory board, in relation to their role and composition, conflicts of interest and independence requirements, board committees and remuneration, shareholders and the general meeting of shareholders, for example, regarding anti-takeover protection and obligations of the company to provide information to its shareholders; and financial reporting, including

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external auditor and internal audit requirements. We do not comply with all the provisions of the DCGC. This may affect your rights as a shareholder and you may not have the same level of protection as a shareholder in a Dutch company that fully complies with the DCGC.

Risks for U.S. Holders

We cannot assure you that we will not be classified as a passive foreign investment company for any taxable year, which may result in adverse U.S. federal income tax consequence to U.S. holders.

Based on our estimated gross income and average value of our gross assets, taking into account the assumed initial public offering price of our shares in this offering and the expected price of our shares following the offering, and the nature of our business, we do not expect to be considered a "passive foreign investment company," or PFIC, for U.S. federal income tax for the 2013 tax year or in the foreseeable future. A corporation organized outside the United States generally will be classified as a PFIC for U.S. federal income tax purposes in any taxable year in which at least 75% of its gross income is passive income or on average at least 50% of the gross value of its assets is attributable to assets that produce passive income or are held for the production of passive income. Passive income for this purpose generally includes dividends, interest, royalties, rents and gains from commodities and securities transactions. Our status in any taxable year will depend on our assets and activities in each year, and because this is a factual determination made annually after the end of each taxable year, there can be no assurance that we will not be considered a PFIC for the current taxable year or any future taxable year. The market value of our assets may be determined in large part by reference to the market price of our ordinary shares, which is likely to fluctuate after the offering, and may fluctuate considerably given that market prices of technology companies have been especially volatile. If we were to be treated as a PFIC for any taxable year during which a U.S. holder held our ordinary shares, however, certain adverse U.S. federal income tax consequences could apply to the U.S. holder. See "Taxation—Taxation in the United States—U.S. federal income tax consequences to U.S. holders—Passive foreign investment company considerations."

Any U.S. or other foreign judgments you may obtain against us may be difficult to enforce against us in the Netherlands.

We are incorporated under the laws of the Netherlands. We currently have only limited operations in the United States. Most of our assets are currently located in the Netherlands.

The majority of our managing directors, supervisory directors and senior management reside outside the United States. As a result, it may not be possible for investors to effect service of process within the United States upon such persons or to enforce against them or us in U.S. courts, including judgments predicated upon the civil liability provisions of the federal securities laws of the United States. In addition, it is not clear whether a Dutch court would impose civil liability on us or any of our managing directors or supervisory directors in an original action based solely upon the federal securities laws of the United States brought in a court of competent jurisdiction in the Netherlands.

The United States and the Netherlands currently do not have a treaty providing for the reciprocal recognition and enforcement of judgments, other than arbitration awards, in civil and commercial matters. Consequently, a final judgment for payment given by a court in the United States, whether or not predicated solely upon U.S. securities laws, would not automatically be recognized or enforceable in the Netherlands. In order to obtain a judgment which is enforceable in the Netherlands, the party in whose favor a final and conclusive judgment of the U.S. court has been rendered will be required to file its claim with a court of competent jurisdiction in the Netherlands. Such party may submit to the Dutch court the final judgment rendered by the U.S. court. If and to the extent that the Dutch court finds that the jurisdiction of the U.S. court has been based on grounds which are internationally acceptable and that proper legal procedures have been observed, the Dutch court will, in principle, give binding effect to the judgment of the U.S. court, unless such judgment contravenes principles of public policy of the Netherlands. Dutch courts may deny

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the recognition and enforcement of punitive damages or other awards. Moreover, a Dutch court may reduce the amount of damages granted by a U.S. court and recognize damages only to the extent that they are necessary to compensate actual losses or damages. Enforcement and recognition of judgments of U.S. courts in the Netherlands are solely governed by the provisions of the Dutch Civil Procedure Code.

Based on the lack of a treaty as described above, U.S. investors may not be able to enforce against us or our management board or supervisory board members, representatives or certain experts named herein who are residents of the Netherlands or countries other than the United States any judgments obtained in U.S. courts in civil and commercial matters, including judgments under the U.S. federal securities laws.

The rights and responsibilities of our shareholders are governed by Dutch law and differ in some important respects from the rights and responsibilities of shareholders under U.S. law.

Our corporate affairs are governed by our articles of association and by the laws governing companies incorporated in the Netherlands. The rights of our shareholders and the responsibilities of members of our supervisory board under Dutch law are different than under the laws of some U.S. jurisdictions. In the performance of its duties, our supervisory board is required by Dutch law to consider the interests of uniQure, its shareholders, its employees and other stakeholders and not only those of our shareholders. Also, as a Dutch company, we are not required to solicit proxies or prepare proxy statements for general meetings of shareholders. Dutch law does not have a regulatory regime for U.S.- style proxy solicitations and, even though Dutch law accommodates voting by proxy, the solicitation of proxies is not a widely used business practice in the Netherlands.

In addition, the rights of holders of shares and many of the rights of shareholders as they relate to, for example, the exercise of shareholder rights, are governed by Dutch law and our articles of association and differ from the rights of shareholders under U.S. law. For example, Dutch law does not grant appraisal rights to a company's shareholders who wish to challenge the consideration to be paid upon a merger or consolidation of the company. See "Description of Share Capital—Differences in Corporate Law."

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