Summary of significant accounting policies (Policies)	9 Months Ended
Summary of significant accounting policies (Policies)	Sep. 30, 2017
Summary of significant accounting policies
Basis of preparation	2.1Basis of preparation The Company prepared these unaudited consolidated financial statements in compliance with generally accepted accounting principles in the United States of America (“U.S. GAAP”) and applicable rules and regulations of the SEC regarding interim financial reporting. Any reference in these notes to applicable guidance is meant to refer to authoritative U.S. GAAP as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Update (“ASU”) of the Financial Accounting Standards Board (“FASB”). The unaudited consolidated financial statements are presented in U.S. dollars, except where otherwise indicated. Transactions denominated in currencies other than U.S. dollars are presented in the transaction currency with the U.S. dollar amount included in parenthesis, converted at the foreign exchange rate as of the transaction date.
Unaudited interim financial information	2.2Unaudited interim financial information The accompanying interim financial statements and related disclosures are unaudited, have been prepared on the same basis as the annual financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary for a fair statement of the financial position, results of operations and changes in financial position for the periods presented. Certain information and footnote disclosures normally included in annual financial statements prepared in accordance with U.S. GAAP have been omitted. The results of operations for the nine months ended September 30, 2017, are not necessarily indicative of the results to be expected for the full year ending December 31, 2017, or for any other future year or interim period. The accompanying financial statements should be read in conjunction with the audited financial statements and the related notes thereto included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2016, filed with the SEC on March 15, 2017.
Use of estimates	2.3Use of estimates The preparation of the financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent assets and liabilities at the date of the financial statements and reported amounts of revenues and expenses during the reporting periods. Actual results could differ from those estimates.
Accounting policies	2.4Accounting policies The principal accounting policies applied in the preparation of these unaudited consolidated financial statements are described in the Company’s audited financial statements as of and for the year ended December 31, 2016, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2016, filed with the SEC on March 15, 2017. There have been no material changes in the Company’s significant accounting policies during the nine months ended September 30, 2017.
Recent accounting pronouncements	2.5Recent accounting pronouncements There have been no new accounting pronouncements or changes to accounting pronouncements during the nine months ended September 30, 2017, compared to the recent accounting pronouncements described in Note 2.3.22 of the Company’s Annual Report on Form 10-K for the year ended December 31, 2016, which could be expected to materially impact the Company’s unaudited condensed consolidated financial statements except the ones discussed below: In May 2017, the FASB issued ASU 2017-09, Compensation-stock compensation (topic 718)- scope of modification accounting (“ASU 2017-09”), which provides clarity regarding the applicability of modification accounting in relation to share-based payment awards. Under the new guidance, modification accounting is required only if the fair value, the vesting conditions, or the classification of the award (as equity or liability) changes because of the change in terms or conditions. The effective date for the standard is for fiscal years beginning after December 15, 2017, which for the Company is January 1, 2018. Early adoption is permitted. The new standard is to be applied prospectively. The Company does not expect ASU 2017-09 to have a material impact on its consolidated financial statements. In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842). Under the new guidance, lessees will be required to recognize the assets and liabilities that arise from operating leases. A lessee should recognize in the statement of financial position a liability to make lease payments (the lease liability) and a right-of-use asset representing its right to use the underlying asset for the lease term. For leases with a term of 12 months or less, a lessee is permitted to make an accounting policy election by class of underlying asset not to recognize lease assets and lease liabilities. Lessees and lessors are required to recognize and measure leases at the beginning of the earliest period presented using a modified retrospective approach. ASU 2016-02 will be effective for the Company beginning in the first quarter of 2019 and early application is permitted. The Company does expect ASU 2016-02 to have a material impact on its consolidated financial statements, primarily from recognition of a right-of-use asset and lease liability in the balance sheet and a shift of cash outflows from operating activities to financing activities. In July 2015, the FASB issued ASU 2015-14, Revenue from Contracts with Customers: Deferral of the Effective Date (“ASU 2015-14”), which deferred the effective date for ASU 2014-09, Revenue from Contracts with Customers (“ASU 2014-09”), by one year. ASU 2014-09 will supersede the revenue recognition requirements in ASC 605, Revenue Recognition, and requires entities to recognize revenue in a way that depicts the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled to in exchange for those goods or services. In 2016, the FASB issued ASU 2016-08, 2016-10 and 2016-12, which provided further clarification on ASU 2014-09. ASU 2014-09 is effective for annual reporting periods beginning after December 15, 2017, including interim periods within that reporting period, which for the Company is January 1, 2018. The Company has one active product development collaboration with Bristol Myers Squibb (“BMS”). ASU 2014-09 provides for two possible implementation methods, (i) full retrospective application to all periods from January 1, 2015 onwards for revenue recognized in relation to collaborations; or (ii) application of the standard from January 1, 2018, onwards to the BMS collaboration with an adjustment to retained earnings as of December 31, 2017, to include the cumulative adjustment to revenue recognized in prior periods in relation to the BMS collaboration. The Company currently accounts for the BMS collaboration agreement as a revenue arrangement with multiple elements. The Company’s substantive deliverables under the BMS collaboration agreement include an exclusive license to its technology in the field of cardiovascular disease, research and development services for specific targets chosen by BMS and general development of the Company’s proprietary vector technology, participation in the Joint Steering Committee, and clinical and commercial manufacturing. The Company is continuing to assess the method of adoption as well as its financial statement disclosures