Spark Therapeutics (ONCE) 10-Q 4 Nov 16 2016 Q3 Quarterly report Financial data

Document and entity information

Document and entity information - shares	9 Months Ended
	Sep. 30, 2016	Nov. 01, 2016
Document And Entity Information [Abstract]
Entity Registrant Name	Spark Therapeutics, Inc.
Entity Central Index Key	1,609,351
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Document Type	10-Q
Trading Symbol	ONCE
Document Period End Date	Sep. 30, 2016
Document Fiscal Year Focus	2,016
Document Fiscal Period Focus (Q1,Q2,Q3,FY)	Q3
Amendment Flag	false
Entity Common Stock, Shares Outstanding		30,773,048

Consolidated balance sheets (un

Consolidated balance sheets (unaudited) - USD ($)	Sep. 30, 2016	Dec. 31, 2015
Current assets:
Cash and cash equivalents	$ 90,615,538	$ 293,530,590
Marketable securities	216,033,237	0
Other receivables	1,020,962	16,944,568
Prepaid expenses and deferred financing costs	1,565,490	1,132,626
Total current assets	309,235,227	311,607,784
Marketable securities	51,920,715	0
Property and equipment, net	19,788,497	16,999,445
Acquired-in-process research and development	15,490,000	0
Goodwill	2,096,119	0
Other assets	1,005,475	1,165,285
Total assets	399,536,033	329,772,514
Current liabilities:
Accounts payable	8,629,440	9,687,594
Accrued expenses and other	9,279,023	6,529,263
Current portion of long-term debt	274,978	0
Current portion of deferred rent	761,236	715,959
Current portion of deferred revenue	5,168,674	5,182,835
Total current liabilities	24,113,351	22,115,651
Long-term debt	1,300,428	0
Long-term deferred rent	7,496,257	8,084,509
Long-term deferred revenue	5,168,674	9,034,559
Deferred tax liability	1,936,250	0
Total liabilities	40,014,960	39,234,719
Stockholders’ equity:
Preferred stock, $0.001 par value. Authorized, 5,000,000 shares; no shares issued or outstanding	0	0
Common stock, $0.001 par value. Authorized, 150,000,000 shares; issued 27,082,493 and 30,766,362 shares at December 31, 2015 and September 30, 2016, respectively; 27,073,287 and 30,757,156 outstanding at December 31, 2015 and September 30, 2016, respectively	30,766	27,083
Additional paid-in capital	575,640,376	419,791,732
Accumulated other comprehensive loss	(53,816)	0
Treasury stock, at cost 9,206 shares at December 31, 2015 and September 30, 2016	(552,636)	(552,636)
Accumulated deficit	(215,543,617)	(128,728,384)
Total stockholders’ equity	359,521,073	290,537,795
Total liabilities and stockholders’ equity	$ 399,536,033	$ 329,772,514

Consolidated balance sheets (u3

Consolidated balance sheets (unaudited) (Parenthetical) - $ / shares	Sep. 30, 2016	Dec. 31, 2015
Statement of Financial Position [Abstract]
Preferred stock , par value (in dollars per share)	$ 0.001	$ 0.001
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, shares authorized	150,000,000	150,000,000
Common stock, shares issued	30,766,362	27,082,493
Common stock, shares outstanding	30,757,156	27,073,287
Treasury stock, shares	9,206	9,206

Consolidated statements of oper

Consolidated statements of operations (unaudited) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Income Statement [Abstract]
Revenues	$ 1,302,789	$ 1,302,789	$ 3,880,046	$ 4,865,885
Operating expenses:
Research and development	22,384,109	11,796,455	60,257,545	29,474,535
General and administrative	12,049,954	6,461,675	31,600,567	16,479,678
Total operating expenses	34,434,063	18,258,130	91,858,112	45,954,213
Loss from operations	(33,131,274)	(16,955,341)	(87,978,066)	(41,088,328)
Interest income	574,934	54,781	1,168,900	117,419
Interest expense	(6,067)	0	(6,067)	0
Net loss	(32,562,407)	(16,900,560)	(86,815,233)	(40,970,909)
Preferred stock dividends	0	0	0	(634,794)
Net loss applicable to common stockholders	$ (32,562,407)	$ (16,900,560)	$ (86,815,233)	$ (41,605,703)
Basic and diluted net loss per common share (in dollars per share)	$ (1.07)	$ (0.70)	$ (3.08)	$ (1.88)
Weighted average basic and diluted common shares outstanding (in shares)	30,368,354	24,138,020	28,218,850	22,078,269
Other comprehensive income (loss):
Unrealized loss on available-for-sale securities	$ (126,943)	$ 0	$ (56,066)	$ 0
Foreign exchange translation adjustments	(1,003)	0	2,250	0
Total comprehensive loss	$ (32,690,353)	$ (16,900,560)	$ (86,869,049)	$ (41,605,703)

Consolidated statements of cash

Consolidated statements of cash flows (unaudited) - USD ($)	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Cash flows from operating activities:
Net loss	$ (86,815,233)	$ (40,970,909)
Adjustments to reconcile net loss to net cash used in operating activities:
Noncash rent expense	(542,975)	362,971
Depreciation expense	2,613,420	1,245,569
Stock-based compensation expense	17,301,617	8,479,278
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(262,144)	(634,856)
Other receivables	15,517,533	(1,006,848)
Accounts payable and accrued expenses	4,221,805	7,654,292
Deferred revenue	(3,880,046)	(4,865,885)
Net cash used in operating activities	(51,846,023)	(29,736,388)
Cash flows from investing activities:
Purchases of marketable securities	(273,010,018)	0
Maturity of marketable securities	5,000,000	0
Payment for acquisition, net of cash acquired	(5,911,243)	0
Purchases of property and equipment	(7,466,103)	(3,077,736)
Net cash used in investing activities	(281,387,364)	(3,077,736)
Cash flows from financing activities:
Proceeds from exercise of stock options	1,834,196	92,924
Proceeds from public offerings of common stock, net	126,908,733	170,313,435
Proceeds from long-term debt	1,600,000	0
Payments on long-term debt	(24,594)	0
Net cash provided by financing activities	130,318,335	170,406,359
Net increase (decrease) in cash and cash equivalents	(202,915,052)	137,592,235
Cash and cash equivalents, beginning of period	293,530,590	74,566,963
Cash and cash equivalents, end of period	90,615,538	212,159,198
Supplemental disclosure of cash flow information:
Property and equipment purchases included in accounts payable and accrued expenses	$ 623,117	$ 834,986

The Company

The Company	9 Months Ended
	Sep. 30, 2016
Accounting Policies [Abstract]
The Company	The Company Spark Therapeutics, Inc. was formed on March 13, 2013 in the state of Delaware as AAVenue Therapeutics, LLC and amended its Certificate of Formation in October 2013 to change its name to Spark Therapeutics LLC. In May 2014, it converted from a limited liability company (LLC) to a C corporation, Spark Therapeutics, Inc. (the Company). The Company is a gene therapy company, seeking to transform the lives of patients suffering from debilitating genetic diseases by developing potentially one-time, life-altering treatments. The Company operates in one segment and has its principal offices in Philadelphia, Pennsylvania. (a) Follow-on Public Offering On June 20, 2016, the Company completed a follow-on public offering, having sold 3,025,000 shares of common stock at an offering price of $45 .00 per share, for aggregate gross proceeds of $136.1 million . The Company received net proceeds from the public offering of $127.6 million , after deducting underwriting discounts and commissions and other offering expenses.

Development-stage risks

Development-stage risks	9 Months Ended
	Sep. 30, 2016
Development-stage Risk [Abstract]
Development-stage risks	Development-stage risks The Company has incurred losses and negative cash flows from operations since inception and had an accumulated deficit of $215.5 million at September 30, 2016 . The Company anticipates incurring additional losses until such time, if ever, that it can generate significant sales of its product candidates in development. The Company will need additional financing to fund its operations and to commercially develop its product candidates. The Company’s future operations are highly dependent on a combination of factors, including: (i) the success of its research and development; (ii) regulatory approval and market acceptance of the Company’s proposed future products; (iii) the timely and successful completion of additional financing; and (iv) the development of competitive therapies by other biotechnology and pharmaceutical companies.

Summary of significant accounti

Summary of significant accounting policies	9 Months Ended
	Sep. 30, 2016
Accounting Policies [Abstract]
Summary of significant accounting policies	Summary of significant accounting policies (a) Basis of presentation The accompanying unaudited interim consolidated financial statements of the Company and its wholly-owned subsidiary have been prepared in accordance with U.S. generally accepted accounting principles (GAAP) for interim financial information. In the opinion of management, the accompanying consolidated financial statements include all normal and recurring adjustments (which consist primarily of accruals, estimates and assumptions that impact the consolidated financial statements) considered necessary to present fairly the Company’s financial position as of September 30, 2016 , its results of operations for the three and nine months ended September 30, 2015 and 2016 and cash flows for the nine months ended September 30, 2015 and 2016 . Operating results for the three and nine months ended September 30, 2016 are not necessarily indicative of the results that may be expected for the year ending December 31, 2016 or any other period. The interim consolidated financial statements presented herein do not contain the required disclosures under U.S. GAAP for annual consolidated financial statements. The accompanying unaudited interim consolidated financial statements should be read in conjunction with the annual audited financial statements and related notes as of and for the year ended December 31, 2015 included in the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2015 . (b) Use of estimates The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from such estimates. (c) Fair value of financial instruments Management believes that the carrying amounts of the Company’s consolidated financial instruments, including cash equivalents, other receivables and accounts payable and accrued expenses, approximate fair value due to the short-term nature of those instruments. Management believes the carrying value of debt approximates fair value as the interest rates are reflective of the rate the Company could obtain on debt with similar terms and conditions. (d) Property and equipment Property and equipment consists of computer and laboratory equipment, software, office equipment, furniture and leasehold improvements and is recorded at cost. Maintenance and repairs that do not improve or extend the lives of the respective assets are expensed to operations as incurred. Upon disposal, retirement or sale, the related cost and accumulated depreciation are removed from the accounts and any resulting gain or loss is included in the results of operations. Property and equipment are depreciated on a straight-line basis over their estimated useful lives. The Company uses a life of three years for computer equipment and software, five years for laboratory and office equipment and seven years for furniture. Leasehold improvements are amortized over the shorter of the lease term or the estimated useful life of the asset . The Company reviews long-lived assets, such as property and equipment, for impairment when events or changes in circumstances indicate the carrying value of the assets may not be recoverable. Recoverability is measured by comparison of the book values of the assets to estimated undiscounted future cash flows that the assets are expected to generate. If the carrying amount of an asset exceeds its estimated future cash flows, then an impairment charge is recognized for the amount by which the carrying value of the asset exceeds the fair value of the asset. (e) Research and development Research and development costs are expensed as incurred. Research and development expenses consist of internal and external expenses. Internal expenses include employee compensation and overhead. External expenses include development, clinical trials, statistical analysis and report writing and regulatory compliance costs incurred with clinical research organizations and other third-party vendors. At the end of the reporting period, the Company compares payments made to third-party service providers to the estimated progress toward completion of the research or development objectives. Such estimates are subject to change as additional information becomes available. Depending on the timing of payments to the service providers and the progress that the Company estimates has been made as a result of the service provided, the Company may record net prepaid or accrued expense relating to these costs. When the Company is reimbursed by a collaboration partner for work performed, the costs incurred are recorded as research and development expenses and the related reimbursement is recorded as a reduction to research and development expenses. Upfront and milestone payments made to third parties who perform research and development services on the Company’s behalf are expensed as services are rendered. (f) Revenue recognition The Company has generated revenue solely through license and collaborative agreements. The Company recognizes revenue in accordance with Financial Accounting Standards Board (FASB) Accounting Standards Codification (ASC) Topic 605-25, Revenue Recognition for Arrangements with Multiple Elements , which addresses the determination of whether an arrangement involving multiple deliverables contains more than one unit of accounting. A delivered item within an arrangement is considered a separate unit of accounting only if both of the following criteria are met: • the delivered item has value to the customer on a stand-alone basis; and • if the arrangement includes a general right of return relative to the delivered item, delivery or performance of the undelivered item is considered probable and substantially in control of the vendor. Under FASB ASC Topic 605-25, if both of the criteria above are not met, then separate accounting for the individual deliverables is not appropriate. Revenue recognition for arrangements with multiple deliverables constituting a single unit of accounting is recognized generally over the greater of the term of the arrangement or the expected period of performance, either on a straight-line basis or on a modified proportional performance method. Milestones related to research and development activities are accounted for in accordance with FASB ASC Topic 605-28, milestone method of revenue recognition. FASB ASC Topic 605-28 allows for the recognition of consideration, which is contingent on the achievement of a substantive milestone in its entirety, in the period the milestone is achieved. A milestone is considered to be substantive if all of the following criteria are met: the milestone is commensurate with either: (1) the performance required to achieve the milestone or (2) the enhancement of the value of the delivered items resulting from the performance required to achieve the milestone; the milestone relates solely to past performance; and the milestone payment is reasonable relative to all of the deliverables and payment terms within the agreement. Nonrefundable license fees are recognized as revenue upon delivery provided there are no undelivered elements in the arrangement. For licenses with no stand-alone value, revenues are recognized on a straight-line basis over the related performance period. Amounts received prior to satisfying the revenue recognition criteria are recorded as deferred revenue on the Company’s consolidated balance sheet. Amounts expected to be recognized as revenue in the next 12 months following the balance sheet date are classified as current liabilities. To date, the Company has not generated any revenues from the commercial sale of products. (g) Net loss per common share Basic and diluted net loss per common share is determined by dividing net loss by the weighted average number of common shares outstanding during the period. For all periods presented unvested restricted shares and common stock options have been excluded from the calculation because their effect would be anti-dilutive. Therefore, the weighted average shares outstanding used to calculate both basic and diluted loss per share are the same. The following potentially dilutive securities have been excluded from the computations of diluted weighted average shares outstanding as of September 30, 2015 and 2016 as they would be anti-dilutive: September 30, 2015 2016 Unvested restricted common shares 391,607 296,621 Options issued and outstanding 3,257,701 4,109,025 (h) Deferred rent Rent expense, including rent holidays and scheduled rent increases, is recorded on a straight-line basis over the term of the lease commencing on the date the Company takes possession of the leased property. Tenant improvement allowances from the lessor are included in the accompanying consolidated balance sheet as deferred rent and are amortized as a reduction of rent expense over the term of the lease from the possession date. Deferred rent as of September 30, 2016 represents the net excess of rent expense over the actual cash paid for rent and the tenant improvement allowances received. (i) Recent accounting pronouncements In May 2014, the FASB issued updated guidance regarding the accounting for, and disclosures of, revenue recognition, with an effective date for annual and interim periods beginning after December 15, 2017. The update provides a single comprehensive model for accounting for revenue from contracts with customers. The model requires that revenue recognized reflect the actual consideration to which the entity expects to be entitled in exchange for the goods or services defined in the contract, including in situations with multiple performance obligations. The Company currently is evaluating the effect that this guidance may have on its consolidated financial statements. In February 2016, the FASB issued ASU 2016-02, “ Leases .” Accounting Standard Update (ASU) 2016-02 requires that lease arrangements longer than 12 months result in an entity recognizing an asset and liability. The updated guidance is effective for interim and annual periods beginning after December 15, 2018, and early adoption is permitted. The Company has not evaluated the impact of the updated guidance on the Company's consolidated financial statements. In March 2016, the FASB issued ASU 2016-09, " Improvements to Employee Share-Based Payment Accounting. " ASU 2016-09 intends to reduce the cost and complexity of accounting for share-based payments. The updated guidance is effective for interim and annual periods beginning after December 15, 2016, and early adoption is permitted. The Company early adopted the guidance as of March 31, 2016 and the impact was immaterial to the Company's consolidated financial statements.

Marketable securities

Marketable securities	9 Months Ended
	Sep. 30, 2016
Investments, Debt and Equity Securities [Abstract]
Marketable securities	Marketable securities The following table summarizes the available-for-sale securities held at September 30, 2016: Description Amortized cost Unrealized gains Unrealized losses Fair value September 30, 2016 U.S. government agency and corporate securities $ 268,010,018 $ 76,975 $ (133,041 ) $ 267,953,952 No available-for-sale securities held as of September 30, 2016 had remaining maturities greater than two years .

Fair value of financial instrum

Fair value of financial instruments	9 Months Ended
	Sep. 30, 2016
Fair Value Disclosures [Abstract]
Fair value of financial instruments	Fair value of financial instruments The Company follows FASB accounting guidance on fair value measurements for financial assets and liabilities measured on a recurring basis. The guidance requires fair value measurements to maximize the use of “observable inputs.” The three-level hierarchy of inputs to measure fair value are as follows: • Level 1: Unadjusted quoted prices in active markets that are accessible at the measurement date for identical, unrestricted assets or liabilities • Level 2: Significant other observable inputs other than Level 1 prices such as quoted prices in markets that are not active, or inputs that are observable, either directly or indirectly, for substantially the full term of the asset or liability • Level 3: Prices or valuation techniques that require inputs that are both significant to the fair value measurement and unobservable (i.e., supported by little or no market activity) The Company has classified assets and liabilities measured at fair value on a recurring basis as follows: Fair value measurements at reporting date using Quoted prices in active markets for identical assets (Level 1) Significant other observable inputs (Level 2) Significant unobservable inputs (Level 3) At December 31, 2015: Assets: Money market funds (included in cash and cash equivalents) $ 293,530,590 — — At September 30, 2016: Assets: Money market funds (included in cash and cash equivalents) $ 90,376,090 — — Marketable securities $ 267,953,952 (a) Cash and cash equivalents The Company considers all highly liquid investments that have maturities of three months or less when acquired to be cash equivalents. Cash equivalents as of September 30, 2016 consisted primarily of money market funds. (b) Marketable securities The amortized cost of available-for-sale marketable securities is adjusted for amortization of premiums and accretion of discounts to maturity. At September 30, 2016, the balance in the Company's accumulated other comprehensive loss was primarily composed of activity related to the Company's available-for-sale marketable securities. There were no realized gains or losses recognized on the sale or maturity of available-for-sale marketable securities during the three or nine months ended September 30, 2016 and, as a result, the Company did not reclassify any amounts out of accumulated other comprehensive loss for the same periods.

Business acquisition of Genable

Business acquisition of Genable	9 Months Ended
	Sep. 30, 2016
Business Combinations [Abstract]
Business acquisition of Genable	Business Acquisition of Genable On March 7, 2016, the Company acquired Genable Technologies, Ltd. (Genable), an Ireland-based private gene therapy company with which the Company had collaborated since 2014 in the development of Genable's therapeutic program targeting a genetic inherited retinal disease (IRD). With the acquisition, the Company acquired RhoNova™, a potential gene therapy targeting rhodopsin-linked autosomal dominant retinitis pigmentosa (RHO-adRP), an IRD that routinely leads to visual impairment and in the most severe cases to blindness. The consideration paid by the Company to Genable shareholders consisted of $6.1 million in cash and 265,000 shares of the Company's common stock with a fair value of $9.2 million , for total consideration of $15.3 million . In connection with the acquistion, a receivable due from Genable also was settled on the date of acquisition for $0.5 million . The Company incurred acquisition-related costs of approximately $0.3 million , which are included in general and administrative expenses on the consolidated statement of operations for the nine months ended September 30, 2016. The Company has accounted for the acquisition as a business combination under the acquisition method of accounting. The Company has preliminarily allocated the purchase price for the purchase of Genable based upon the estimated fair value of net assets acquired and liabilities assumed at the date of acquisition. The Company expects to finalize the allocation of the purchase price upon receipt of the final valuation for the acquired in-process research and development and goodwill and final resolution of post-closing working capital adjustments. The completion and filing of federal and state tax returns for the purchased entity may result in adjustments to the carrying value of assets and liabilities. Any adjustments to the preliminary fair values will be made as soon as practicable, but not later than one year from the March 7, 2016 acquisition date. Recognition and measurement of assets acquired and liabilities assumed The following table summarizes the preliminary fair values of the tangible and intangible assets acquired and liabilities assumed at the acquisition date, net of cash acquired at the acquisition date: Cash acquired $ 196,307 Other current assets 102,506 Acquired in-process research and development 15,490,000 Goodwill 2,096,119 Total assets assumed 17,884,932 Other non-current liabilities 254,753 Deferred tax liability 1,936,250 Total liabilities assumed 2,191,003 Total allocation of purchase price $ 15,693,929 Acquired in-process research and development The Company’s preliminary allocation of purchase price to acquired in-process research and development was $15.5 million . The estimated fair value of the in-process research and development was determined using the “income approach,” which is a valuation technique that provides an estimate of the fair value of an asset based on market participant expectations of the cash flows an asset would generate over its remaining useful life. Some of the more significant assumptions inherent in the development of those asset valuations include the estimated net cash flows for each year for the asset or product (including net revenues, cost of sales, research and development costs, selling and marketing costs and working capital/asset contributory asset charges), the appropriate discount rate to select in order to measure the risk inherent in each future cash flow stream, the assessment of the asset’s life cycle, the potential regulatory and commercial success risks, competitive trends impacting the asset cash flow stream as well as other factors. No assurances can be given that the underlying assumptions used to prepare the discounted cash flow analysis will not change. For these and other reasons, actual results may vary significantly from estimated results.

Accrued expenses

Accrued expenses	9 Months Ended
	Sep. 30, 2016
Payables and Accruals [Abstract]
Accrued expenses	Accrued expenses Accrued expenses consist of the following: December 31, September 30, Compensation and benefits $ 4,880,239 $ 5,455,096 Consulting and professional fees 432,346 1,506,794 Research and development 978,156 1,677,539 Other 238,522 639,594 Total accrued expenses $ 6,529,263 $ 9,279,023

Stockholders' equity

Stockholders' equity	9 Months Ended
	Sep. 30, 2016
Equity [Abstract]
Stockholders' equity	Stockholders’ equity The Company’s certificate of incorporation and bylaws contain the rights, preferences and privileges of the Company’s stockholders and their respective shares. The Company has authorized 150,000,000 shares of common stock and 5,000,000 shares of preferred stock. (a) Convertible preferred In 2013 and 2014 the Company issued shares of Series A Stock and Series B Stock convertible preferred stock, respectively. Every five shares of Series A Stock and Series B Stock were to automatically convert into one share of common stock at a qualified Initial Public Offering, or IPO, as defined, or upon approval by holders of at least 87.5% of the Series B Stock, subject to certain customary antidilution adjustments contained in the Company’s certificate of incorporation. The Series A Stock and Series B Stock were entitled to receive cumulative dividends at 8% per annum, which accrued from day to day beginning November 23, 2014 and were payable upon conversion, an event of liquidation or a qualified IPO, in each case, in shares of Series A Stock and Series B Stock, as applicable. As of February 4, 2015, dividends of $1.3 million had accumulated, and in connection with the IPO, were declared and converted along with all outstanding shares of Series A Stock and Series B Stock into an aggregate of 10,200,050 shares of common stock. (b) Common In 2013 and 2014, the Company issued restricted stock to various employees, directors and consultants of the Company. The vesting terms of the restricted stock issued varied, but primarily, shares vested 25% on the first anniversary of the vesting commencement date and then quarterly over three years , with accelerated vesting in the event of a change in control, as defined. Any unvested shares are forfeited in the event that the individual ceases to provide services to the Company. For the nine months ended September 30, 2015 , the Company recorded compensation expense of $47,559 and $1.4 million in general and administrative expense and research and development expense, respectively, related to the restricted shares. For the nine months ended September 30, 2016, the Company recorded compensation expense of $ 47,481 and $1.3 million in general and administrative expense and research and development expense, respectively, related to the restricted shares. At September 30, 2016 , there was $1.1 million of unrecognized compensation expense related to restricted common shares which is expected to be recognized over a weighted-average period of 0.8 years . The following table summarizes restricted stock activity: Number of shares Weighted- average grant date fair value Nonvested shares at December 31, 2015 348,555 $ 4.83 Shares vested (129,167 ) $ 3.90 Nonvested shares at September 30, 2016 219,388 $ 5.38 In February 2015, the Company completed its IPO, having sold 8,050,000 shares of common stock at an IPO price of $23.00 per share, for aggregate gross proceeds of $185.2 million . The Company received net proceeds from the IPO of $168.9 million , after deducting underwriting discounts and commissions and other offering expenses. As part of the IPO, all of the outstanding shares of preferred stock, including shares of preferred stock issued as accrued dividends, were converted into an aggregate of 10,200,050 shares of common stock. In December 2015, the Company completed a follow-on public offering, having sold 2,266,995 shares of common stock at a public offering price of $47.00 per share, for aggregate gross proceeds of $106.5 million . The Company received net proceeds from the public offering of $99.4 million , after deducting underwriting discounts and commissions and other offering expenses. In June 2016, the Company completed a follow-on public offering, having sold 3,025,000 shares of common stock at a public offering price of $45.00 per share, for aggregate gross proceeds of $136.1 million . The Company received net proceeds from the public offering of $127.6 million , after deducting underwriting discounts and commissions and other offering expenses.

Stock incentive plans

Stock incentive plans	9 Months Ended
	Sep. 30, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock incentive plans	Stock incentive plans The Company's 2015 Stock Incentive Plan (the 2015 Plan) provides for the grant of incentive stock options, nonstatutory stock options, stock appreciation rights, restricted stock awards, restricted stock units and other stock-based awards to employees, officers, directors, consultants and advisors. In January 2016, the number of shares of common stock authorized for issuance under the 2015 Plan automatically increased, pursuant to the terms of the 2015 Plan, by 1,083,313 shares. As of September 30, 2016 , 602,566 shares were available for future grants under the 2015 Plan. In January 2016, the number of shares of common stock authorized for issuance under the 2015 Employee Stock Purchase Plan (the 2015 ESPP) automatically increased, pursuant to the terms of the 2015 ESPP, by 270,828 shares. The 2015 ESPP will provide participating employees with the opportunity to purchase an aggregate of 490,828 shares of common stock. As of September 30, 2016, no shares were issued under the 2015 ESPP. The following table summarizes stock option activity: Number of options Weighted- average exercise price Outstanding at December 31, 2015 3,071,372 $ 22.99 Granted 1,419,250 $ 41.59 Exercised (354,569 ) $ 5.17 Canceled (27,028 ) $ 32.10 Outstanding at September 30, 2016 4,109,025 $ 30.89 Vested at September 30, 2016 1,026,013 $ 23.51 Vested at September 30, 2016 and expected to vest 4,109,025 $ 30.89 The weighted average remaining contractual term of options outstanding as of September 30, 2016 is 8.6 years. The weighted average remaining contractual term of options vested as of September 30, 2016 is 8.1 years. During the nine months ended September 30, 2015, the Company recorded compensation expense of $2.9 million and $4.0 million in research and development expense and general and administrative expense, respectively, related to stock options. During the nine months ended September 30, 2016, the Company recorded compensation expense of $6.7 million and $8.9 million in research and development expense and general and administrative expense, respectively, related to stock options. At September 30, 2016 , there was $61.6 million of unrecognized compensation expense related to stock options, which is expected to be recognized over a weighted average period of 2.9 years. The weighted average grant date fair value of the options granted during the nine months ended September 30, 2016 was $27.51 per share using the Black-Scholes option-pricing model with the following weighted-average assumptions: Expected volatility 74.5 % Risk-free interest rate 1.70 % Expected term (in years) 6.11 Expected dividend yield 0.0 % The following table summarizes non-vested restricted common stock activity under the 2015 Plan: Number Weighted- Nonvested shares at December 31, 2015 25,100 $ 60.03 Shares granted 59,500 $ 44.35 Shares canceled (700 ) $ 60.03 Shares vested (6,667 ) $ 37.06 Nonvested shares at September 30, 2016 77,233 $ 49.94 Included in the above table are non-vested restricted stock grants to certain employees and consultants totaling 43,900 shares for which the vesting provisions are based on the achievement of certain Company milestones. During the nine months ended September 30, 2016, the Company recorded compensation expense of $49,073 and $0.2 million in research and development expense and general and administrative expense, respectively, related to restricted common stock under the 2015 Plan. At September 30, 2016, there was $2.5 million of unrecognized compensation expense related to the restricted common stock.

Related-party transactions

Related-party transactions	9 Months Ended
	Sep. 30, 2016
Related Party Transactions [Abstract]
Related-party transactions	Related-party transactions As of December 31, 2015 and September 30, 2016 , CHOP was considered a significant equity holder. In October 2013, the Company entered into technology and license agreements with CHOP for certain commercialization licenses to be provided to the Company in order to develop services, methods and marketable products for commercialization. The license agreement requires the Company to reimburse CHOP for the patent costs related to the underlying licensed rights incurred after the effective date. For the three months ended September 30, 2015 and 2016 , the Company recorded $0.1 million and $0.2 million , respectively, of general and administrative expense related to the reimbursement of such patent costs in the accompanying consolidated statements of operations. For the nine months ended September 30, 2015 and 2016, the Company recorded $0.5 million and $0.7 million , respectively, of general and administrative expense related to the reimbursement of such patent costs in the accompanying consolidated statements of operations. In 2013, the Company entered into a number of services agreements with CHOP. The Master Research Services Agreement provides for certain research, development, and manufacturing services to be provided to the Company by CHOP. A separate Services Agreement provides for clinical, technical, and administrative services to be provided by CHOP to the Company. For the three months ended September 30, 2015 and 2016 , the Company recorded $1.8 million and $2.1 million , respectively, as research and development expense. For the nine months ended September 30, 2015 and 2016, the Company recorded $4.0 million and $5.4 million , respectively, as research and development expense. As of December 31, 2015 , $0.2 million and $1.7 million were recorded in accrued expenses and accounts payable, respectively, as amounts due to CHOP. As of September 30, 2016 , $0.9 million and $1.2 million were recorded in accrued expenses and accounts payable, respectively, as amounts due to CHOP.

Collaboration and license agree

Collaboration and license agreements	9 Months Ended
	Sep. 30, 2016
Collaboration and License Agreements [Abstract]
Collaboration and license agreements	Collaboration and license agreements In April 2014, the Company began discussions with a biopharmaceutical company concerning a potential manufacturing technology agreement. The Company received a one-time, nonrefundable payment of $1.0 million to engage in due diligence. In March 2015, the Company concluded discussions on a potential arrangement with the biopharmaceutical company and, as a result, the Company recognized the nonrefundable payment of $1.0 million as revenue during the nine months ended September 30, 2015. In December 2014, the Company entered into a global collaboration agreement with Pfizer Inc. (Pfizer), for the development and commercialization of SPK-FIX product candidates for the treatment of hemophilia B. Under the agreement, the Company granted Pfizer an exclusive worldwide license to any Factor IX gene therapy that it develops, manufactures or commercializes prior to December 31, 2024. The Company is primarily responsible for conducting all research and development activities through completion of Phase 1/2 clinical trials of hemophilia B product candidates. Pfizer and the Company will share development costs incurred under an agreed product development plan for each product candidate with the Company’s share of development costs under the agreement limited to $10.6 million . Following the completion of Phase 1/2 clinical trials, Pfizer will be primarily responsible for development, manufacture, regulatory approval and commercialization, including all costs associated therewith. In connection with this agreement, the Company received a $20.0 million upfront payment for the license in December 2014. As there is no stand-alone value for the license, the Company is recognizing revenue through the estimated completion date of Phase 1/2 clinical trials. During each of the nine month periods ended September 30, 2015 and 2016, the Company recognized $3.9 million of revenue related to the upfront payment. As of September 30, 2016 , there is $5.2 million and $5.2 million of current and long term deferred revenue, respectively, related to this payment. During the nine month periods ended September 30, 2015 and 2016, the Company recorded $1.1 million and $1.2 million , respectively, as a reduction to research and development expenses for the reimbursement of costs from Pfizer. The Company is eligible to receive up to an additional $245.0 million in aggregate milestone payments, $125.0 million of which relate to potential development, regulatory and commercial milestones for the first product candidate to achieve each milestone and $120.0 million of which relate to potential regulatory milestones for additional product candidates. In addition, the Company is entitled to receive royalties calculated as a low-teen percentage of net sales of licensed products. The royalties may be subject to certain reductions, including for a specified portion of royalty payments that Pfizer may become required to pay under any third-party license agreements, subject to a minimum royalty. Under the agreement, the Company remains solely responsible for the payment of license payments payable by the Company under specified license agreements. The agreement will expire on a country-by-country basis upon the latest of: (i) the expiration of the last-to-expire valid claim, as defined in the agreement, in licensed patent rights covering a licensed product; (ii) the expiration of the last-to-expire regulatory exclusivity granted with respect to a licensed product; or (iii) 15 years after the first commercial sale of the last licensed product to be launched, in each case, in the applicable country. Pfizer may terminate the agreement on a licensed product-by-licensed product and country-by-country basis, or in its entirety, for any or no reason subject to notice requirements.

Summary of significant accoun17

Summary of significant accounting policies (Policies)	9 Months Ended
	Sep. 30, 2016
Accounting Policies [Abstract]
Basis of presentation	Basis of presentation The accompanying unaudited interim consolidated financial statements of the Company and its wholly-owned subsidiary have been prepared in accordance with U.S. generally accepted accounting principles (GAAP) for interim financial information. In the opinion of management, the accompanying consolidated financial statements include all normal and recurring adjustments (which consist primarily of accruals, estimates and assumptions that impact the consolidated financial statements) considered necessary to present fairly the Company’s financial position as of September 30, 2016 , its results of operations for the three and nine months ended September 30, 2015 and 2016 and cash flows for the nine months ended September 30, 2015 and 2016 . Operating results for the three and nine months ended September 30, 2016 are not necessarily indicative of the results that may be expected for the year ending December 31, 2016 or any other period. The interim consolidated financial statements presented herein do not contain the required disclosures under U.S. GAAP for annual consolidated financial statements. The accompanying unaudited interim consolidated financial statements should be read in conjunction with the annual audited financial statements and related notes as of and for the year ended December 31, 2015 included in the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2015 .
Use of estimates	Use of estimates The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from such estimates.
Fair value of financial instruments	Fair value of financial instruments Management believes that the carrying amounts of the Company’s consolidated financial instruments, including cash equivalents, other receivables and accounts payable and accrued expenses, approximate fair value due to the short-term nature of those instruments.
Property and equipment	Property and equipment Property and equipment consists of computer and laboratory equipment, software, office equipment, furniture and leasehold improvements and is recorded at cost. Maintenance and repairs that do not improve or extend the lives of the respective assets are expensed to operations as incurred. Upon disposal, retirement or sale, the related cost and accumulated depreciation are removed from the accounts and any resulting gain or loss is included in the results of operations. Property and equipment are depreciated on a straight-line basis over their estimated useful lives. The Company uses a life of three years for computer equipment and software, five years for laboratory and office equipment and seven years for furniture. Leasehold improvements are amortized over the shorter of the lease term or the estimated useful life of the asset . The Company reviews long-lived assets, such as property and equipment, for impairment when events or changes in circumstances indicate the carrying value of the assets may not be recoverable. Recoverability is measured by comparison of the book values of the assets to estimated undiscounted future cash flows that the assets are expected to generate. If the carrying amount of an asset exceeds its estimated future cash flows, then an impairment charge is recognized for the amount by which the carrying value of the asset exceeds the fair value of the asset.
Research and development	Research and development Research and development costs are expensed as incurred. Research and development expenses consist of internal and external expenses. Internal expenses include employee compensation and overhead. External expenses include development, clinical trials, statistical analysis and report writing and regulatory compliance costs incurred with clinical research organizations and other third-party vendors. At the end of the reporting period, the Company compares payments made to third-party service providers to the estimated progress toward completion of the research or development objectives. Such estimates are subject to change as additional information becomes available. Depending on the timing of payments to the service providers and the progress that the Company estimates has been made as a result of the service provided, the Company may record net prepaid or accrued expense relating to these costs. When the Company is reimbursed by a collaboration partner for work performed, the costs incurred are recorded as research and development expenses and the related reimbursement is recorded as a reduction to research and development expenses. Upfront and milestone payments made to third parties who perform research and development services on the Company’s behalf are expensed as services are rendered.
Revenue recognition	Revenue recognition The Company has generated revenue solely through license and collaborative agreements. The Company recognizes revenue in accordance with Financial Accounting Standards Board (FASB) Accounting Standards Codification (ASC) Topic 605-25, Revenue Recognition for Arrangements with Multiple Elements , which addresses the determination of whether an arrangement involving multiple deliverables contains more than one unit of accounting. A delivered item within an arrangement is considered a separate unit of accounting only if both of the following criteria are met: • the delivered item has value to the customer on a stand-alone basis; and • if the arrangement includes a general right of return relative to the delivered item, delivery or performance of the undelivered item is considered probable and substantially in control of the vendor. Under FASB ASC Topic 605-25, if both of the criteria above are not met, then separate accounting for the individual deliverables is not appropriate. Revenue recognition for arrangements with multiple deliverables constituting a single unit of accounting is recognized generally over the greater of the term of the arrangement or the expected period of performance, either on a straight-line basis or on a modified proportional performance method. Milestones related to research and development activities are accounted for in accordance with FASB ASC Topic 605-28, milestone method of revenue recognition. FASB ASC Topic 605-28 allows for the recognition of consideration, which is contingent on the achievement of a substantive milestone in its entirety, in the period the milestone is achieved. A milestone is considered to be substantive if all of the following criteria are met: the milestone is commensurate with either: (1) the performance required to achieve the milestone or (2) the enhancement of the value of the delivered items resulting from the performance required to achieve the milestone; the milestone relates solely to past performance; and the milestone payment is reasonable relative to all of the deliverables and payment terms within the agreement. Nonrefundable license fees are recognized as revenue upon delivery provided there are no undelivered elements in the arrangement. For licenses with no stand-alone value, revenues are recognized on a straight-line basis over the related performance period. Amounts received prior to satisfying the revenue recognition criteria are recorded as deferred revenue on the Company’s consolidated balance sheet. Amounts expected to be recognized as revenue in the next 12 months following the balance sheet date are classified as current liabilities. To date, the Company has not generated any revenues from the commercial sale of products.
Net loss per common share	Net loss per common share Basic and diluted net loss per common share is determined by dividing net loss by the weighted average number of common shares outstanding during the period. For all periods presented unvested restricted shares and common stock options have been excluded from the calculation because their effect would be anti-dilutive. Therefore, the weighted average shares outstanding used to calculate both basic and diluted loss per share are the same.
Deferred rent	Deferred rent Rent expense, including rent holidays and scheduled rent increases, is recorded on a straight-line basis over the term of the lease commencing on the date the Company takes possession of the leased property. Tenant improvement allowances from the lessor are included in the accompanying consolidated balance sheet as deferred rent and are amortized as a reduction of rent expense over the term of the lease from the possession date. Deferred rent as of September 30, 2016 represents the net excess of rent expense over the actual cash paid for rent and the tenant improvement allowances received.
Recent accounting pronouncements	Recent accounting pronouncements In May 2014, the FASB issued updated guidance regarding the accounting for, and disclosures of, revenue recognition, with an effective date for annual and interim periods beginning after December 15, 2017. The update provides a single comprehensive model for accounting for revenue from contracts with customers. The model requires that revenue recognized reflect the actual consideration to which the entity expects to be entitled in exchange for the goods or services defined in the contract, including in situations with multiple performance obligations. The Company currently is evaluating the effect that this guidance may have on its consolidated financial statements. In February 2016, the FASB issued ASU 2016-02, “ Leases .” Accounting Standard Update (ASU) 2016-02 requires that lease arrangements longer than 12 months result in an entity recognizing an asset and liability. The updated guidance is effective for interim and annual periods beginning after December 15, 2018, and early adoption is permitted. The Company has not evaluated the impact of the updated guidance on the Company's consolidated financial statements. In March 2016, the FASB issued ASU 2016-09, " Improvements to Employee Share-Based Payment Accounting. " ASU 2016-09 intends to reduce the cost and complexity of accounting for share-based payments. The updated guidance is effective for interim and annual periods beginning after December 15, 2016, and early adoption is permitted. The Company early adopted the guidance as of March 31, 2016 and the impact was immaterial to the Company's consolidated financial statements.
Cash and cash equivalents	Cash and cash equivalents The Company considers all highly liquid investments that have maturities of three months or less when acquired to be cash equivalents. Cash equivalents as of September 30, 2016 consisted primarily of money market funds.
Marketable securities	Marketable securities The amortized cost of available-for-sale marketable securities is adjusted for amortization of premiums and accretion of discounts to maturity. At September 30, 2016, the balance in the Company's accumulated other comprehensive loss was primarily composed of activity related to the Company's available-for-sale marketable securities. There were no realized gains or losses recognized on the sale or maturity of available-for-sale marketable securities during the three or nine months ended September 30, 2016 and, as a result, the Company did not reclassify any amounts out of accumulated other comprehensive loss for the same periods.

Summary of significant accoun18

Summary of significant accounting policies (Tables)	9 Months Ended
	Sep. 30, 2016
Accounting Policies [Abstract]
Schedule of potentially dilutive securities excluded from computation of diluted weighted average shares outstanding	The following potentially dilutive securities have been excluded from the computations of diluted weighted average shares outstanding as of September 30, 2015 and 2016 as they would be anti-dilutive: September 30, 2015 2016 Unvested restricted common shares 391,607 296,621 Options issued and outstanding 3,257,701 4,109,025

Marketable securities (Tables)

Marketable securities (Tables)	9 Months Ended
	Sep. 30, 2016
Investments, Debt and Equity Securities [Abstract]
Schedule of available-for-sale securities	The following table summarizes the available-for-sale securities held at September 30, 2016: Description Amortized cost Unrealized gains Unrealized losses Fair value September 30, 2016 U.S. government agency and corporate securities $ 268,010,018 $ 76,975 $ (133,041 ) $ 267,953,952

Fair value of financial instr20

Fair value of financial instruments (Tables)	9 Months Ended
	Sep. 30, 2016
Fair Value Disclosures [Abstract]
Summary of assets and liabilities measured at fair value on recurring basis	The Company has classified assets and liabilities measured at fair value on a recurring basis as follows: Fair value measurements at reporting date using Quoted prices in active markets for identical assets (Level 1) Significant other observable inputs (Level 2) Significant unobservable inputs (Level 3) At December 31, 2015: Assets: Money market funds (included in cash and cash equivalents) $ 293,530,590 — — At September 30, 2016: Assets: Money market funds (included in cash and cash equivalents) $ 90,376,090 — — Marketable securities $ 267,953,952

Business acquisition of Genab21

Business acquisition of Genable (Tables)	9 Months Ended
	Sep. 30, 2016
Business Combinations [Abstract]
Summary of recognized identified assets acquired and liabilities assumed	The following table summarizes the preliminary fair values of the tangible and intangible assets acquired and liabilities assumed at the acquisition date, net of cash acquired at the acquisition date: Cash acquired $ 196,307 Other current assets 102,506 Acquired in-process research and development 15,490,000 Goodwill 2,096,119 Total assets assumed 17,884,932 Other non-current liabilities 254,753 Deferred tax liability 1,936,250 Total liabilities assumed 2,191,003 Total allocation of purchase price $ 15,693,929

Accrued expenses (Tables)

Accrued expenses (Tables)	9 Months Ended
	Sep. 30, 2016
Payables and Accruals [Abstract]
Schedule of accrued expenses	Accrued expenses consist of the following: December 31, September 30, Compensation and benefits $ 4,880,239 $ 5,455,096 Consulting and professional fees 432,346 1,506,794 Research and development 978,156 1,677,539 Other 238,522 639,594 Total accrued expenses $ 6,529,263 $ 9,279,023

Stockholders' equity (Tables)

Stockholders' equity (Tables)	9 Months Ended
	Sep. 30, 2016
Equity [Abstract]
Summary of restricted stock activity	The following table summarizes restricted stock activity: Number of shares Weighted- average grant date fair value Nonvested shares at December 31, 2015 348,555 $ 4.83 Shares vested (129,167 ) $ 3.90 Nonvested shares at September 30, 2016 219,388 $ 5.38 The following table summarizes non-vested restricted common stock activity under the 2015 Plan: Number Weighted- Nonvested shares at December 31, 2015 25,100 $ 60.03 Shares granted 59,500 $ 44.35 Shares canceled (700 ) $ 60.03 Shares vested (6,667 ) $ 37.06 Nonvested shares at September 30, 2016 77,233 $ 49.94

Stock incentive plans (Tables)

Stock incentive plans (Tables)	9 Months Ended
	Sep. 30, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Summary of stock option activity	The following table summarizes stock option activity: Number of options Weighted- average exercise price Outstanding at December 31, 2015 3,071,372 $ 22.99 Granted 1,419,250 $ 41.59 Exercised (354,569 ) $ 5.17 Canceled (27,028 ) $ 32.10 Outstanding at September 30, 2016 4,109,025 $ 30.89 Vested at September 30, 2016 1,026,013 $ 23.51 Vested at September 30, 2016 and expected to vest 4,109,025 $ 30.89
Summary of weighted-average assumptions	The weighted average grant date fair value of the options granted during the nine months ended September 30, 2016 was $27.51 per share using the Black-Scholes option-pricing model with the following weighted-average assumptions: Expected volatility 74.5 % Risk-free interest rate 1.70 % Expected term (in years) 6.11 Expected dividend yield 0.0 %
Summary of restricted common stock activity	The following table summarizes restricted stock activity: Number of shares Weighted- average grant date fair value Nonvested shares at December 31, 2015 348,555 $ 4.83 Shares vested (129,167 ) $ 3.90 Nonvested shares at September 30, 2016 219,388 $ 5.38 The following table summarizes non-vested restricted common stock activity under the 2015 Plan: Number Weighted- Nonvested shares at December 31, 2015 25,100 $ 60.03 Shares granted 59,500 $ 44.35 Shares canceled (700 ) $ 60.03 Shares vested (6,667 ) $ 37.06 Nonvested shares at September 30, 2016 77,233 $ 49.94

The Company - Additional Inform

The Company - Additional Information (Detail) $ / shares in Units, $ in Millions	Jun. 20, 2016USD ($)$ / sharesshares	Dec. 31, 2015USD ($)$ / sharesshares	Sep. 30, 2016segment
Class of Stock [Line Items]
Number of operating segments | segment			1
Common Stock [Member]
Class of Stock [Line Items]
Issuance of common stock, net of issuance costs (in shares) | shares	3,025,000	2,266,995
Stock issuance price per share (in dollars per share) | $ / shares	$ 45	$ 47
Sale of stock, aggregate gross proceeds	$ 136.1	$ 106.5
Sale of stock, net proceeds	$ 127.6	$ 99.4

Development-stage risks - Addit

Development-stage risks - Additional Information (Detail) - USD ($)	Sep. 30, 2016	Dec. 31, 2015
Development-stage Risk [Abstract]
Accumulated deficit	$ (215,543,617)	$ (128,728,384)

Summary of significant accoun27

Summary of significant accounting policies - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2016
Computers [Member]
Property, Plant and Equipment [Line Items]
Property and equipment useful life	3 years
Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment useful life	3 years
Laboratory and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment useful life	5 years
Furniture & Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment useful life	7 years
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment estimated useful lives	Leasehold improvements are amortized over the shorter of the lease term or the estimated useful life of the asset

Summary of significant accoun28

Summary of significant accounting policies - Schedule of Antidilutive Securities Excluded from Computation of Diluted Weighted Average Shares Outstanding (Detail) - shares	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Unvested Restricted Common Shares [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of diluted weighted average shares outstanding	296,621	391,607
Options Issued and Outstanding [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of diluted weighted average shares outstanding	4,109,025	3,257,701

Marketable securities - Availab

Marketable securities - Available-for-sale Securities (Details)	9 Months Ended
	Sep. 30, 2016USD ($)
U.S. government agency and corporate securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 268,010,018
Unrealized gains	76,975
Unrealized losses	(133,041)
Fair value	$ 267,953,952
Maximum [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, remaining maturity period	2 years

Fair value of financial instr30

Fair value of financial instruments - Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - Fair Value Measurements Recurring [Member] - USD ($)	Sep. 30, 2016	Dec. 31, 2015
Money market funds (included in cash and cash equivalents) [Member] | Quoted prices in active markets for identical assets (Level 1) [Member]
Assets:
Money market funds (included in cash and cash equivalents)	$ 90,376,090	$ 293,530,590
Money market funds (included in cash and cash equivalents) [Member] | Significant other observable inputs (Level 2) [Member]
Assets:
Money market funds (included in cash and cash equivalents)	0	0
Money market funds (included in cash and cash equivalents) [Member] | Significant unobservable inputs (Level 3) [Member]
Assets:
Money market funds (included in cash and cash equivalents)	0	$ 0
Marketable securities [Member] | Quoted prices in active markets for identical assets (Level 1) [Member]
Assets:
Investments	$ 267,953,952

Business acquisition of Genab31

Business acquisition of Genable - Additional Information (Details) - USD ($)	Mar. 07, 2016	Sep. 30, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Acquired-in-process research and development		$ 15,490,000	$ 0
Genable [Member]
Business Acquisition [Line Items]
Consideration paid, cash	$ 6,100,000
Consideration paid, value of shares	9,200,000
Total consideration	15,300,000
Acquisition-related costs	300,000
Acquired-in-process research and development	$ 15,500,000
Common Stock [Member] | Genable [Member]
Business Acquisition [Line Items]
Consideration paid, in shares	265,000
Genable [Member] | Genable [Member]
Business Acquisition [Line Items]
Receivable due settlement amount	$ 500,000

Business acquisition of Genab32

Business acquisition of Genable - Schedule of Tangible and Intangible Assets Acquired and Liabilities Assumed (Details) - USD ($)	Sep. 30, 2016	Mar. 07, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Goodwill	$ 2,096,119		$ 0
Genable [Member]
Business Acquisition [Line Items]
Cash acquired		$ 196,307
Other current assets		102,506
Acquired in-process research and development		15,490,000
Goodwill		2,096,119
Total assets assumed		17,884,932
Other non-current liabilities		254,753
Deferred tax liability		1,936,250
Total liabilities assumed		2,191,003
Total allocation of purchase price		$ 15,693,929

Accrued expenses - Schedule of

Accrued expenses - Schedule of Accrued Expenses (Detail) - USD ($)	Sep. 30, 2016	Dec. 31, 2015
Payables and Accruals [Abstract]
Compensation and benefits	$ 5,455,096	$ 4,880,239
Consulting and professional fees	1,506,794	432,346
Research and development	1,677,539	978,156
Other	639,594	238,522
Total accrued expenses	$ 9,279,023	$ 6,529,263

Stockholders' equity - Addition

Stockholders' equity - Additional Information (Detail) - USD ($)	Jun. 20, 2016	Feb. 04, 2015	Dec. 31, 2015	Sep. 30, 2016	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2014	Nov. 23, 2014	Dec. 31, 2013
Class of Stock [Line Items]
Common stock, shares authorized			150,000,000	150,000,000	150,000,000
Preferred stock, shares authorized			5,000,000	5,000,000	5,000,000
Accumulated dividends		$ 1,300,000
Common Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares issued upon conversion (in shares)		10,200,050
Issuance of common stock, net of issuance costs (in shares)	3,025,000		2,266,995
Stock issuance price per share (in dollars per share)	$ 45		$ 47
Proceeds from public offerings of common stock, net		$ 185,200,000
Sale of stock, net proceeds	$ 127,600,000		$ 99,400,000
Sale of stock, aggregate gross proceeds	$ 136,100,000		$ 106,500,000
Series A Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock dividend rate								8.00%
Series A Preferred Stock [Member] | Common Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares issued upon conversion (in shares)									0.2
Series B Preferred Stock [Member]
Class of Stock [Line Items]
Convertible preferred stock terms of conversion					Every five shares of Series A Stock and Series B Stock were to automatically convert into one share of common stock at a qualified IPO, as defined, or upon approval by at least 87.5% of the Series B Stock holders
Minimum percentage of approval by outstanding preferred share holders for preferred stock conversion to common stock		87.50%
Preferred stock dividend rate								8.00%
Series B Preferred Stock [Member] | Common Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares issued upon conversion (in shares)							0.2
Initial Public Offering [Member] | Common Stock [Member]
Class of Stock [Line Items]
Issuance of common stock, net of issuance costs (in shares)		8,050,000
Stock issuance price per share (in dollars per share)		$ 23
Sale of stock, net proceeds		$ 168,900,000
Restricted Stock [Member]
Class of Stock [Line Items]
Unrecognized compensation expense related to restricted common shares				$ 1,100,000	$ 1,100,000
Unrecognized compensation expense related to restricted common shares expected to be recognized, weighted-average period					9 months 5 days
Restricted Stock [Member] | Share-based Compensation Award, Tranche One [Member]
Class of Stock [Line Items]
Percentage of vesting of units on first anniversary				25.00%
Restricted Stock [Member] | Share-based Compensation Award, Tranche Two [Member]
Class of Stock [Line Items]
Shares, vesting period				3 years
Restricted Stock [Member] | General and Administrative Expense [Member]
Class of Stock [Line Items]
Compensation expense					$ 47,481	$ 47,559
Restricted Stock [Member] | Research and Development Expense [Member]
Class of Stock [Line Items]
Compensation expense					$ 1,300,000	$ 1,400,000

Stockholders' equity - Summary

Stockholders' equity - Summary of Restricted Stock Activity (Detail) - Restricted Stock [Member]	9 Months Ended
	Sep. 30, 2016$ / sharesshares
Number of shares (in shares):
Nonvested shares beginning balance (in shares) | shares	348,555
Shares vested (in shares) | shares	(129,167)
Nonvested shares ending balance (in shares) | shares	219,388
Weighted-average grant date fair value (in dollars per share):
Nonvested shares beginning balance (in dollars per share) | $ / shares	$ 4.83
Shares vested (in dollars per share) | $ / shares	3.90
Nonvested shares ending balance (in dollars per share) | $ / shares	$ 5.38

Stock incentive plans - Additio

Stock incentive plans - Additional Information (Detail) - USD ($)	1 Months Ended	9 Months Ended
	Jan. 31, 2016	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015	Jan. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average remaining contractual term of options exercisable		8 years 7 months
Weighted average remaining contractual term of options outstanding		8 years 1 month 10 days
Unrecognized compensation expense related to stock options		$ 61,600,000
Weighted average grant date fair value of options (in dollars per share)		$ 27.51
Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average period recognition for unrecognized compensation expense		2 years 11 months 5 days
Employee Stock Option [Member] | Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense		$ 6,700,000	$ 2,900,000
Employee Stock Option [Member] | General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense		$ 8,900,000	4,000,000
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average period recognition for unrecognized compensation expense		9 months 5 days
Stock grants issued (in shares)		219,388		348,555
Restricted Stock [Member] | Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense		$ 1,300,000	1,400,000
Restricted Stock [Member] | General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense		$ 47,481	$ 47,559
Time-based and Performance-based Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock grants issued (in shares)		43,900
2015 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of additional shares authorized (in share)	1,083,313
Shares available for future grants (in shares)		602,566
2015 Stock Incentive Plan [Member] | Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense		$ 2,500,000
2015 Stock Incentive Plan [Member] | Restricted Stock [Member] | Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense		49,073
2015 Stock Incentive Plan [Member] | Restricted Stock [Member] | General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense		$ 200,000
2015 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of additional shares authorized (in share)	270,828
Common stock reserved for issuance (in shares)					490,828
Number of shares issued (in shares)		0

Stock incentive plans - Summary

Stock incentive plans - Summary of Stock Option Activity (Detail)	9 Months Ended
	Sep. 30, 2016$ / sharesshares
Number of shares (in shares):
Outstanding at beginning of period (in shares) | shares	3,071,372
Granted (in shares) | shares	1,419,250
Exercised (in shares) | shares	(354,569)
Canceled (in shares) | shares	(27,028)
Outstanding at end of period (in shares) | shares	4,109,025
Vested at end of period (in shares) | shares	1,026,013
Vested and expected to vest at end of period (in shares) | shares	4,109,025
Weighted-average exercise price (in dollars per share):
Outstanding at beginning of period (in dollars per share) | $ / shares	$ 22.99
Granted (in dollars per share) | $ / shares	41.59
Exercised (in dollars per share) | $ / shares	5.17
Canceled (in dollars per share) | $ / shares	32.10
Outstanding at end of period (in dollars per share) | $ / shares	30.89
Vested at end of period (in dollars per share) | $ / shares	23.51
Vested and expected to vest at end of period (in dollars per share) | $ / shares	$ 30.89

Stock incentive plans - Summa38

Stock incentive plans - Summary of Weighted-Average Assumptions (Detail)	9 Months Ended
	Sep. 30, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Expected volatility	74.50%
Risk-free interest rate	1.70%
Expected term	6 years 1 month 8 days
Expected dividend yield	0.00%

Stock incentive plans - Summa39

Stock incentive plans - Summary of Restricted Common Stock Activity (Details) - Restricted Stock [Member] - USD ($)	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Number of shares (in shares):
Nonvested shares beginning balance (in shares)	348,555
Shares vested (in shares)	(129,167)
Nonvested shares ending balance (in shares)	219,388
Weighted-average grant date fair value (in dollars per share):
Nonvested shares beginning balance (in dollars per share)	$ 4.83
Shares vested (in dollars per share)	3.90
Nonvested shares ending balance (in dollars per share)	$ 5.38
Common Stock [Member]
Number of shares (in shares):
Nonvested shares beginning balance (in shares)	25,100
Shares granted (in shares)	59,500
Shares canceled (in shares)	(700)
Shares vested (in shares)	(6,667)
Nonvested shares ending balance (in shares)	77,233
Weighted-average grant date fair value (in dollars per share):
Nonvested shares beginning balance (in dollars per share)	$ 60.03
Shares granted (in dollars per share)	44.35
Shares canceled (in dollars per share)	60.03
Shares vested (in dollars per share)	37.06
Nonvested shares ending balance (in dollars per share)	$ 49.94
Research and Development Expense [Member]
Weighted-average grant date fair value (in dollars per share):
Compensation expense	$ 1,300,000	$ 1,400,000
General and Administrative Expense [Member]
Weighted-average grant date fair value (in dollars per share):
Compensation expense	47,481	$ 47,559
2015 Stock Incentive Plan [Member]
Weighted-average grant date fair value (in dollars per share):
Unrecognized compensation expense	2,500,000
2015 Stock Incentive Plan [Member] | Research and Development Expense [Member]
Weighted-average grant date fair value (in dollars per share):
Compensation expense	49,073
2015 Stock Incentive Plan [Member] | General and Administrative Expense [Member]
Weighted-average grant date fair value (in dollars per share):
Compensation expense	$ 200,000

Related-party transactions - Ad

Related-party transactions - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Related Party Transaction [Line Items]
Research and development expense related to agreements	$ 22,384,109	$ 11,796,455	$ 60,257,545	$ 29,474,535
CHOP [Member]
Related Party Transaction [Line Items]
Accrued expenses due to related party	900,000		900,000		$ 200,000
Accounts payable due to related party	1,200,000		1,200,000		$ 1,700,000
CHOP [Member] | General and Administrative Expense [Member] | Technology and License Agreement [Member]
Related Party Transaction [Line Items]
Reimbursement of patent costs	200,000	100,000	700,000	500,000
CHOP [Member] | Research and Development Expense [Member] | Master Research Services Agreement [Member]
Related Party Transaction [Line Items]
Research and development expense related to agreements	$ 2,100,000	$ 1,800,000	$ 5,400,000	$ 4,000,000

Collaboration and license agr41

Collaboration and license agreements - Additional Information (Detail) - USD ($)	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Dec. 31, 2014	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2014	Dec. 31, 2015	Apr. 30, 2014
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Nonrefundable payment received								$ 1,000,000
Recognition of nonrefundable payment as revenue					$ 1,000,000
Development costs		$ 22,384,109	$ 11,796,455	$ 60,257,545	29,474,535
Current portion of deferred revenue		5,168,674		5,168,674			$ 5,182,835
Long-term deferred revenue		5,168,674		5,168,674			$ 9,034,559
Milestone payment, additional revenue potential to be earned				$ 245,000,000
Pfizer Collaboration and License Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Collaborative arrangement rights and obligations				Under the agreement, the Company granted Pfizer an exclusive worldwide license to any Factor IX gene therapy that it develops, manufactures or commercializes prior to December 31, 2024.
Development costs						$ 10,600,000
Upfront payment received for license	$ 20,000,000
Licenses revenue				$ 3,900,000	3,900,000
Current portion of deferred revenue		5,200,000		5,200,000
Long-term deferred revenue		$ 5,200,000		$ 5,200,000
Collaboration agreement expiration, period				15 years
Collaboration and license agreement expiration description				The agreement will expire on a country-by-country basis upon the latest of: (i) the expiration of the last-to-expire valid claim, as defined in the agreement, in licensed patent rights covering a licensed product; (ii) the expiration of the last-to-expire regulatory exclusivity granted with respect to a licensed product or; (iii) 15 years after the first commercial sale of the last licensed product to be launched, in each case, in the applicable country. Pfizer may terminate the agreement on a licensed product-by-licensed product and country-by-country basis, or in its entirety, for any or no reason subject to notice requirements.
First Product Candidate [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone payment, additional revenue potential to be earned				$ 125,000,000
Additional Product Candidates [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone payment, additional revenue potential to be earned				120,000,000
Research and Development Expense [Member] | Pfizer Collaboration and License Agreement [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Reimbursed costs				$ 1,200,000	$ 1,100,000