REGENXBIO (RGNX) 10-Q 8 Aug 18 2018 Q2 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2018	Aug. 03, 2018
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q2
Trading Symbol	RGNX
Entity Registrant Name	REGENXBIO Inc.
Entity Central Index Key	1,590,877
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		32,308,644

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Current assets
Cash and cash equivalents	$ 106,889	$ 46,656
Marketable securities	179,605	114,122
Accounts receivable	739	473
Prepaid expenses	3,690	5,334
Other current assets	2,347	1,412
Total current assets	293,270	167,997
Marketable securities	19,795	15,616
Accounts receivable	4,485
Property and equipment, net	16,698	13,977
Restricted cash	225	225
Other assets	1,514	862
Total assets	335,987	198,677
Current liabilities
Accounts payable	4,230	4,832
Accrued expenses and other current liabilities	12,023	9,605
Deferred revenue	600
Total current liabilities	16,853	14,437
Deferred rent, net of current portion	1,192	1,211
Other liabilities	720
Total liabilities	18,765	15,648
Commitments and contingencies (Note 6)
Stockholders’ equity
Preferred stock; $0.0001 par value; 10,000 shares authorized, and no shares issued and outstanding at June 30, 2018 and December 31, 2017
Common stock; $0.0001 par value; 100,000 shares authorized at June 30, 2018 and December 31, 2017; 32,275 and 31,295 shares issued and outstanding at June 30, 2018 and December 31, 2017, respectively	3	3
Additional paid-in capital	386,110	371,497
Accumulated other comprehensive loss	(771)	(715)
Accumulated deficit	(68,120)	(187,756)
Total stockholders’ equity	317,222	183,029
Total liabilities and stockholders’ equity	$ 335,987	$ 198,677

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Jun. 30, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	100,000,000	100,000,000
Common stock, shares issued	32,275,000	31,295,000
Common stock, shares outstanding	32,275,000	31,295,000

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Income (Loss) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenues
Revenues	$ 40,031	$ 6,562	$ 172,422	$ 7,017
Costs of revenues
Research and development	21,486	13,917	41,036	30,536
General and administrative	8,318	6,355	16,698	12,977
Other operating expenses	5	29	33	74
Total operating expenses	33,681	21,618	64,047	44,995
Income (loss) from operations	6,350	(15,056)	108,375	(37,978)
Other Income
Interest income from licensing	6,898		8,253
Investment income	1,196	583	2,055	1,512
Total other income	8,094	583	10,308	1,512
Income (loss) before income taxes	14,444	(14,473)	118,683	(36,466)
Income Tax Expense	(3,850)		(3,850)
Net income (loss)	10,594	(14,473)	114,833	(36,466)
Other Comprehensive Income (Loss)
Unrealized gain (loss) on available-for-sale securities, net of reclassifications and income tax expense	132	(74)	(56)	(613)
Total other comprehensive income (loss)	132	(74)	(56)	(613)
Comprehensive income (loss)	10,726	(14,547)	114,777	(37,079)
Net income (loss) applicable to common stockholders	$ 10,594	$ (14,473)	$ 114,833	$ (36,466)
Net income (loss) per share:
Basic	$ 0.33	$ (0.47)	$ 3.60	$ (1.27)
Diluted	$ 0.30	$ (0.47)	$ 3.29	$ (1.27)
Weighted-average common shares outstanding:
Basic	32,082	30,662	31,858	28,678
Diluted	35,272	30,662	34,884	28,678
License [Member]
Revenues
Revenues	$ 40,031	$ 6,555	$ 172,422	$ 7,010
Costs of revenues
Costs of revenues	$ 3,872	1,311	$ 6,280	1,402
Other [Member]
Revenues
Revenues		7		7
Costs of revenues
Costs of revenues		$ 6		$ 6

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Cash flows from operating activities
Net income (loss)	$ 114,833	$ (36,466)
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities
Stock-based compensation expense	7,272	5,074
Net amortization of premiums and accretion of discounts on marketable debt securities	718	945
Depreciation and amortization	1,730	1,257
Net realized gains on sales of marketable securities		(480)
Imputed interest income from licensing	(8,253)
Other non-cash adjustments	13	40
Changes in operating assets and liabilities
Accounts receivable	8,879	982
Prepaid expenses	1,644	(657)
Other current assets	(585)	(242)
Other assets	(652)	(86)
Accounts payable	(340)	2,723
Accrued expenses and other current liabilities	2,566	(31)
Deferred revenue	600
Deferred rent	19	(89)
Other liabilities	(99)
Net cash provided by (used in) operating activities	128,345	(27,030)
Cash flows from investing activities
Purchases of marketable securities	(139,081)	(46,593)
Maturities of marketable securities	68,645	28,010
Sales of marketable securities		780
Purchases of property and equipment	(5,017)	(4,609)
Net cash used in investing activities	(75,453)	(22,412)
Cash flows from financing activities
Proceeds from exercise of stock options	6,999	329
Proceeds from issuance of common stock under employee stock purchase plan	342	147
Proceeds from public offering of common stock, net of underwriting discounts and commissions		81,994
Issuance costs for public offering of common stock		(219)
Net cash provided by financing activities	7,341	82,251
Net increase in cash and cash equivalents and restricted cash	60,233	32,809
Cash and cash equivalents and restricted cash
Beginning of period	46,881	25,065
End of period	$ 107,114	57,874
Supplemental disclosures of non-cash investing and financing activities
Issuance costs for public offering of common stock in accounts payable and accrued expenses		$ 193

Nature of Business

Nature of Business	6 Months Ended
	Jun. 30, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Nature of Business	1. Nature of Business REGENXBIO Inc. (the Company) is a leading clinical-stage biotechnology company seeking to improve lives through the curative potential of gene therapy. The Company’s proprietary adeno-associated virus (AAV) gene delivery platform (NAV Technology Platform) consists of exclusive rights to over 100 novel AAV vectors, including AAV7, AAV8, AAV9 and AAVrh10. The Company’s NAV® Technology Platform is being applied by the Company, as well as by third-party licensees (NAV Technology Licensees), in the development of product candidates for a variety of diseases with unmet needs. The Company was formed in 2008 in the State of Delaware and is headquartered in Rockville, Maryland. Liquidity and Risks As of June 30, 2018, the Company had generated an accumulated deficit of $68.1 million since inception. As the Company has incurred cumulative losses since inception, transition to recurring profitability is dependent upon the successful development, approval and commercialization of its product candidates and achieving a level of revenues adequate to support the Company’s cost structure. The Company may never achieve recurring profitability, and unless and until it does, the Company will continue to need to raise additional capital. As of June 30, 2018, the Company had cash, cash equivalents and marketable securities of $306.3 million, which management believes is sufficient to fund operations for at least the next 12 months from the date these consolidated financial statements were issued. The Company is subject to risks common to companies in the biotechnology industry, including, but not limited to, development by the Company or its competitors of technological innovations, risks of failure of clinical trials, dependence on key personnel, protection of proprietary technology, compliance with government regulations and ability to transition from clinical manufacturing to the commercial production of products.

Summary of Significant Accounti

Summary of Significant Accounting Policies	6 Months Ended
	Jun. 30, 2018
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation and Principles of Consolidation The accompanying consolidated financial statements are unaudited and have been prepared in conformity with accounting principles generally accepted in the United States of America (GAAP). Any reference in these notes to applicable guidance is meant to refer to the authoritative GAAP as found in the Accounting Standards Codification (ASC) and Accounting Standards Update (ASU) of the Financial Accounting Standards Board (FASB). The interim unaudited consolidated financial statements have been prepared on the same basis as the annual audited consolidated financial statements as of and for the year ended December 31, 2017 included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2017, which was filed with the SEC on March 6, 2018. Certain information and footnote disclosures required by GAAP which are normally included in the Company’s annual consolidated financial statements have been omitted pursuant to SEC rules and regulations for interim reporting. In the opinion of management, the accompanying consolidated financial statements reflect all adjustments, which include all normal and recurring adjustments necessary for the fair statement of the Company’s financial position as of June 30, 2018, and the results of its operations and its cash flows for the interim periods ended June 30, 2018 and 2017. The results of operations for the interim periods are not necessarily indicative of the results of operations to be expected for the full year, any other interim periods, or any future year or period. These interim consolidated financial statements should be read in conjunction with the audited consolidated financial statements as of and for the year ended December 31, 2017, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K. The accompanying consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation. Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts in the financial statements and accompanying notes. Actual results could differ materially from those estimates. Management considers many factors in selecting appropriate financial accounting policies and controls, and in developing the estimates and assumptions that are used in the preparation of these consolidated financial statements. Management must apply significant judgment in this process. In addition, other factors may affect estimates, including: expected business and operational changes, sensitivity and volatility associated with the assumptions used in developing estimates and whether historical trends are expected to be representative of future trends. The estimation process often may yield a range of potentially reasonable estimates of the ultimate future outcomes and management must select an amount that falls within that range of reasonable estimates. This process may result in actual results differing materially from those estimated amounts used in the preparation of the consolidated financial statements. Estimates are used in the following areas, among others: revenue, stock-based compensation expense, accrued research and development expenses and other accrued expenses, income taxes and the fair value of financial instruments. Accounts Receivable Accounts receivable primarily consist of consideration due to the Company resulting from its license agreements with NAV Technology Licensees. Accounts receivable include amounts invoiced to licensees as well as rights to consideration which have not yet been invoiced to licensees and for which payment is conditional solely upon the passage of time. If a licensee elects to terminate a license prior to the end of the license term, the licensed intellectual property is returned to the Company and any accounts receivable from the licensee which are not contractually payable to the Company are charged off as a reduction of license revenue in the period of the termination. Accounts receivable from licensees which are not expected to be received by the Company within 12 months from the reporting date are stated net of a discount to present value and recorded as non-current assets on the consolidated balance sheets. Receivables are stated net of an allowance for doubtful accounts, if deemed necessary based on the Company’s evaluation of collectability using specific identification of account balances, the credit profile of its customers and historical information regarding write-offs. Account balances are charged off against the allowance when the potential for recovery is considered remote. The Company did not record an allowance for doubtful accounts as of June 30, 2018 or December 31, 2017. Non-marketable Equity Securities The Company’s non-marketable equity securities do not have readily determinable fair values and consist of equity investments in other entities in which the Company’s ownership interest is below 20% and the Company does not have significant influence over the operations of the entity. Prior to January 1, 2018, non-marketable equity securities were accounted for using the cost method and measured at cost less impairment. Beginning January 1, 2018, upon the Company’s adoption of ASU 2016-01, Financial Instruments—Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities , non-marketable equity securities are measured at cost less impairment, adjusted for observable price changes for identical or similar investments of the same issuer. Fair Value of Financial Instruments The Company is required to disclose information on all assets and liabilities reported at fair value that enables an assessment of the inputs used in determining the reported fair values. FASB ASC Topic 820, Fair Value Measurements and Disclosures • Level 1—Valuations based on unadjusted quoted prices in active markets for identical assets or liabilities that the Company has the ability to access at the measurement date. • Level 2—Valuations based on quoted prices for similar assets or liabilities in markets that are not active or for which all significant inputs are observable, either directly or indirectly. • Level 3—Valuations that require inputs that reflect the Company’s own assumptions that are both significant to the fair value measurement and unobservable. To the extent that valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. The fair values of the Company’s Level 2 instruments are based on quoted market prices or broker or dealer quotations for similar assets. These investments are initially valued at the transaction price and subsequently valued utilizing third party pricing providers or other market observable data. Please refer to Note 4 for further information on the fair value measurement of the Company’s financial instruments. Revenue Recognition Effective January 1, 2018, the Company adopted ASU 2014-09, Revenue from Contracts with Customers Revenue Recognition Topic 606 requires entities to recognize revenue when control of the promised goods or services is transferred to customers at an amount that reflects the consideration to which the entity expects to be entitled to in exchange for those goods or services. The following five steps are performed to determine the appropriate revenue recognition for arrangements within the scope of Topic 606: (i) identify the contract(s) with a customer, (ii) identify the performance obligations in the contract, (iii) determine the transaction price, (iv) allocate the transaction price to the performance obligations in the contract and (v) recognize revenue when (or as) the entity satisfies the performance obligations. The Company applies the five-step model to contracts that are within the scope of Topic 606 only when it is probable that the Company will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. At contract inception, for contracts within the scope of Topic 606, the Company assesses the goods or services promised within each contract and determined those that are performance obligations and whether each promised good or service is distinct. The Company then recognizes as revenue the amount of the transaction price that is allocated to respective performance obligations when (or as) the respective performance obligations are satisfied. The Company evaluates its contracts for the presence of significant financing components. If a significant financing component is identified in a contract and provides a financing benefit to the customer, the transaction price for the contract is adjusted to account for the financing portion of the arrangement, which is recognized as interest income over the financing term using the effective interest method. In determining the appropriate interest rates for significant financing components, the Company evaluates the credit profile of the customer and prevailing market interest rates and selects an interest rate in which it believes would be charged to the customer in a separate financing arrangement over a similar financing term. License revenue The Company licenses its NAV Technology Platform to other biotechnology and pharmaceutical companies. The terms of the licenses vary, however licenses may be exclusive or non-exclusive and may be sublicensable by the licensee. Licenses may grant intellectual property rights for purposes of internal and preclinical research and development only, or may include the rights, or options to obtain future rights, to commercialize drug therapies for specific diseases using the Company’s NAV Technology Platform. License agreements generally have a term at least equal to the life of the underlying patents, but are terminable at the option of the licensee. Consideration to the Company under its license agreements may include: (i) up-front fees, (ii) option fees to obtain additional licenses, (iii) annual maintenance fees, (iv) milestone payments based on the achievement of certain development and sales-based milestones by licensees, (v) sublicense fees and (vi) royalties on sales of licensed products. The Company has determined that all of its license agreements are contracts with customers within the scope of Topic 606. Although licenses are terminable at the option of licensee, the Company has determined that there is a substantive termination penalty associated with the termination of each license. Due to the substantive termination penalty, the contract term for purposes of applying Topic 606 is equal to the stated term of the license agreement, which is the life of the underlying licensed patents. The Company’s performance obligations under its license agreements include the delivery of intellectual property licenses to licensees as well as options granted to licensees to acquire future licenses to the extent the options represent material rights to the licensee. The transaction price for each license agreement is allocated to these performance obligations and recognized as revenue when the performance obligations are satisfied. Consideration allocated to performance obligations for the delivery of intellectual property licenses is recognized as revenue upon the delivery of the license(s) to the licensee, which generally occurs upon the execution of the license agreement. Consideration allocated to performance obligations for license options is recognized as revenue upon the earlier of the option exercise or expiration. For license agreements which contain options for the licensee to purchase additional licenses in the future, the Company evaluates the options at the inception of the agreement to determine if they provide a material right to the licensee. In making this determination, the Company considers whether the optional licenses are priced at a discount to the standalone selling price for the licenses. For options granted which are deemed to be material rights to the licensee, the Company allocates a portion of the transaction price to the performance obligation for the option and recognizes that consideration as revenue at the earlier of option exercise or expiration. Options which are not material rights to licensees are not considered performance obligations and are not accounted for as part of the license agreement until exercised by the licensee. Consideration contingent upon the exercise of options by licensees is excluded from the transaction price and not accounted for as part of the license agreement until the option is exercised. Upon the exercise of an option by a licensee, the additional consideration related to the option exercise is added to the transaction price and recognized as revenue upon the delivery of the newly purchased license. The Company evaluates the transaction price for its license agreements at each reporting date. The transaction price for each license includes all fixed consideration, as well as variable consideration to the extent that it is probable that a significant reversal of revenue will not occur in the future. Fixed consideration under the Company’s license agreements includes up-front fees and annual maintenance fees. Variable consideration under the Company’s license agreements includes development and sales-based milestone payments, sublicense fees and royalties on sales of licensed products. Up-front license fees are included in the transaction price and recognized as revenue upon the delivery of the license. If up-front license fees are payable to Company in periods beyond 12 months from the delivery of the license, a significant financing component is deemed to exist and the Company adjusts the transaction price to include only the present value of the license fees. The discounted portion of the license fees is recognized as interest income in the consolidated statements of operations over the term of the financing period. Annual maintenance fees are generally payable to the Company on each anniversary date over the term of the license agreement. The Company has determined that the payment of annual maintenance fees by licensees in future periods represents a significant financing component to the license since the delivery of the license occurs at the inception of the agreement. The present value of aggregate annual maintenance fees payable to the Company over the term of the license is included in the transaction price and recognized as revenue upon the delivery of the license. The discounted portion of the annual maintenance fees is recognized as interest income in the consolidated statements of operations over the term of the license. Development milestone payments are payable to the Company upon the achievement of specified development milestones by licensees. At the inception of each license agreement that contains development milestone payments, the Company evaluates whether the milestones are considered probable of achievement and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal will not occur in the future, milestone payments are included in the transaction price and recognized as revenue upon the delivery of the license. Milestone payments contingent on the achievement of development milestones that are not within the control of the Company or the licensee, such as regulatory approvals, are not considered probable of being achieved and are excluded from the transaction price until the milestone is achieved. At each reporting date, the Company re-evaluates the probability of achievement of outstanding development milestones and, if necessary, adjusts the transaction price for any milestones for which the probability of achievement has changed due to current facts and circumstances. Any such adjustments are recorded on a cumulative catch-up basis and recorded as license revenue in the period of the adjustment. Royalties on sales of licensed products, sales-based milestone payments and sublicense fees based on the receipt of certain fees by licensees from any sublicensees are excluded from the transaction price for each license and recognized as revenue in the period that the related sales or sublicenses occur, provided that the associated license has been delivered to the licensee. To date the Company has not recognized any revenue from royalties on sales of licensed products, the achievement of sales-based milestones or sublicense fees. The Company receives payments from licensees based on the billing schedules established in each license agreement. Amounts recognized as revenue which have not yet been received from licensees are recorded as accounts receivable when the Company’s rights to the consideration are conditional solely upon the passage of time. Amounts recognized as revenue which have not yet been received from licensees are recorded as contract assets when the Company’s rights to the consideration are not unconditional. Contract assets are recorded as other current assets on the consolidated balance sheets. If a licensee elects to terminate a license prior to the end of the license term, the licensed intellectual property is returned to the Company and any consideration recorded as accounts receivable or contract assets which is not contractually payable by the licensee is charged off as a reduction of license revenue in the period of the termination. Amounts received by the Company prior to the delivery of underlying performance obligations are deferred and recognized as revenue upon the satisfaction of the performance obligations by the Company. Costs of Revenues Licensing costs consist of sublicense fees incurred by the Company to its licensors as a result of license revenues generated by the Company. Sublicense fees are based on a percentage of license fees received by the Company from licensees as specified in the Company’s agreements with its licensors. The Company recognizes sublicense fees in the period that the underlying license revenue is recognized. Sublicense fees payable by the Company to licensors in periods beyond 12 months from the reporting are recorded as non-current liabilities on the consolidated balance sheets. Net Income (Loss) Per Share Basic net income (loss) per share is calculated by dividing net income (loss) applicable to common stockholders by the weighted-average common shares outstanding during the period, without consideration for common stock equivalents. Diluted net income (loss) per share is calculated by adjusting the weighted-average common shares outstanding for the dilutive effect of common stock equivalents outstanding for the period, determined using the treasury-stock method. Contingently convertible shares in which conversion is based on non-market-priced contingencies are excluded from the calculations of both basic and diluted net income (loss) per share until the contingency has been fully met. For purposes of the diluted net income (loss) per share calculation, common stock equivalents are excluded from the calculation of diluted net income (loss) per share if their effect would be anti-dilutive. Recent Accounting Pronouncements Adoption of ASU 2014-09, Revenue from Contracts with Customers In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers Revenue Recognition The Company recorded a net reduction in opening accumulated deficit of $4.8 million as of January 1, 2018 for the cumulative impact of adoption of Topic 606, which was primarily the result of accelerated recognition of license revenue due to annual maintenance fees under Topic 606. Under Topic 605, annual maintenance fees payable to the Company by licensees were recognized as license revenue annually when the amounts became fixed or determinable. Under Topic 606, the present value of aggregate annual maintenance fees over the term of the license agreement are recognized as revenue upon the delivery of the license to the licensee. The impact of the accelerated recognition of license revenue upon adoption was partially offset by the accelerated recognition of licensing costs to the Company’s licensors. The Company recognizes sublicense fees to its licensors in the period the underlying license revenue is recognized. The cumulative adjustment for the adoption of Topic 606 had the following effects on the Company’s consolidated balance sheet as of January 1, 2018 (in thousands): Cumulative Adjustment for Balance at Adoption of Balance at December 31, 2017 Topic 606 January 1, 2018 Consolidated Balance Sheet Assets: Accounts receivable, current $ 473 $ 527 $ 1,000 Accounts receivable, non-current $ — $ 4,850 $ 4,850 Other current assets $ 1,412 $ 350 $ 1,762 Liabilities: Accrued expenses and other current liabilities $ 9,605 $ 105 $ 9,710 Other liabilities $ — $ 819 $ 819 Stockholdersʼ Equity: Accumulated deficit $ (187,756 ) $ 4,803 $ (182,953 ) The following tables present the effects of the adoption of Topic 606 on each financial statement line item of the Company’s financial statements for the interim periods ended June 30, 2018 (in thousands, except per share data): As of June 30, 2018 Impact of Results Without Adoption of Adoption of As Reported Topic 606 Topic 606 Consolidated Balance Sheet Assets: Accounts receivable, current $ 739 $ 567 $ 172 Accounts receivable, non-current $ 4,485 $ 4,485 $ — Prepaid expenses $ 3,690 $ 60 $ 3,630 Liabilities: Accrued expenses and other current liabilities $ 12,023 $ 100 $ 11,923 Deferred revenue $ 600 $ 600 $ — Other liabilities $ 720 $ 720 $ — Stockholdersʼ Equity: Accumulated deficit $ (68,120 ) $ 3,692 $ (71,812 ) Three Months Ended June 30, 2018 Six Months Ended June 30, 2018 Impact of Results Without Impact of Results Without Adoption of Adoption of Adoption of Adoption of As Reported Topic 606 Topic 606 As Reported Topic 606 Topic 606 Consolidated Statement of Operations Revenues: License revenue $ 40,031 $ (61,244 ) $ 101,275 $ 172,422 $ (9,529 ) $ 181,951 Expenses: Licensing costs $ 3,872 $ (1,220 ) $ 5,092 $ 6,280 $ (165 ) $ 6,445 Other Income: Interest income from licensing $ 6,898 $ 6,898 $ — $ 8,253 $ 8,253 $ — Net Income $ 10,594 $ (53,126 ) $ 63,720 $ 114,833 $ (1,111 ) $ 115,944 Net Income Per Share: Basic $ 0.33 $ (1.66 ) $ 1.99 $ 3.60 $ (0.04 ) $ 3.64 Diluted $ 0.30 $ (1.51 ) $ 1.81 $ 3.29 $ (0.03 ) $ 3.32 Six Months Ended June 30, 2018 Impact of Results Without Adoption of Adoption of As Reported Topic 606 Topic 606 Consolidated Statement of Cash Flows Cash Flows from Operating Activities: Net income $ 114,833 $ (1,111 ) $ 115,944 Imputed interest income from licensing $ (8,253 ) $ (8,253 ) $ — Changes in accounts receivable $ 8,879 $ 8,578 $ 301 Changes in prepaid expenses $ 1,644 $ (60 ) $ 1,704 Changes in other current assets $ (585 ) $ 350 $ (935 ) Changes in accrued expenses and other current liabilities $ 2,566 $ (5 ) $ 2,571 Changes in deferred revenue $ 600 $ 600 $ — Changes in other liabilities $ (99 ) $ (99 ) $ — The most significant effect that the adoption of Topic 606 had on the results of operations for the three and six months ended June 30, 2018, as compared to what results would have been if Topic 605 had continued to be applied, is related to the amount of revenue and interest income from licensing recognized under the Company’s January 2018 amendment to its license agreement with AveXis, Inc. (AveXis) for the development and commercialization of treatments for spinal muscular atrophy (SMA). Under Topic 606, the Company recognized the present value of all fixed consideration under the amendment as revenue upon the delivery of the license to AveXis in January 2018, including the present value of the two $30.0 million payments originally due to the Company in January 2019 and January 2020. The present value discount, which represents the financing portion of the consideration under Topic 606, was recognized as interest income from licensing over the financing term of the agreement. Under Topic 605, the Company would not have recognized such revenue until it became fixed and determinable and collectability was reasonably assured, and the Company would not have recognized any interest income from significant financing components under the license agreement. Under the requirements of Topic 606, the Company recognized license revenue of $40.0 million and $172.1 million, and interest income from licensing of $6.8 million and $8.0 million, during the three and six months ended June 30, 2018, respectively, related to its amended license agreement with AveXis. If the requirements of Topic 605 had been applied during the three and six months ended June 30, 2018, the Company would have recognized license revenue of $100.0 million and $180.0 million, respectively, and interest income from licensing of $0, related to its amended license agreement with AveXis. Please refer to Note 7 for further information on license revenue and the Company’s accounting analysis for the amended license with AveXis. Other recently adopted accounting pronouncements In May 2017, the FASB issued ASU 2017-09, Compensation—Stock Compensation (Topic 718): Scope of Modification Accounting In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash The Company’s restricted cash includes money market mutual funds used to collateralize an irrevocable letter of credit as required by the Company’s lease agreement for its office space in New York, New York. The following table provides a reconciliation of cash and cash equivalents and restricted cash as reported on the consolidated balance sheets to the total of these amounts as reported at the end of the period in the consolidated statements of cash flows (in thousands): June 30, 2018 June 30, 2017 Cash and cash equivalents $ 106,889 $ 57,649 Restricted cash 225 225 Total cash and cash equivalents and restricted cash $ 107,114 $ 57,874 In January 2016, the FASB issued ASU 2016-01, Financial Instruments—Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities, Technical Corrections and Improvements to Financial Instruments—Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities Recent accounting pronouncements not yet adopted In June 2018, the FASB issued ASU 2018-07, Compensation—Stock Compensation (Topic 718): Improvements to Nonemployee Share-based Payment Accounting Equity—Equity-based Payments to Nonemployees. In February 2018, the FASB issued ASU 2018-02, Income Statement—Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income Tax Cuts and Jobs Act of 2017 (the TCJA) In April 2017, the FASB issued ASU 2017-08, Receivables—Nonrefundable Fees and Other Costs (Subtopic 310-20) In June 2016, the FASB issued ASU 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) Leases (Topic 840) Codification Improvements to Topic 842, Leases

Marketable Securities

Marketable Securities	6 Months Ended
	Jun. 30, 2018
Investments Debt And Equity Securities [Abstract]
Marketable Securities	3. Marketable Securities The following tables present a summary of the Company’s marketable securities, which consist solely of available-for-sale debt securities (in thousands): Amortized Cost Unrealized Gains Unrealized Losses Fair Value June 30, 2018 U.S. government and federal agency securities $ 19,280 $ — $ (14 ) $ 19,266 Certificates of deposit 735 — — 735 Corporate bonds 179,721 3 (325 ) 179,399 $ 199,736 $ 3 $ (339 ) $ 199,400 Amortized Cost Unrealized Gains Unrealized Losses Fair Value December 31, 2017 Corporate bonds $ 130,018 $ 2 $ (282 ) $ 129,738 $ 130,018 $ 2 $ (282 ) $ 129,738 As of June 30, 2018 and December 31, 2017, no available-for-sale securities had remaining maturities greater than three years. The amortized cost of available-for-sale securities is adjusted for amortization of premiums and accretion of discounts to maturity. As of June 30, 2018 and December 31, 2017, the balance in the Company’s accumulated other comprehensive loss consisted solely of net unrealized gains and losses on available-for-sale securities, net of income tax effects and reclassification adjustments for realized gains and losses. During the three months and six months ended June 30, 2018, the Company recognized net unrealized gains (losses) on available-for-sale securities of $0.1 million and $(0.1) million, respectively, and income tax expense of $0 in other comprehensive income (loss) for the period. The Company did not recognize any realized gains or losses on the sale or maturity of available-for-sale securities during the three and six months ended June 30, 2018. During the three and six months ended June 30, 2017, the Company recognized net unrealized losses on available-for-sale securities of $0.1 million and $0.1 million, respectively, and income tax expense of $0 in other comprehensive income (loss) for the period. The Company recognized net realized gains of $0 and $0.5 million on the sale or maturity of marketable securities during the three and six months ended June 30, 2017, respectively, which were reclassified out of accumulated other comprehensive loss during the period and are included in investment income in the consolidated statements of operations and comprehensive income (loss). The following tables present the fair values and unrealized losses of marketable securities held by the Company in an unrealized loss position for less than 12 months and 12 months or greater (in thousands): Less than 12 Months 12 Months or Greater Total Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized Losses June 30, 2018 U.S. government and federal agency securities $ 19,266 $ (14 ) $ — $ — $ 19,266 $ (14 ) Corporate bonds 143,503 (239 ) 21,066 (86 ) 164,569 (325 ) $ 162,769 $ (253 ) $ 21,066 $ (86 ) $ 183,835 $ (339 ) Less than 12 Months 12 Months or Greater Total Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized Losses December 31, 2017 Corporate bonds $ 109,238 $ (180 ) $ 17,124 $ (102 ) $ 126,362 $ (282 ) $ 109,238 $ (180 ) $ 17,124 $ (102 ) $ 126,362 $ (282 ) As of June 30, 2018, securities held by the Company which were in an unrealized loss position consisted of 62 investment grade fixed income security positions. The Company has the intent and ability to hold such securities until recovery and has determined that none of its investments were other-than-temporarily impaired as of June 30, 2018 or December 31, 2017.

Fair Value of Financial Instrum

Fair Value of Financial Instruments	6 Months Ended
	Jun. 30, 2018
Fair Value Disclosures [Abstract]
Fair Value of Financial Instruments	4. Fair Value of Financial Instruments Financial instruments reported at fair value on a recurring basis include cash equivalents and marketable securities. Cash equivalents consist of money market mutual funds and marketable securities consist of fixed income debt securities. The following tables present the fair value of cash equivalents and marketable securities in accordance with the hierarchy discussed in Note 2 (in thousands): Quoted Significant prices other Significant in active observable unobservable markets inputs inputs (Level 1) (Level 2) (Level 3) Total June 30, 2018 Money market mutual funds (cash equivalents) $ — $ 106,867 $ — $ 106,867 U.S. government and federal agency securities (marketable securities) — 19,266 — 19,266 Certificates of deposit (marketable securities) — 735 — 735 Corporate bonds (marketable securities) — 179,399 — 179,399 $ — $ 306,267 $ — $ 306,267 Quoted Significant prices other Significant in active observable unobservable markets inputs inputs (Level 1) (Level 2) (Level 3) Total December 31, 2017 Money market mutual funds (cash equivalents) $ — $ 46,646 $ — $ 46,646 Corporate bonds (marketable securities) — 129,738 — 129,738 $ — $ 176,384 $ — $ 176,384 There were no transfers of financial instruments between levels of the fair value hierarchy during the six months ended June 30, 2018. Management estimates that the carrying amounts of its current accounts receivable, accounts payable and accrued expenses and other current liabilities approximate fair value due to the short-term nature of those instruments. Non-current accounts receivable are recorded at their present values using a discount rate that is based on prevailing market rates and the credit profile of the licensee on the date the amounts are initially recorded. Management does not believe there have been any significant changes in market conditions that would cause the discount rates used to be significantly different from those that would be used as of June 30, 2018 to determine the present value of the receivables. Accordingly, management estimates that the carrying value of its non-current accounts receivable approximates the fair value of those instruments. The Company’s non-marketable equity securities are measured at cost less impairment, adjusted for observable price changes for identical or similar investments of the same issuer. As of June 30, 2018 and December 31, 2017, non-marketable equity securities had carrying values of $0.4 million and are included in other assets on the consolidated balance sheets. Since the acquisition of the securities, the Company has not identified any observable price changes or changes in circumstances that would have an adverse effect on the fair value of its non-marketable equity securities as of June 30, 2018. No impairment losses on non-marketable equity securities were recorded during the three and six months ended June 30, 2018 and 2017.

Property and Equipment, Net

Property and Equipment, Net	6 Months Ended
	Jun. 30, 2018
Property Plant And Equipment [Abstract]
Property and Equipment, Net	5. Property and Equipment, Net Property and equipment, net consists of the following (in thousands): June 30, 2018 December 31, 2017 Computer equipment and software $ 1,814 $ 1,481 Lab equipment 9,496 8,561 Furniture and fixtures 1,695 1,384 Leasehold improvements 8,680 5,828 Total property and equipment 21,685 17,254 Accumulated depreciation and amortization (4,987 ) (3,277 ) Property and equipment, net $ 16,698 $ 13,977

Commitments and Contingencies

Commitments and Contingencies	6 Months Ended
	Jun. 30, 2018
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	6. Commitments and Contingencies Lease Agreements The Company recognizes rent expense on a straight-line basis over the term of its operating leases commencing on the date the Company takes possession of the leased property. Tenant improvement allowances that are considered to be lease incentives from the lessor are recorded as deferred rent and amortized as a reduction of rent expense over the term of the lease from the possession date. In March 2015, the Company entered into a non-cancelable operating lease for office space at 9712 Medical Center Drive in Rockville, Maryland (the Medical Center Drive Lease). The lease term commenced in April 2015. Monthly payments under the lease began in October 2015 and escalate annually in accordance with the lease agreement. In September 2015, November 2015, July 2017 and April 2018, the Company amended the Medical Center Drive Lease to include additional office and laboratory space at 9714 Medical Center Drive, and ultimately extend the term of the lease to September 2021. The Company has options to extend the term of the Medical Center Drive Lease for up to six additional years. Under the amended lease, the Company has received a $0.4 million tenant improvement allowance from the landlord which will be deferred and amortized on a straight-line basis as a reduction of rent expense over the term of the lease. In January 2016, the Company entered into a 7.5-year, non-cancelable operating lease for its corporate headquarters at 9600 Blackwell Road in Rockville, Maryland (the Blackwell Road Lease). The lease commenced in February 2016, and expires in September 2023. The Company has an option to extend the term of the lease for an additional five years. In November 2017, the Blackwell Road Lease was amended to include additional office space for the remainder of the lease term. Monthly payments under the lease began in September 2016 and escalate annually in accordance with the lease agreement. The Company received a $0.8 million tenant improvement allowance from the landlord which will be deferred and amortized on a straight-line basis as a reduction of rent expense over the term of lease. In May 2016, the Company entered into a 51-month, non-cancelable operating lease for additional office space at 400 Madison Avenue in New York, New York. The lease commenced in July 2016, and expires in October 2020. Monthly payments under the lease began in October 2016 and escalate annually in accordance with the lease agreement. Under the terms of the lease agreement, the Company has provided the landlord with an irrevocable letter of credit of $0.2 million which the landlord may draw upon in the event of any uncured default by the Company under the terms of the lease. As of June 30, 2018, the Company has recorded restricted cash of $0.2 million as collateral to the financial institution which issued the letter of credit. As of June 30, 2018, future minimum lease payments under non-cancelable operating leases are as follows (in thousands): Operating Leases 2018 (remainder of year) $ 1,193 2019 2,395 2020 2,411 2021 1,797 2022 621 Thereafter 479 Total minimum lease payments $ 8,896 Licenses Granted to the Company Licenses granted to the Company may require the Company to make future payments relating to sublicense fees, milestone fees for milestones achieved in the future and royalties on future sales of licensed products. Additionally, the Company may be responsible for the cost of the maintenance of the intellectual property as incurred by its licensors. Up-front fees to obtain licensed technology are included in research and development expenses and patent maintenance costs are included in general and administrative expenses in the consolidated statements of operations and comprehensive income (loss). Sublicense fees are based on a specified percentage of license fees earned by the Company and are included in licensing costs in the consolidated statements of operations and comprehensive income (loss). Milestone fees are included in licensing costs in the consolidated statements of operations and comprehensive loss if the underlying milestone is achieved by a licensee, or in research and development expense if the underlying milestone is achieved by the Company as a result of the development of its product candidates. Royalties on sales of licensed reagents for use in research and development are included in other costs of revenue in the consolidated statements of operations and comprehensive income (loss). The Company has not commercialized any product candidates or paid any royalties under these agreements other than for the sales of licensed reagents. The Trustees of the University of Pennsylvania . In February 2009, the Company entered into a license agreement, which has been amended from time to time, with The Trustees of the University of Pennsylvania (together with the University of Pennsylvania, Penn) for exclusive, worldwide rights to certain patents owned by Penn underlying the Company’s NAV Technology Platform. Under the terms of the agreement, in consideration for the license, the Company issued to Penn a 24.5% equity interest in the Company on a fully diluted basis after issuance. The Company is obligated to pay Penn royalties on net sales and sublicense fees, if any. Additionally, the Company is obligated to reimburse Penn for certain costs incurred related to the maintenance of the licensed patents. In April 2016, the Company entered into an agreement with Penn whereby the Company will fund clinical trial activities performed by Penn for RGX-501, the Company’s product candidate for the treatment of homozygous familial hypercholesterolemia (HoFH). In connection with the agreement, the Company amended its license from Penn to include exclusive license rights to data, results and other information generated in connection with the RGX-501 clinical trial. Expenses incurred by the Company related to its license from Penn were as follows (in thousands): Three Months Ended June 30, Six Months Ended June 30, 2018 2017 2018 2017 Sublicense fees $ — $ 656 $ — $ 701 Royalties on sales of reagents — 4 — 4 Maintenance of licensed patents 98 85 117 169 $ 98 $ 745 $ 117 $ 874 As of June 30, 2018 and December 31, 2017, the Company had accrued $0.1 million and less than $0.1 million, respectively, in expenses payable to Penn under the license agreement, which are included in accounts payable, accrued expenses and other current liabilities and other liabilities on the Company’s consolidated balance sheets. GlaxoSmithKline LLC. In March 2009, the Company entered into a license agreement, which was amended in April 2009, with GlaxoSmithKline LLC (GSK) for exclusive, worldwide rights to certain patents underlying the Company’s NAV Technology Platform which are owned by Penn and exclusively licensed to GSK. Under the terms of the agreement, in consideration for the license, the Company issued to GSK a 19.9% equity interest in the Company on a fully diluted basis after issuance. The Company is obligated to pay GSK royalties on net sales and sublicense fees, if any. Additionally, the Company is obligated to reimburse GSK for certain costs incurred and invoiced to the Company related to the maintenance of the licensed patents. The Company is also obligated to pay GSK up to $1.5 million upon the achievement of various milestones. From the inception of the agreement through June 30, 2018, the Company has incurred $0.5 million for milestones that have been achieved or are deemed probable of achievement. Expenses incurred by the Company related to its license from GSK were as follows (in thousands): Three Months Ended June 30, Six Months Ended June 30, 2018 2017 2018 2017 Sublicense fees $ 3,998 $ 656 $ 6,030 $ 701 Royalties on sales of reagents — 2 — 2 Maintenance of licensed patents 97 14 490 159 $ 4,095 $ 672 $ 6,520 $ 862 As of June 30, 2018 and December 31, 2017, the Company had accrued $1.3 million and $0.3 million, respectively, in expenses payable to GSK under the license agreement, which are included in accounts payable, accrued expenses and other current liabilities and other liabilities on the Company’s consolidated balance sheets. Regents of the University of Minnesota. In November 2014, the Company entered into a license agreement, which was amended in November 2016, with Regents of the University of Minnesota (Minnesota), for an exclusive license under certain patent rights to commercialize products covered by the licensed patent rights in any country or territory in which a licensed patent has been issued and is unexpired, or a licensed patent application is pending. In consideration for the license, the Company paid an up-front fee, and reimbursed Minnesota for patent maintenance expenses, for a total of less than $0.1 million. Under the terms of the agreement, the Company is obligated to pay Minnesota annual maintenance fees of less than $0.1 million per year on each anniversary date of the agreement. Additionally, the Company is obligated to pay royalties on net sales and sublicense fees, if any, and up to $0.1 million per licensed product upon the achievement of various milestones. In November 2016, the license with Minnesota was amended to include additional patent rights. In consideration for the additional patent rights, the Company paid an up-front fee of less than $0.1 million. From the inception of the agreement through June 30, 2018, the Company has incurred less than $0.1 million for milestones that have been achieved or are deemed probable of achievement. Expenses incurred by the Company related to its license from Minnesota were as follows (in thousands): Three Months Ended June 30, Six Months Ended June 30, 2018 2017 2018 2017 Sublicense fees $ (125 ) $ — $ 250 $ — Maintenance of licensed patents 130 11 142 16 $ 5 $ 11 $ 392 $ 16 As of June 30, 2018 and December 31, 2017, the Company had accrued $0.3 million and $0.1 million, respectively, in expenses payable to Minnesota under the license agreement, which are included in accounts payable and accrued expenses on the Company’s consolidated balance sheets. Other Funding Commitments In the normal course of business, the Company enters into agreements with contract research organizations, contract manufacturing organizations and other third-parties for services to be provided to the Company. Generally, these agreements provide for termination upon notice, with specified amounts due upon termination based on the timing of termination and the terms of the agreement. The actual amounts and timing of payments under these agreements are uncertain and contingent upon the initiation and completion of services to be provided to the Company. Guarantees and Indemnifications In the normal course of business, the Company enters into agreements that contain a variety of representations and provide for general indemnification. The Company’s potential exposure under these agreements is unknown because it involves claims that may be made against the Company in the future. To date, the Company has not paid any claims or been required to defend any action related to its indemnification obligations. As of June 30, 2018 and December 31, 2017, the Company did not have any material indemnification claims that were probable or reasonably possible and consequently has not recorded any related liabilities. European Patent Office Proceeding In June 2017, a third party filed an opposition with the European Patent Office (EPO) challenging the validity of a European patent owned by Penn for the AAV8 vector, which the Company has exclusively licensed. The Company is unable to estimate the outcome of this matter but intends to assist Penn in vigorously defending this patent. The EPO has scheduled oral proceedings to begin on October 26, 2018. As of June 30, 2018, the Company has not recorded any liabilities related to this matter.

License Revenue

License Revenue	6 Months Ended
	Jun. 30, 2018
License Agreement Revenue Recognition [Abstract]
License Revenue	7. License Revenue Effective January 1, 2018, the Company adopted Topic 606 using the modified retrospective transition method and has applied the new standard to all of its license agreements in effect as of January 1, 2018. Please refer to Note 2 for additional information regarding the adoption of Topic 606. License revenue for periods ending after January 1, 2018 is presented in accordance with the requirements of Topic 606, while prior period amounts are not adjusted and continue to be reported in accordance with Topic 605. As of June 30, 2018, the Company’s NAV Technology Platform was being applied in the development of more than 20 partnered product candidates by its NAV Technology Licensees. Consideration to the Company under its license agreements may include: (i) up-front fees, (ii) option fees to obtain additional licenses, (iii) annual maintenance fees, (iv) milestone payments based on the achievement of certain development and sales-based milestones by licensees, (v) sublicense fees and (vi) royalties on sales of licensed products. Sublicense fees vary by license and range from a mid-single digit percentage to a low-double digit percentage of license fees received by licensees as a result of sublicenses. Royalties on net sales of commercialized products vary by license and range from a mid-single digit percentage to a low double-digit percentage of net sales by licensees. To date the Company has not recognized any revenue from the achievement of sales-based milestones, royalties on sales of licensed products or sublicense fees. Development milestone payments are only included in the transaction price of each license and recognized as revenue to the extent they are considered probable of achievement. Sales-based milestones are excluded from the transaction price of each license agreement and recognized as revenue in the period of achievement. As of June 30, 2018, the Company’s license agreements, excluding additional licenses that could be granted upon the exercise of options by licensees, could result in aggregate milestone fees payable to the Company of up to $25.9 million upon the commencement of various stages of clinical trials, $46.0 million upon the submission of regulatory approval filings, $105.5 million upon the approval of commercial products by regulatory agencies and $172.0 million upon the achievement of specified sales targets for licensed products. The achievement of milestones by licensees is highly dependent on the successful development and commercialization of licensed products and it is at least reasonably possible that some or all of the milestone fees will not be realized by the Company. The following table presents changes in the balances of the Company’s receivables, contract assets and contract liabilities during the periods presented (in thousands): Balance at Beginning Net Additions Balance at of Period (Deductions) End of Period Three Months Ended June 30, 2018 Receivables and contract assets: Accounts receivable, current and non-current $ 58,621 $ (53,397 ) $ 5,224 Contract assets $ 250 $ (250 ) $ — Contract liabilities: Deferred revenue $ — $ 600 $ 600 Balance at Beginning Net Additions Balance at of Period (Deductions) End of Period Six Months Ended June 30, 2018 Receivables and contract assets: Accounts receivable, current and non-current $ 5,850 $ (626 ) $ 5,224 Contract assets $ 350 $ (350 ) $ — Contract liabilities: Deferred revenue $ — $ 600 $ 600 The change in the balance of accounts receivable during the three months ended June 30, 2018 is primarily attributable to the acceleration and collection of $100.0 million in license fees due from AveXis as a result of their acquisition by Novartis AG (Novartis). Of the $100.0 million received, $53.3 million was previously recorded as accounts receivable at the beginning of the period. As of June 30, 2018, the Company had recorded deferred revenue of $0.6 million which represents consideration received from licensees for performance obligations that have not yet been satisfied by the Company. Unsatisfied performance obligations consist of options granted to licensees that provide material rights to the licensee to acquire future licenses from the Company. These performance obligations will be satisfied, and underlying revenue will be recognized, upon the exercise or expiration of the options. During the three and six months ended June 30, 2018, the Company recognized license revenue of $39.9 million and $0.2 million, respectively, from licenses delivered to licensees in prior periods as a result of changes in the transaction prices of its license agreements. Changes in the transaction price were primarily attributable to development milestones achieved or deemed probable of achievement during the period that were previously not considered probable of achievement, as well as the acquisition of AveXis by Novartis as discussed below. Additionally, the Company recognized $6.9 million and $0.3 million of interest income from licensing during the three and six months ended June 30, 2018, respectively, from licenses delivered in prior periods which contained significant financing components. As of June 30, 2018, the Company had not recognized any impairment losses on its receivables or contract assets from contracts with customers. AveXis, Inc. March 2014 License and January 2018 Amendment In March 2014, the Company entered into an exclusive license agreement (the March 2014 License) with AveXis. Under the license, the Company granted AveXis an exclusive, worldwide commercial license, with rights to sublicense, to the NAV AAV9 vector for the treatment of SMA in humans by in vivo gene therapy. mid-single to low double-digit royalties on net sales of licensed products, In January 2018, the Company entered into an amendment (the January 2018 Amendment) to the March 2014 License with AveXis. Under the January 2018 Amendment, the licensed intellectual property was expanded to include, in addition to the NAV AAV9 vector previously licensed, any other recombinant AAV vector in the Company’s intellectual property portfolio during a period of 14 years from the effective date of the January 2018 Amendment, for the treatment of SMA in humans by in vivo The January 2018 Amendment also modified the terms and conditions of the March 2014 License relating to assignment. Under the amended assignment provision, AveXis is permitted to transfer the March 2014 License, as amended, without the Company’s consent in connection with a change of control of AveXis, subject to the transferee or successor agreeing in writing to be bound by the terms of the March 2014 License, as amended, and the payment to the Company of certain fees due upon such change of control, as described below. Under the original March 2014 License, any assignment by AveXis without the Company’s prior written consent had been prohibited. Pursuant to the January 2018 Amendment, in consideration for the additional rights granted thereunder and in addition to any consideration owed under the original March 2014 License, AveXis paid to the Company a fee of $80.0 million upon entry into the January 2018 Amendment. In addition, AveXis was obligated to pay the Company (i) $30.0 million on the first anniversary of the effective date of the January 2018 Amendment, (ii) $30.0 million on the second anniversary of the effective date of the January 2018 Amendment and (iii) potential sales-based milestone payments of up to $120.0 million. In the event of a change of control of AveXis, to the extent that any fee described in (i) or (ii) above, or the first $40.0 million of sales-based milestone payments described in (iii) above, had not yet been paid to the Company, the January 2018 Amendment obliged AveXis to pay any such unpaid fee to the Company upon the change of control. For any product developed for the treatment of SMA using the NAV AAV9 vector, AveXis will continue to be obligated to pay to the Company mid-single to low double-digit royalties on net sales as defined in the March 2014 License, and for any product developed for the treatment of SMA using a licensed vector other than NAV AAV9, the Company will receive a low double-digit royalty on net sales. In May 2018, AveXis was acquired by Novartis, which qualified as a change of control of AveXis under the January 2018 Amendment. Pursuant to the January 2018 Amendment, AveXis paid the Company $100.0 million in accelerated license payments as a result of the change of control. Accounting Analysis The January 2018 Amendment was accounted for under Topic 606 as a modification of the license agreement resulting in a new and separate contract from the original March 2014 License for revenue recognition purposes. The only material performance obligation of the Company under the January 2018 Amendment is for the delivery of the modified license, which occurred upon the execution of the amendment in January 2018. As of June 30, 2018, the transaction price of the original March 2014 License was $3.5 million. The transaction price of $3.5 million includes (i) the up-front payment in March 2014 of $2.0 million, (ii) the present value of aggregate annual maintenance fees payable to the Company over the term of the license and (iii) the development milestones that had been achieved to date. The discounted portion of the annual maintenance fees represents the financing benefit provided to AveXis and is recognized as interest income from licensing over the term of the license. Variable consideration under the original March 2014 License, which has been excluded from the transaction price, includes payments for remaining development milestones that have not yet been achieved and are not considered probable of achievement, as well as any potential sublicense fees or royalties on sales of licensed products, which will be recognized in the period of the underlying sales or sublicenses, if any. Upon its execution, the transaction price of the January 2018 Amendment was $132.1 million, which was fully recognized as license revenue upon the delivery of the modified license in January 2018. In May 2018, as a result of the acquisition of AveXis by Novartis, the transaction price was increased by $40.0 million to account for the acceleration of the sale-based milestone which was previously excluded from the transaction price. The $40.0 million increase in the transaction price was recognized as license revenue upon the completion of the change of control in May 2018 since the amended license had been fully delivered to AveXis. Additionally, due to the acceleration of the two $30.0 million payments originally due in January 2019 and January 2020, the Company recognized $6.1 million of interest income from licensing upon the completion of the change of control of AveXis, which represents the remaining present value discount on such payments as of the date of the change of control of AveXis. The transaction price of $172.1 million as of June 30, 2018 includes the following fixed consideration: (i) the $80.0 million payment in January 2018, (ii) the present value, as of the date of the January 2018 Amendment, of the two $30.0 million payments originally due in January 2019 and January 2020 and (iii) the $40.0 million sales-based milestone which was accelerated upon the change of control in May 2018. Variable consideration under the January 2018 Amendment, which has been excluded from the transaction price, includes the remaining sales-based milestone payment of $80.0 million, as well as any potential sublicense fees or royalties on sales of licensed products, which will be recognized in the period of the underlying sales or sublicenses, if any. During the three and six months ended June 30, 2018, the Company recognized license revenue of $40.0 million and $172.1 million, respectively, and interest income from licensing of $6.8 million and $8.0 million, respectively, from the March 2014 License, as amended, with AveXis, which includes the amounts from both the original March 2014 License and the January 2018 Amendment. As of June 30, 2018, the Company had recorded $0.2 million of accounts receivable from AveXis under the March 2014 License, as amended, of which less than $0.1 million are included in current assets and $0.2 million are included in non-current assets on the consolidated balance sheets. During the three and six months ended June 30, 2017, the Company recognized license revenue of $0 and $0.1 million, respectively, from the March 2014 License which was recognized under the requirements of Topic 605. As of December 31, 2017, the Company had no amounts receivable from AveXis related to the March 2014 License under the requirements of Topic 605. AveXis, Inc. June 2017 License In June 2017, the Company entered into an exclusive license agreement (the June 2017 License) with AveXis. Under the license, the Company granted AveXis an exclusive, worldwide commercial license, with rights to sublicense, to the NAV AAV9 vector for the treatment of Rett Syndrome and in humans by in vivo gene therapy. a low double-digit royalty percentage on net sales of licensed products, During the three and six months ended June 30, 2018, the Company recognized license revenue of $0 and interest income from licensing of less than $0.1 million, from the June 2017 License with AveXis. As of June 30, 2018, the Company had recorded $0.7 million of accounts receivable from AveXis under the June 2017 License, of which $0.1 million are included in current assets and $0.6 million are included in non-current assets on the consolidated balance sheets. During the three and six months ended June 30, 2017, the Company recognized license revenue of $6.0 million from the June 2017 License which was recognized under the requirements of Topic 605. As of December 31, 2017, the Company had no amounts receivable from AveXis related to the June 2017 License under the requirements of Topic 605.

Stock-based Compensation

Stock-based Compensation	6 Months Ended
	Jun. 30, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-based Compensation	8. Stock-based Compensation In January 2018, an additional 1,251,810 shares became available for issuance under the 2015 Equity Incentive Plan (the 2015 Plan). As of June 30, 2018, the total number of shares of common stock authorized for issuance under the 2015 Plan and 2014 Stock Plan (the 2014 Plan) was 9,488,413, of which 2,236,302 remain available for future grants under the 2015 Plan. Stock-based Compensation Expense The Company’s stock-based compensation expense by award type is as follows (in thousands): Three Months Ended June 30, Six Months Ended June 30, 2018 2017 2018 2017 Stock options $ 3,818 $ 2,332 $ 6,939 $ 4,771 Restricted stock units 69 69 136 138 Employee stock purchase plan 95 82 197 165 $ 3,982 $ 2,483 $ 7,272 $ 5,074 As of June 30, 2018, the Company had $39.7 million of unrecognized stock-based compensation expense related to stock options, restricted stock units and the 2015 Employee Stock Purchase Plan (the 2015 ESPP), which is expected to be recognized over a weighted-average period of 2.7 years. The Company has recorded aggregate stock-based compensation expense in the consolidated statements of operations and comprehensive income (loss) as follows (in thousands): Three Months Ended June 30, Six Months Ended June 30, 2018 2017 2018 2017 Research and development $ 1,895 $ 1,137 $ 3,424 $ 2,374 General and administrative 2,087 1,346 3,848 2,700 $ 3,982 $ 2,483 $ 7,272 $ 5,074 Stock Options The following table summarizes stock option activity under the 2014 Plan and 2015 Plan (in thousands, except per share data): Weighted- average Weighted- Remaining average Contractual Aggregate Exercise Life Intrinsic Shares Price (Years) Value (a) Outstanding at December 31, 2017 5,468 $ 10.25 7.9 $ 125,738 Granted 1,137 $ 34.39 Exercised (961 ) $ 7.32 Cancelled or forfeited (199 ) $ 15.88 Outstanding at June 30, 2018 5,445 $ 15.61 7.9 $ 305,721 Exercisable at June 30, 2018 2,748 $ 7.88 7.1 $ 175,499 Vested and expected to vest at June 30, 2018 5,424 $ 15.66 7.9 $ 304,215 (a) The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the fair value of the common stock for the options that were in the money at the dates reported. The weighted-average grant date fair value per share of options granted during the six months ended June 30, 2018 was $22.88. During the six months ended June 30, 2018, the total number of stock options exercised was 960,838, resulting in total proceeds of $7.0 million. The total intrinsic value of options exercised during the six months ended June 30, 2018 was $28.1 million. Restricted Stock Units The following table summarizes restricted stock unit activity under the 2015 Plan (in thousands, except per share data): Weighted- average Grant Date Shares Fair Value Unvested balance at December 31, 2017 40 $ 20.90 Granted — $ — Vested — $ — Forfeited — $ — Unvested balance at June 30, 2018 40 $ 20.90 Employee Stock Purchase Plan As of June 30, 2018, the total number of shares of common stock authorized for issuance under the 2015 ESPP was 254,000, of which 186,752 remain available for future issuance. During the six months ended June 30, 2018, 19,528 shares of common stock were issued under the 2015 ESPP.

Income Taxes

Income Taxes	6 Months Ended
	Jun. 30, 2018
Income Tax Disclosure [Abstract]
Income Taxes	9. Income Taxes The TCJA was signed into law in December 2017, and has resulted in significant changes to the U.S. corporate income tax system. In December 2017, the SEC staff issued Staff Accounting Bulletin No. 118 (SAB 118), which allows the Company to record provisional amounts for the effects of the TCJA in the period it was enacted for a measurement period not extend beyond one year from the enactment date. In accordance with SAB 118, the Company has determined that the impact of the TCJA to its deferred tax assets and liabilities and valuation allowance as of June 30, 2018 and December 31, 2017 is an estimate and provisional amount. The final impact of the TCJA may differ from this provisional amount due to changes in the Company’s estimates and the issuance of additional regulatory or other guidance. The Company expects to complete its assessment of the final impact of the TCJA within the required measurement period under SAB 118. The Company’s effective tax rate for the three and six months ended June 30, 2018 differed from the U.S. federal statutory rate of 21%, primarily due to tax credits generated, tax windfall benefits from share-based payments and the expected utilization of U.S. federal net operating loss (NOL) carryforwards. These benefits were partially offset by state taxes and non-deductible expenses. The Company’s net deferred tax assets decreased d uring the six months ended June 30, 2018, primarily as a result of the expected utilization of U.S. federal and state NOL carryforwards. The decrease in deferred tax assets was offset by a corresponding decrease in the Company’s valuation allowance resulting in no impact on the Company’s tax provision for the period. The Company has evaluated the positive and negative evidence bearing upon the realizability of its deferred tax assets. Based on the Company’s history of operating losses, the Company has concluded that it is more likely than not that the benefit of its deferred tax assets will not be realized. Accordingly, the Company has provided a full valuation allowance for its net deferred tax assets as of June 30, 2018 and December 31, 2017.

Related Party Transactions

Related Party Transactions	6 Months Ended
	Jun. 30, 2018
Related Party Transactions [Abstract]
Related Party Transactions	10. Related Party Transactions FOXKISER LLP Effective January 2017, the Company entered into a Professional Services Agreement with FOXKISER LLP (FOXKISER), an affiliate of certain stockholders of the Company and an affiliate of a member of the Company’s Board of Directors, pursuant to which the Company pays a fixed monthly fee in consideration for certain strategic planning, development and regulatory services provided by FOXKISER. The agreement expired in December 2017, and effective January 2018 the Company entered into a new Professional Services Agreement with FOXKISER, which has a term of one year and is terminable by either party, at any time, upon 60 days’ prior written notice to the other party. Costs incurred under the agreements with FOXKISER for the three and six months ended June 30, 2018 were $0.5 million and $1.1 million, respectively. Costs incurred under the agreements with FOXKISER for the three and six months ended June 30, 2017 were $0.4 million and $0.8 million, respectively. Costs incurred under the agreements with FOXKISER are recorded as research and development expenses in the consolidated statements of operations and comprehensive income (loss). Scientific Founder and Special Advisor In September 2014, the Company entered into an advisory agreement, as amended, with James M. Wilson, M.D., Ph.D., who was formerly and currently and Special Advisor During the three months ended March 31, 2017, the Company incurred advisory fees of $0.1 million under the agreement, which are recorded as research and development expenses in the consolidated statements of operations and comprehensive income (loss).

Net Income (Loss) Per Share

Net Income (Loss) Per Share	6 Months Ended
	Jun. 30, 2018
Earnings Per Share [Abstract]
Net Income (Loss) Per Share	11. Net Income (Loss) Per Share The computations of basic and diluted net income (loss) per share are as follows (in thousands, except per share data): Three Months Ended June 30, Six Months Ended June 30, 2018 2017 2018 2017 Basic net income (loss) per share: Net income (loss) applicable to common stockholders $ 10,594 $ (14,473 ) $ 114,833 $ (36,466 ) Shares used in computation: Weighted-average common shares outstanding 32,082 30,662 31,858 28,678 Basic net income (loss) per share $ 0.33 $ (0.47 ) $ 3.60 $ (1.27 ) Diluted net income (loss) per share: Net income (loss) applicable to common stockholders $ 10,594 $ (14,473 ) $ 114,833 $ (36,466 ) Shares used in computation: Weighted-average common shares outstanding 32,082 30,662 31,858 28,678 Stock options 3,153 — 2,995 — Restricted stock units 30 — 27 — Employee stock purchase plan 7 — 4 — Weighted-average diluted common shares 35,272 30,662 34,884 28,678 Diluted net income (loss) per share $ 0.30 $ (0.47 ) $ 3.29 $ (1.27 ) For periods in which the Company incurred net losses applicable to common stockholders, common stock equivalents are excluded from the calculation of diluted net loss per share as their effect would be anti-dilutive, and accordingly, basic and diluted net loss per share are the same for such periods. Outstanding stock options with exercise prices greater than the average market price of common stock are excluded from the calculation of diluted net income (loss) per share as their effect would be anti-dilutive. The following potentially dilutive common stock equivalents outstanding at the end of the period were excluded from the computations of weighted-average diluted common shares for the periods indicated as their effects would be anti-dilutive (in thousands): Three Months Ended June 30, Six Months Ended June 30, 2018 2017 2018 2017 Stock options issued and outstanding 804 5,835 1,236 5,835 Unvested restricted stock units outstanding — 40 — 40 Employee stock purchase plan — 26 — 26 804 5,901 1,236 5,901

Supplemental Disclosures

Supplemental Disclosures	6 Months Ended
	Jun. 30, 2018
Payables And Accruals [Abstract]
Supplemental Disclosures	12. Supplemental Disclosures Accrued expenses and other current liabilities consists of the following (in thousands): June 30, 2018 December 31, 2017 Accrued personnel costs $ 4,902 $ 5,789 Accrued external research and development expenses 2,602 2,072 Accrued income taxes payable 2,439 — Accrued external general and administrative expenses 1,203 1,078 Accrued licensing costs 350 — Accrued purchases of property and equipment 139 430 Other accrued expenses and current liabilities 388 236 $ 12,023 $ 9,605

Summary of Significant Accoun18

Summary of Significant Accounting Policies (Policies)	6 Months Ended
	Jun. 30, 2018
Accounting Policies [Abstract]
Basis of Presentation and Principles of Consolidation	Basis of Presentation and Principles of Consolidation The accompanying consolidated financial statements are unaudited and have been prepared in conformity with accounting principles generally accepted in the United States of America (GAAP). Any reference in these notes to applicable guidance is meant to refer to the authoritative GAAP as found in the Accounting Standards Codification (ASC) and Accounting Standards Update (ASU) of the Financial Accounting Standards Board (FASB). The interim unaudited consolidated financial statements have been prepared on the same basis as the annual audited consolidated financial statements as of and for the year ended December 31, 2017 included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2017, which was filed with the SEC on March 6, 2018. Certain information and footnote disclosures required by GAAP which are normally included in the Company’s annual consolidated financial statements have been omitted pursuant to SEC rules and regulations for interim reporting. In the opinion of management, the accompanying consolidated financial statements reflect all adjustments, which include all normal and recurring adjustments necessary for the fair statement of the Company’s financial position as of June 30, 2018, and the results of its operations and its cash flows for the interim periods ended June 30, 2018 and 2017. The results of operations for the interim periods are not necessarily indicative of the results of operations to be expected for the full year, any other interim periods, or any future year or period. These interim consolidated financial statements should be read in conjunction with the audited consolidated financial statements as of and for the year ended December 31, 2017, and the notes thereto, which are included in the Company’s Annual Report on Form 10-K. The accompanying consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts in the financial statements and accompanying notes. Actual results could differ materially from those estimates. Management considers many factors in selecting appropriate financial accounting policies and controls, and in developing the estimates and assumptions that are used in the preparation of these consolidated financial statements. Management must apply significant judgment in this process. In addition, other factors may affect estimates, including: expected business and operational changes, sensitivity and volatility associated with the assumptions used in developing estimates and whether historical trends are expected to be representative of future trends. The estimation process often may yield a range of potentially reasonable estimates of the ultimate future outcomes and management must select an amount that falls within that range of reasonable estimates. This process may result in actual results differing materially from those estimated amounts used in the preparation of the consolidated financial statements. Estimates are used in the following areas, among others: revenue, stock-based compensation expense, accrued research and development expenses and other accrued expenses, income taxes and the fair value of financial instruments.
Accounts Receivable	Accounts Receivable Accounts receivable primarily consist of consideration due to the Company resulting from its license agreements with NAV Technology Licensees. Accounts receivable include amounts invoiced to licensees as well as rights to consideration which have not yet been invoiced to licensees and for which payment is conditional solely upon the passage of time. If a licensee elects to terminate a license prior to the end of the license term, the licensed intellectual property is returned to the Company and any accounts receivable from the licensee which are not contractually payable to the Company are charged off as a reduction of license revenue in the period of the termination. Accounts receivable from licensees which are not expected to be received by the Company within 12 months from the reporting date are stated net of a discount to present value and recorded as non-current assets on the consolidated balance sheets. Receivables are stated net of an allowance for doubtful accounts, if deemed necessary based on the Company’s evaluation of collectability using specific identification of account balances, the credit profile of its customers and historical information regarding write-offs. Account balances are charged off against the allowance when the potential for recovery is considered remote. The Company did not record an allowance for doubtful accounts as of June 30, 2018 or December 31, 2017.
Non-marketable Equity Securities	Non-marketable Equity Securities The Company’s non-marketable equity securities do not have readily determinable fair values and consist of equity investments in other entities in which the Company’s ownership interest is below 20% and the Company does not have significant influence over the operations of the entity. Prior to January 1, 2018, non-marketable equity securities were accounted for using the cost method and measured at cost less impairment. Beginning January 1, 2018, upon the Company’s adoption of ASU 2016-01, Financial Instruments—Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities , non-marketable equity securities are measured at cost less impairment, adjusted for observable price changes for identical or similar investments of the same issuer.
Fair Value of Financial Instruments	Fair Value of Financial Instruments The Company is required to disclose information on all assets and liabilities reported at fair value that enables an assessment of the inputs used in determining the reported fair values. FASB ASC Topic 820, Fair Value Measurements and Disclosures • Level 1—Valuations based on unadjusted quoted prices in active markets for identical assets or liabilities that the Company has the ability to access at the measurement date. • Level 2—Valuations based on quoted prices for similar assets or liabilities in markets that are not active or for which all significant inputs are observable, either directly or indirectly. • Level 3—Valuations that require inputs that reflect the Company’s own assumptions that are both significant to the fair value measurement and unobservable. To the extent that valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. The fair values of the Company’s Level 2 instruments are based on quoted market prices or broker or dealer quotations for similar assets. These investments are initially valued at the transaction price and subsequently valued utilizing third party pricing providers or other market observable data. Please refer to Note 4 for further information on the fair value measurement of the Company’s financial instruments.
Revenue Recognition	Revenue Recognition Effective January 1, 2018, the Company adopted ASU 2014-09, Revenue from Contracts with Customers Revenue Recognition Topic 606 requires entities to recognize revenue when control of the promised goods or services is transferred to customers at an amount that reflects the consideration to which the entity expects to be entitled to in exchange for those goods or services. The following five steps are performed to determine the appropriate revenue recognition for arrangements within the scope of Topic 606: (i) identify the contract(s) with a customer, (ii) identify the performance obligations in the contract, (iii) determine the transaction price, (iv) allocate the transaction price to the performance obligations in the contract and (v) recognize revenue when (or as) the entity satisfies the performance obligations. The Company applies the five-step model to contracts that are within the scope of Topic 606 only when it is probable that the Company will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. At contract inception, for contracts within the scope of Topic 606, the Company assesses the goods or services promised within each contract and determined those that are performance obligations and whether each promised good or service is distinct. The Company then recognizes as revenue the amount of the transaction price that is allocated to respective performance obligations when (or as) the respective performance obligations are satisfied. The Company evaluates its contracts for the presence of significant financing components. If a significant financing component is identified in a contract and provides a financing benefit to the customer, the transaction price for the contract is adjusted to account for the financing portion of the arrangement, which is recognized as interest income over the financing term using the effective interest method. In determining the appropriate interest rates for significant financing components, the Company evaluates the credit profile of the customer and prevailing market interest rates and selects an interest rate in which it believes would be charged to the customer in a separate financing arrangement over a similar financing term. License revenue The Company licenses its NAV Technology Platform to other biotechnology and pharmaceutical companies. The terms of the licenses vary, however licenses may be exclusive or non-exclusive and may be sublicensable by the licensee. Licenses may grant intellectual property rights for purposes of internal and preclinical research and development only, or may include the rights, or options to obtain future rights, to commercialize drug therapies for specific diseases using the Company’s NAV Technology Platform. License agreements generally have a term at least equal to the life of the underlying patents, but are terminable at the option of the licensee. Consideration to the Company under its license agreements may include: (i) up-front fees, (ii) option fees to obtain additional licenses, (iii) annual maintenance fees, (iv) milestone payments based on the achievement of certain development and sales-based milestones by licensees, (v) sublicense fees and (vi) royalties on sales of licensed products. The Company has determined that all of its license agreements are contracts with customers within the scope of Topic 606. Although licenses are terminable at the option of licensee, the Company has determined that there is a substantive termination penalty associated with the termination of each license. Due to the substantive termination penalty, the contract term for purposes of applying Topic 606 is equal to the stated term of the license agreement, which is the life of the underlying licensed patents. The Company’s performance obligations under its license agreements include the delivery of intellectual property licenses to licensees as well as options granted to licensees to acquire future licenses to the extent the options represent material rights to the licensee. The transaction price for each license agreement is allocated to these performance obligations and recognized as revenue when the performance obligations are satisfied. Consideration allocated to performance obligations for the delivery of intellectual property licenses is recognized as revenue upon the delivery of the license(s) to the licensee, which generally occurs upon the execution of the license agreement. Consideration allocated to performance obligations for license options is recognized as revenue upon the earlier of the option exercise or expiration. For license agreements which contain options for the licensee to purchase additional licenses in the future, the Company evaluates the options at the inception of the agreement to determine if they provide a material right to the licensee. In making this determination, the Company considers whether the optional licenses are priced at a discount to the standalone selling price for the licenses. For options granted which are deemed to be material rights to the licensee, the Company allocates a portion of the transaction price to the performance obligation for the option and recognizes that consideration as revenue at the earlier of option exercise or expiration. Options which are not material rights to licensees are not considered performance obligations and are not accounted for as part of the license agreement until exercised by the licensee. Consideration contingent upon the exercise of options by licensees is excluded from the transaction price and not accounted for as part of the license agreement until the option is exercised. Upon the exercise of an option by a licensee, the additional consideration related to the option exercise is added to the transaction price and recognized as revenue upon the delivery of the newly purchased license. The Company evaluates the transaction price for its license agreements at each reporting date. The transaction price for each license includes all fixed consideration, as well as variable consideration to the extent that it is probable that a significant reversal of revenue will not occur in the future. Fixed consideration under the Company’s license agreements includes up-front fees and annual maintenance fees. Variable consideration under the Company’s license agreements includes development and sales-based milestone payments, sublicense fees and royalties on sales of licensed products. Up-front license fees are included in the transaction price and recognized as revenue upon the delivery of the license. If up-front license fees are payable to Company in periods beyond 12 months from the delivery of the license, a significant financing component is deemed to exist and the Company adjusts the transaction price to include only the present value of the license fees. The discounted portion of the license fees is recognized as interest income in the consolidated statements of operations over the term of the financing period. Annual maintenance fees are generally payable to the Company on each anniversary date over the term of the license agreement. The Company has determined that the payment of annual maintenance fees by licensees in future periods represents a significant financing component to the license since the delivery of the license occurs at the inception of the agreement. The present value of aggregate annual maintenance fees payable to the Company over the term of the license is included in the transaction price and recognized as revenue upon the delivery of the license. The discounted portion of the annual maintenance fees is recognized as interest income in the consolidated statements of operations over the term of the license. Development milestone payments are payable to the Company upon the achievement of specified development milestones by licensees. At the inception of each license agreement that contains development milestone payments, the Company evaluates whether the milestones are considered probable of achievement and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal will not occur in the future, milestone payments are included in the transaction price and recognized as revenue upon the delivery of the license. Milestone payments contingent on the achievement of development milestones that are not within the control of the Company or the licensee, such as regulatory approvals, are not considered probable of being achieved and are excluded from the transaction price until the milestone is achieved. At each reporting date, the Company re-evaluates the probability of achievement of outstanding development milestones and, if necessary, adjusts the transaction price for any milestones for which the probability of achievement has changed due to current facts and circumstances. Any such adjustments are recorded on a cumulative catch-up basis and recorded as license revenue in the period of the adjustment. Royalties on sales of licensed products, sales-based milestone payments and sublicense fees based on the receipt of certain fees by licensees from any sublicensees are excluded from the transaction price for each license and recognized as revenue in the period that the related sales or sublicenses occur, provided that the associated license has been delivered to the licensee. To date the Company has not recognized any revenue from royalties on sales of licensed products, the achievement of sales-based milestones or sublicense fees. The Company receives payments from licensees based on the billing schedules established in each license agreement. Amounts recognized as revenue which have not yet been received from licensees are recorded as accounts receivable when the Company’s rights to the consideration are conditional solely upon the passage of time. Amounts recognized as revenue which have not yet been received from licensees are recorded as contract assets when the Company’s rights to the consideration are not unconditional. Contract assets are recorded as other current assets on the consolidated balance sheets. If a licensee elects to terminate a license prior to the end of the license term, the licensed intellectual property is returned to the Company and any consideration recorded as accounts receivable or contract assets which is not contractually payable by the licensee is charged off as a reduction of license revenue in the period of the termination. Amounts received by the Company prior to the delivery of underlying performance obligations are deferred and recognized as revenue upon the satisfaction of the performance obligations by the Company.
Cost of Revenues	Costs of Revenues Licensing costs consist of sublicense fees incurred by the Company to its licensors as a result of license revenues generated by the Company. Sublicense fees are based on a percentage of license fees received by the Company from licensees as specified in the Company’s agreements with its licensors. The Company recognizes sublicense fees in the period that the underlying license revenue is recognized. Sublicense fees payable by the Company to licensors in periods beyond 12 months from the reporting are recorded as non-current liabilities on the consolidated balance sheets.
Net Income (Loss) Per Share	Net Income (Loss) Per Share Basic net income (loss) per share is calculated by dividing net income (loss) applicable to common stockholders by the weighted-average common shares outstanding during the period, without consideration for common stock equivalents. Diluted net income (loss) per share is calculated by adjusting the weighted-average common shares outstanding for the dilutive effect of common stock equivalents outstanding for the period, determined using the treasury-stock method. Contingently convertible shares in which conversion is based on non-market-priced contingencies are excluded from the calculations of both basic and diluted net income (loss) per share until the contingency has been fully met. For purposes of the diluted net income (loss) per share calculation, common stock equivalents are excluded from the calculation of diluted net income (loss) per share if their effect would be anti-dilutive.
Recent Accounting Pronouncements	Recent Accounting Pronouncements Adoption of ASU 2014-09, Revenue from Contracts with Customers In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers Revenue Recognition The Company recorded a net reduction in opening accumulated deficit of $4.8 million as of January 1, 2018 for the cumulative impact of adoption of Topic 606, which was primarily the result of accelerated recognition of license revenue due to annual maintenance fees under Topic 606. Under Topic 605, annual maintenance fees payable to the Company by licensees were recognized as license revenue annually when the amounts became fixed or determinable. Under Topic 606, the present value of aggregate annual maintenance fees over the term of the license agreement are recognized as revenue upon the delivery of the license to the licensee. The impact of the accelerated recognition of license revenue upon adoption was partially offset by the accelerated recognition of licensing costs to the Company’s licensors. The Company recognizes sublicense fees to its licensors in the period the underlying license revenue is recognized. The cumulative adjustment for the adoption of Topic 606 had the following effects on the Company’s consolidated balance sheet as of January 1, 2018 (in thousands): Cumulative Adjustment for Balance at Adoption of Balance at December 31, 2017 Topic 606 January 1, 2018 Consolidated Balance Sheet Assets: Accounts receivable, current $ 473 $ 527 $ 1,000 Accounts receivable, non-current $ — $ 4,850 $ 4,850 Other current assets $ 1,412 $ 350 $ 1,762 Liabilities: Accrued expenses and other current liabilities $ 9,605 $ 105 $ 9,710 Other liabilities $ — $ 819 $ 819 Stockholdersʼ Equity: Accumulated deficit $ (187,756 ) $ 4,803 $ (182,953 ) The following tables present the effects of the adoption of Topic 606 on each financial statement line item of the Company’s financial statements for the interim periods ended June 30, 2018 (in thousands, except per share data): As of June 30, 2018 Impact of Results Without Adoption of Adoption of As Reported Topic 606 Topic 606 Consolidated Balance Sheet Assets: Accounts receivable, current $ 739 $ 567 $ 172 Accounts receivable, non-current $ 4,485 $ 4,485 $ — Prepaid expenses $ 3,690 $ 60 $ 3,630 Liabilities: Accrued expenses and other current liabilities $ 12,023 $ 100 $ 11,923 Deferred revenue $ 600 $ 600 $ — Other liabilities $ 720 $ 720 $ — Stockholdersʼ Equity: Accumulated deficit $ (68,120 ) $ 3,692 $ (71,812 ) Three Months Ended June 30, 2018 Six Months Ended June 30, 2018 Impact of Results Without Impact of Results Without Adoption of Adoption of Adoption of Adoption of As Reported Topic 606 Topic 606 As Reported Topic 606 Topic 606 Consolidated Statement of Operations Revenues: License revenue $ 40,031 $ (61,244 ) $ 101,275 $ 172,422 $ (9,529 ) $ 181,951 Expenses: Licensing costs $ 3,872 $ (1,220 ) $ 5,092 $ 6,280 $ (165 ) $ 6,445 Other Income: Interest income from licensing $ 6,898 $ 6,898 $ — $ 8,253 $ 8,253 $ — Net Income $ 10,594 $ (53,126 ) $ 63,720 $ 114,833 $ (1,111 ) $ 115,944 Net Income Per Share: Basic $ 0.33 $ (1.66 ) $ 1.99 $ 3.60 $ (0.04 ) $ 3.64 Diluted $ 0.30 $ (1.51 ) $ 1.81 $ 3.29 $ (0.03 ) $ 3.32 Six Months Ended June 30, 2018 Impact of Results Without Adoption of Adoption of As Reported Topic 606 Topic 606 Consolidated Statement of Cash Flows Cash Flows from Operating Activities: Net income $ 114,833 $ (1,111 ) $ 115,944 Imputed interest income from licensing $ (8,253 ) $ (8,253 ) $ — Changes in accounts receivable $ 8,879 $ 8,578 $ 301 Changes in prepaid expenses $ 1,644 $ (60 ) $ 1,704 Changes in other current assets $ (585 ) $ 350 $ (935 ) Changes in accrued expenses and other current liabilities $ 2,566 $ (5 ) $ 2,571 Changes in deferred revenue $ 600 $ 600 $ — Changes in other liabilities $ (99 ) $ (99 ) $ — The most significant effect that the adoption of Topic 606 had on the results of operations for the three and six months ended June 30, 2018, as compared to what results would have been if Topic 605 had continued to be applied, is related to the amount of revenue and interest income from licensing recognized under the Company’s January 2018 amendment to its license agreement with AveXis, Inc. (AveXis) for the development and commercialization of treatments for spinal muscular atrophy (SMA). Under Topic 606, the Company recognized the present value of all fixed consideration under the amendment as revenue upon the delivery of the license to AveXis in January 2018, including the present value of the two $30.0 million payments originally due to the Company in January 2019 and January 2020. The present value discount, which represents the financing portion of the consideration under Topic 606, was recognized as interest income from licensing over the financing term of the agreement. Under Topic 605, the Company would not have recognized such revenue until it became fixed and determinable and collectability was reasonably assured, and the Company would not have recognized any interest income from significant financing components under the license agreement. Under the requirements of Topic 606, the Company recognized license revenue of $40.0 million and $172.1 million, and interest income from licensing of $6.8 million and $8.0 million, during the three and six months ended June 30, 2018, respectively, related to its amended license agreement with AveXis. If the requirements of Topic 605 had been applied during the three and six months ended June 30, 2018, the Company would have recognized license revenue of $100.0 million and $180.0 million, respectively, and interest income from licensing of $0, related to its amended license agreement with AveXis. Please refer to Note 7 for further information on license revenue and the Company’s accounting analysis for the amended license with AveXis. Other recently adopted accounting pronouncements In May 2017, the FASB issued ASU 2017-09, Compensation—Stock Compensation (Topic 718): Scope of Modification Accounting In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash The Company’s restricted cash includes money market mutual funds used to collateralize an irrevocable letter of credit as required by the Company’s lease agreement for its office space in New York, New York. The following table provides a reconciliation of cash and cash equivalents and restricted cash as reported on the consolidated balance sheets to the total of these amounts as reported at the end of the period in the consolidated statements of cash flows (in thousands): June 30, 2018 June 30, 2017 Cash and cash equivalents $ 106,889 $ 57,649 Restricted cash 225 225 Total cash and cash equivalents and restricted cash $ 107,114 $ 57,874 In January 2016, the FASB issued ASU 2016-01, Financial Instruments—Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities, Technical Corrections and Improvements to Financial Instruments—Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities Recent accounting pronouncements not yet adopted In June 2018, the FASB issued ASU 2018-07, Compensation—Stock Compensation (Topic 718): Improvements to Nonemployee Share-based Payment Accounting Equity—Equity-based Payments to Nonemployees. In February 2018, the FASB issued ASU 2018-02, Income Statement—Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income Tax Cuts and Jobs Act of 2017 (the TCJA) In April 2017, the FASB issued ASU 2017-08, Receivables—Nonrefundable Fees and Other Costs (Subtopic 310-20) In June 2016, the FASB issued ASU 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) Leases (Topic 840) Codification Improvements to Topic 842, Leases

Summary of Significant Accoun19

Summary of Significant Accounting Policies (Tables)	6 Months Ended
	Jun. 30, 2018
Summary of Reconciliation of Cash and Cash Equivalents and Restricted Cash	The Company’s restricted cash includes money market mutual funds used to collateralize an irrevocable letter of credit as required by the Company’s lease agreement for its office space in New York, New York. The following table provides a reconciliation of cash and cash equivalents and restricted cash as reported on the consolidated balance sheets to the total of these amounts as reported at the end of the period in the consolidated statements of cash flows (in thousands): June 30, 2018 June 30, 2017 Cash and cash equivalents $ 106,889 $ 57,649 Restricted cash 225 225 Total cash and cash equivalents and restricted cash $ 107,114 $ 57,874
ASU 2014-09 [Member]
Schedule of Impact on Financial Statements on Adoption of Topic 606	The cumulative adjustment for the adoption of Topic 606 had the following effects on the Company’s consolidated balance sheet as of January 1, 2018 (in thousands): Cumulative Adjustment for Balance at Adoption of Balance at December 31, 2017 Topic 606 January 1, 2018 Consolidated Balance Sheet Assets: Accounts receivable, current $ 473 $ 527 $ 1,000 Accounts receivable, non-current $ — $ 4,850 $ 4,850 Other current assets $ 1,412 $ 350 $ 1,762 Liabilities: Accrued expenses and other current liabilities $ 9,605 $ 105 $ 9,710 Other liabilities $ — $ 819 $ 819 Stockholdersʼ Equity: Accumulated deficit $ (187,756 ) $ 4,803 $ (182,953 ) The following tables present the effects of the adoption of Topic 606 on each financial statement line item of the Company’s financial statements for the interim periods ended June 30, 2018 (in thousands, except per share data): As of June 30, 2018 Impact of Results Without Adoption of Adoption of As Reported Topic 606 Topic 606 Consolidated Balance Sheet Assets: Accounts receivable, current $ 739 $ 567 $ 172 Accounts receivable, non-current $ 4,485 $ 4,485 $ — Prepaid expenses $ 3,690 $ 60 $ 3,630 Liabilities: Accrued expenses and other current liabilities $ 12,023 $ 100 $ 11,923 Deferred revenue $ 600 $ 600 $ — Other liabilities $ 720 $ 720 $ — Stockholdersʼ Equity: Accumulated deficit $ (68,120 ) $ 3,692 $ (71,812 ) Three Months Ended June 30, 2018 Six Months Ended June 30, 2018 Impact of Results Without Impact of Results Without Adoption of Adoption of Adoption of Adoption of As Reported Topic 606 Topic 606 As Reported Topic 606 Topic 606 Consolidated Statement of Operations Revenues: License revenue $ 40,031 $ (61,244 ) $ 101,275 $ 172,422 $ (9,529 ) $ 181,951 Expenses: Licensing costs $ 3,872 $ (1,220 ) $ 5,092 $ 6,280 $ (165 ) $ 6,445 Other Income: Interest income from licensing $ 6,898 $ 6,898 $ — $ 8,253 $ 8,253 $ — Net Income $ 10,594 $ (53,126 ) $ 63,720 $ 114,833 $ (1,111 ) $ 115,944 Net Income Per Share: Basic $ 0.33 $ (1.66 ) $ 1.99 $ 3.60 $ (0.04 ) $ 3.64 Diluted $ 0.30 $ (1.51 ) $ 1.81 $ 3.29 $ (0.03 ) $ 3.32 Six Months Ended June 30, 2018 Impact of Results Without Adoption of Adoption of As Reported Topic 606 Topic 606 Consolidated Statement of Cash Flows Cash Flows from Operating Activities: Net income $ 114,833 $ (1,111 ) $ 115,944 Imputed interest income from licensing $ (8,253 ) $ (8,253 ) $ — Changes in accounts receivable $ 8,879 $ 8,578 $ 301 Changes in prepaid expenses $ 1,644 $ (60 ) $ 1,704 Changes in other current assets $ (585 ) $ 350 $ (935 ) Changes in accrued expenses and other current liabilities $ 2,566 $ (5 ) $ 2,571 Changes in deferred revenue $ 600 $ 600 $ — Changes in other liabilities $ (99 ) $ (99 ) $ —

Marketable Securities (Tables)

Marketable Securities (Tables)	6 Months Ended
	Jun. 30, 2018
Investments Debt And Equity Securities [Abstract]
Summary of Company Marketable Securities	The following tables present a summary of the Company’s marketable securities, which consist solely of available-for-sale debt securities (in thousands): Amortized Cost Unrealized Gains Unrealized Losses Fair Value June 30, 2018 U.S. government and federal agency securities $ 19,280 $ — $ (14 ) $ 19,266 Certificates of deposit 735 — — 735 Corporate bonds 179,721 3 (325 ) 179,399 $ 199,736 $ 3 $ (339 ) $ 199,400 Amortized Cost Unrealized Gains Unrealized Losses Fair Value December 31, 2017 Corporate bonds $ 130,018 $ 2 $ (282 ) $ 129,738 $ 130,018 $ 2 $ (282 ) $ 129,738
Summary of Fair Values and Unrealized Losses of Marketable Securities Held by the Company in an Unrealized Loss Position for Less Than 12 months and 12 Months or Greater	The following tables present the fair values and unrealized losses of marketable securities held by the Company in an unrealized loss position for less than 12 months and 12 months or greater (in thousands): Less than 12 Months 12 Months or Greater Total Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized Losses June 30, 2018 U.S. government and federal agency securities $ 19,266 $ (14 ) $ — $ — $ 19,266 $ (14 ) Corporate bonds 143,503 (239 ) 21,066 (86 ) 164,569 (325 ) $ 162,769 $ (253 ) $ 21,066 $ (86 ) $ 183,835 $ (339 ) Less than 12 Months 12 Months or Greater Total Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized Losses December 31, 2017 Corporate bonds $ 109,238 $ (180 ) $ 17,124 $ (102 ) $ 126,362 $ (282 ) $ 109,238 $ (180 ) $ 17,124 $ (102 ) $ 126,362 $ (282 )

Fair Value of Financial Instr21

Fair Value of Financial Instruments (Tables)	6 Months Ended
	Jun. 30, 2018
Fair Value Disclosures [Abstract]
Schedule of Fair Value of Cash Equivalents and Marketable Securities	The following tables present the fair value of cash equivalents and marketable securities in accordance with the hierarchy discussed in Note 2 (in thousands): Quoted Significant prices other Significant in active observable unobservable markets inputs inputs (Level 1) (Level 2) (Level 3) Total June 30, 2018 Money market mutual funds (cash equivalents) $ — $ 106,867 $ — $ 106,867 U.S. government and federal agency securities (marketable securities) — 19,266 — 19,266 Certificates of deposit (marketable securities) — 735 — 735 Corporate bonds (marketable securities) — 179,399 — 179,399 $ — $ 306,267 $ — $ 306,267 Quoted Significant prices other Significant in active observable unobservable markets inputs inputs (Level 1) (Level 2) (Level 3) Total December 31, 2017 Money market mutual funds (cash equivalents) $ — $ 46,646 $ — $ 46,646 Corporate bonds (marketable securities) — 129,738 — 129,738 $ — $ 176,384 $ — $ 176,384

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	6 Months Ended
	Jun. 30, 2018
Property Plant And Equipment [Abstract]
Schedule of Property and Equipment	Property and equipment, net consists of the following (in thousands): June 30, 2018 December 31, 2017 Computer equipment and software $ 1,814 $ 1,481 Lab equipment 9,496 8,561 Furniture and fixtures 1,695 1,384 Leasehold improvements 8,680 5,828 Total property and equipment 21,685 17,254 Accumulated depreciation and amortization (4,987 ) (3,277 ) Property and equipment, net $ 16,698 $ 13,977

Commitments and Contingencies (

Commitments and Contingencies (Tables)	6 Months Ended
	Jun. 30, 2018
Schedule of Future Minimum Lease Payments Under Non-Cancelable Operating Leases	As of June 30, 2018, future minimum lease payments under non-cancelable operating leases are as follows (in thousands): Operating Leases 2018 (remainder of year) $ 1,193 2019 2,395 2020 2,411 2021 1,797 2022 621 Thereafter 479 Total minimum lease payments $ 8,896
The Trustees of the University of Pennsylvania [Member]
Summary of Expenses Incurred by Company	Expenses incurred by the Company related to its license from Penn were as follows (in thousands): Three Months Ended June 30, Six Months Ended June 30, 2018 2017 2018 2017 Sublicense fees $ — $ 656 $ — $ 701 Royalties on sales of reagents — 4 — 4 Maintenance of licensed patents 98 85 117 169 $ 98 $ 745 $ 117 $ 874
GlaxoSmithKline LLC [Member]
Summary of Expenses Incurred by Company	Expenses incurred by the Company related to its license from GSK were as follows (in thousands): Three Months Ended June 30, Six Months Ended June 30, 2018 2017 2018 2017 Sublicense fees $ 3,998 $ 656 $ 6,030 $ 701 Royalties on sales of reagents — 2 — 2 Maintenance of licensed patents 97 14 490 159 $ 4,095 $ 672 $ 6,520 $ 862
Regents of the University of Minnesota [Member]
Summary of Expenses Incurred by Company	Expenses incurred by the Company related to its license from Minnesota were as follows (in thousands): Three Months Ended June 30, Six Months Ended June 30, 2018 2017 2018 2017 Sublicense fees $ (125 ) $ — $ 250 $ — Maintenance of licensed patents 130 11 142 16 $ 5 $ 11 $ 392 $ 16

License Revenue (Tables)

License Revenue (Tables)	6 Months Ended
	Jun. 30, 2018
License Agreement Revenue Recognition [Abstract]
Summary of Changes in Balances of Receivables, Contract Assets and Contract Liabilities	The following table presents changes in the balances of the Company’s receivables, contract assets and contract liabilities during the periods presented (in thousands): Balance at Beginning Net Additions Balance at of Period (Deductions) End of Period Three Months Ended June 30, 2018 Receivables and contract assets: Accounts receivable, current and non-current $ 58,621 $ (53,397 ) $ 5,224 Contract assets $ 250 $ (250 ) $ — Contract liabilities: Deferred revenue $ — $ 600 $ 600 Balance at Beginning Net Additions Balance at of Period (Deductions) End of Period Six Months Ended June 30, 2018 Receivables and contract assets: Accounts receivable, current and non-current $ 5,850 $ (626 ) $ 5,224 Contract assets $ 350 $ (350 ) $ — Contract liabilities: Deferred revenue $ — $ 600 $ 600

Stock-based Compensation (Table

Stock-based Compensation (Tables)	6 Months Ended
	Jun. 30, 2018
Stock-Based Compensation Expense by Award Type	The Company’s stock-based compensation expense by award type is as follows (in thousands): Three Months Ended June 30, Six Months Ended June 30, 2018 2017 2018 2017 Stock options $ 3,818 $ 2,332 $ 6,939 $ 4,771 Restricted stock units 69 69 136 138 Employee stock purchase plan 95 82 197 165 $ 3,982 $ 2,483 $ 7,272 $ 5,074
Stock-Based Compensation Expense	The Company has recorded aggregate stock-based compensation expense in the consolidated statements of operations and comprehensive income (loss) as follows (in thousands): Three Months Ended June 30, Six Months Ended June 30, 2018 2017 2018 2017 Research and development $ 1,895 $ 1,137 $ 3,424 $ 2,374 General and administrative 2,087 1,346 3,848 2,700 $ 3,982 $ 2,483 $ 7,272 $ 5,074
Summary of Unvested RSUs Activity Under 2015 Plan	The following table summarizes restricted stock unit activity under the 2015 Plan (in thousands, except per share data): Weighted- average Grant Date Shares Fair Value Unvested balance at December 31, 2017 40 $ 20.90 Granted — $ — Vested — $ — Forfeited — $ — Unvested balance at June 30, 2018 40 $ 20.90
2014 and 2015 Equity Incentive Plan [Member]
Summary of Stock Option Activity	The following table summarizes stock option activity under the 2014 Plan and 2015 Plan (in thousands, except per share data): Weighted- average Weighted- Remaining average Contractual Aggregate Exercise Life Intrinsic Shares Price (Years) Value (a) Outstanding at December 31, 2017 5,468 $ 10.25 7.9 $ 125,738 Granted 1,137 $ 34.39 Exercised (961 ) $ 7.32 Cancelled or forfeited (199 ) $ 15.88 Outstanding at June 30, 2018 5,445 $ 15.61 7.9 $ 305,721 Exercisable at June 30, 2018 2,748 $ 7.88 7.1 $ 175,499 Vested and expected to vest at June 30, 2018 5,424 $ 15.66 7.9 $ 304,215 (a) The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the fair value of the common stock for the options that were in the money at the dates reported.

Net Income (Loss) Per Share (Ta

Net Income (Loss) Per Share (Tables)	6 Months Ended
	Jun. 30, 2018
Earnings Per Share [Abstract]
Computations of Basic and Diluted Net Income (Loss) Per Share	The computations of basic and diluted net income (loss) per share are as follows (in thousands, except per share data): Three Months Ended June 30, Six Months Ended June 30, 2018 2017 2018 2017 Basic net income (loss) per share: Net income (loss) applicable to common stockholders $ 10,594 $ (14,473 ) $ 114,833 $ (36,466 ) Shares used in computation: Weighted-average common shares outstanding 32,082 30,662 31,858 28,678 Basic net income (loss) per share $ 0.33 $ (0.47 ) $ 3.60 $ (1.27 ) Diluted net income (loss) per share: Net income (loss) applicable to common stockholders $ 10,594 $ (14,473 ) $ 114,833 $ (36,466 ) Shares used in computation: Weighted-average common shares outstanding 32,082 30,662 31,858 28,678 Stock options 3,153 — 2,995 — Restricted stock units 30 — 27 — Employee stock purchase plan 7 — 4 — Weighted-average diluted common shares 35,272 30,662 34,884 28,678 Diluted net income (loss) per share $ 0.30 $ (0.47 ) $ 3.29 $ (1.27 )
Schedules for Computation of Diluted Weighted-Average Shares Outstanding	The following potentially dilutive common stock equivalents outstanding at the end of the period were excluded from the computations of weighted-average diluted common shares for the periods indicated as their effects would be anti-dilutive (in thousands): Three Months Ended June 30, Six Months Ended June 30, 2018 2017 2018 2017 Stock options issued and outstanding 804 5,835 1,236 5,835 Unvested restricted stock units outstanding — 40 — 40 Employee stock purchase plan — 26 — 26 804 5,901 1,236 5,901

Supplemental Disclosures (Table

Supplemental Disclosures (Tables)	6 Months Ended
	Jun. 30, 2018
Payables And Accruals [Abstract]
Schedules of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consists of the following (in thousands): June 30, 2018 December 31, 2017 Accrued personnel costs $ 4,902 $ 5,789 Accrued external research and development expenses 2,602 2,072 Accrued income taxes payable 2,439 — Accrued external general and administrative expenses 1,203 1,078 Accrued licensing costs 350 — Accrued purchases of property and equipment 139 430 Other accrued expenses and current liabilities 388 236 $ 12,023 $ 9,605

Nature of Business - Additional

Nature of Business - Additional Information (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Variable Interests And Equity Method Investments Disclosure [Abstract]
Accumulated deficit	$ (68,120)	$ (187,756)
Cash, cash equivalents and marketable securities	$ 306,300

Summary of Significant Accoun29

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Significant Accounting Policies [Line Items]
Percentage of ownership interest	20.00%		20.00%
Accumulated deficit	$ (68,120)		$ (68,120)			$ (187,756)
License revenue	40,031	$ 6,562	172,422	$ 7,017
Interest income from licensing	6,898		8,253
License [Member]
Significant Accounting Policies [Line Items]
License revenue	40,031	$ 6,555	172,422	$ 7,010
ASU 2014-09 [Member]
Significant Accounting Policies [Line Items]
Accumulated deficit					$ (182,953)
ASU 2014-09 [Member] | AveXis, Inc. [Member]
Significant Accounting Policies [Line Items]
Payments due from related party	30,000		$ 30,000
Consideration payment due period one			2019-01
Consideration payment due period two			2020-01
Interest income from licensing	6,800		$ 8,000
ASU 2014-09 [Member] | AveXis, Inc. [Member] | License [Member]
Significant Accounting Policies [Line Items]
License revenue	40,000		172,100
Reduction in Accumulated Deficit [Member] | ASU 2014-09 [Member]
Significant Accounting Policies [Line Items]
Accumulated deficit	3,692		3,692		$ 4,803
Reduction in Accumulated Deficit [Member] | ASU 2014-09 [Member] | License [Member]
Significant Accounting Policies [Line Items]
License revenue	(61,244)		(9,529)
Results Without Adoption of Topic 606 | ASU 2014-09 [Member]
Significant Accounting Policies [Line Items]
Accumulated deficit	(71,812)		(71,812)
Results Without Adoption of Topic 606 | ASU 2014-09 [Member] | License [Member]
Significant Accounting Policies [Line Items]
License revenue	101,275		181,951
Results Without Adoption of Topic 606 | ASU 2014-09 [Member] | AveXis, Inc. [Member]
Significant Accounting Policies [Line Items]
Interest income from licensing	0		0
Results Without Adoption of Topic 606 | ASU 2014-09 [Member] | AveXis, Inc. [Member] | License [Member]
Significant Accounting Policies [Line Items]
License revenue	$ 100,000		$ 180,000

Summary of Significant Accoun30

Summary of Significant Accounting Policies - Schedule of Impact on Consolidated Balance Sheet on Adoption of Topic 606 (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Assets
Accounts receivable, current	$ 739		$ 473
Accounts receivable, non-current	4,485
Other current assets	2,347		1,412
Prepaid expenses	3,690		5,334
Liabilities:
Accrued expenses and other current liabilities	12,023		9,605
Deferred revenue	600
Other liabilities	720
Stockholders’ equity
Accumulated deficit	(68,120)		$ (187,756)
ASU 2014-09 [Member]
Assets
Accounts receivable, current		$ 1,000
Accounts receivable, non-current		4,850
Other current assets		1,762
Liabilities:
Accrued expenses and other current liabilities		9,710
Other liabilities		819
Stockholders’ equity
Accumulated deficit		(182,953)
Cumulative Adjustment for Adoption of Topic 606 [Member] | ASU 2014-09 [Member]
Assets
Accounts receivable, current	567	527
Accounts receivable, non-current	4,485	4,850
Other current assets		350
Prepaid expenses	60
Liabilities:
Accrued expenses and other current liabilities	100	105
Deferred revenue	600
Other liabilities	720	819
Stockholders’ equity
Accumulated deficit	3,692	$ 4,803
Results Without Adoption of Topic 606 | ASU 2014-09 [Member]
Assets
Accounts receivable, current	172
Prepaid expenses	3,630
Liabilities:
Accrued expenses and other current liabilities	11,923
Stockholders’ equity
Accumulated deficit	$ (71,812)

Summary of Significant Accoun31

Summary of Significant Accounting Policies - Schedule of Impact on Consolidated Statement of Operations on Adoption of Topic 606 (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenues
License revenue	$ 40,031	$ 6,562	$ 172,422	$ 7,017
Other Income
Interest income from licensing	6,898		8,253
Net Income	$ 10,594	$ (14,473)	$ 114,833	$ (36,466)
Net income (loss) per share:
Basic	$ 0.33	$ (0.47)	$ 3.60	$ (1.27)
Diluted	$ 0.30	$ (0.47)	$ 3.29	$ (1.27)
License [Member]
Revenues
License revenue	$ 40,031	$ 6,555	$ 172,422	$ 7,010
Expenses
Licensing costs	3,872	$ 1,311	6,280	$ 1,402
Cumulative Adjustment for Adoption of Topic 606 [Member] | ASU 2014-09 [Member]
Other Income
Interest income from licensing	6,898		8,253
Net Income	$ (53,126)		$ (1,111)
Net income (loss) per share:
Basic	$ (1.66)		$ (0.04)
Diluted	$ (1.51)		$ (0.03)
Cumulative Adjustment for Adoption of Topic 606 [Member] | License [Member] | ASU 2014-09 [Member]
Revenues
License revenue	$ (61,244)		$ (9,529)
Expenses
Licensing costs	(1,220)		(165)
Results Without Adoption of Topic 606 | ASU 2014-09 [Member]
Other Income
Net Income	$ 63,720		$ 115,944
Net income (loss) per share:
Basic	$ 1.99		$ 3.64
Diluted	$ 1.81		$ 3.32
Results Without Adoption of Topic 606 | License [Member] | ASU 2014-09 [Member]
Revenues
License revenue	$ 101,275		$ 181,951
Expenses
Licensing costs	$ 5,092		$ 6,445

Summary of Significant Accoun32

Summary of Significant Accounting Policies - Schedule of Impact on Consolidated Statement of Cash Flows on Adoption of Topic 606 (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Cash Flows from Operating Activities:
Net income	$ 114,833	$ (36,466)
Imputed interest income from licensing	(8,253)
Changes in accounts receivable	8,879	982
Changes in prepaid expenses	1,644	(657)
Changes in other current assets	(585)	(242)
Changes in accrued expenses and other current liabilities	2,566	$ (31)
Changes in deferred revenue	600
Changes in other liabilities	(99)
Cumulative Adjustment for Adoption of Topic 606 [Member] | ASU 2014-09 [Member]
Cash Flows from Operating Activities:
Net income	(1,111)
Imputed interest income from licensing	(8,253)
Changes in accounts receivable	8,578
Changes in prepaid expenses	(60)
Changes in other current assets	350
Changes in accrued expenses and other current liabilities	(5)
Changes in deferred revenue	600
Changes in other liabilities	(99)
Results Without Adoption of Topic 606 | ASU 2014-09 [Member]
Cash Flows from Operating Activities:
Net income	115,944
Changes in accounts receivable	301
Changes in prepaid expenses	1,704
Changes in other current assets	(935)
Changes in accrued expenses and other current liabilities	$ 2,571

Summary of Significant Accoun33

Summary of Significant Accounting Policies - Summary of Reconciliation of Cash and Cash Equivalents and Restricted Cash (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Accounting Policies [Abstract]
Cash and cash equivalents	$ 106,889	$ 46,656	$ 57,649
Restricted cash	225	$ 225	225
Total cash and cash equivalents and restricted cash	$ 107,114		$ 57,874

Marketable Securities - Summary

Marketable Securities - Summary of Company Marketable Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 199,736	$ 130,018
Unrealized Gains	3	2
Unrealized Losses	(339)	(282)
Fair Value	199,400	129,738
U.S. Government and Federal Agency Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	19,280
Unrealized Losses	(14)
Fair Value	19,266
Certificates of Deposit [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	735
Fair Value	735
Corporate Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	179,721	130,018
Unrealized Gains	3	2
Unrealized Losses	(325)	(282)
Fair Value	$ 179,399	$ 129,738

Marketable Securities - Additio

Marketable Securities - Additional Information (Detail)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018USD ($)Security	Jun. 30, 2017USD ($)	Jun. 30, 2018USD ($)Security	Jun. 30, 2017USD ($)	Dec. 31, 2017USD ($)
Schedule Of Available For Sale Securities [Line Items]
Available for sale securities remaining maturities greater than three years	$ 0		$ 0		$ 0
Unrealized gains (losses) on available-for-sale securities, before tax	100,000	$ (100,000)	(100,000)	$ (100,000)
Unrealized gains (losses) on available-for-sale securities, income tax expense	$ 0	0	$ 0	0
Number of investment grade fixed income security | Security	62		62
Other-than-temporary impaired			$ 0		$ 0
Debt Securities [Member] | Investment Income [Member]
Schedule Of Available For Sale Securities [Line Items]
Net realized gains or losses available for sale securities	$ 0	$ 0	$ 0	$ 500,000

Marketable Securities - Summa36

Marketable Securities - Summary of Fair Values and Unrealized Losses of Marketable Securities Held by the Company in an Unrealized Loss Position for Less Than 12 months and 12 Months or Greater (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Less than 12 Months, Fair Value	$ 162,769
Less than 12 Months, Unrealized Losses	(253)	$ (180)
12 Months or Greater, Fair Value	21,066
12 Months or Greater, Unrealized Losses	(86)	(102)
Total, Fair Value	183,835
Total, Unrealized Losses	(339)	(282)
Less than 12 Months, Fair Value		109,238
12 Months or Greater, Fair Value		17,124
Total, Fair Value		126,362
U.S. Government and Federal Agency Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Less than 12 Months, Fair Value	19,266
Less than 12 Months, Unrealized Losses	(14)
Total, Fair Value	19,266
Total, Unrealized Losses	(14)
Corporate Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Less than 12 Months, Fair Value	143,503
Less than 12 Months, Unrealized Losses	(239)	(180)
12 Months or Greater, Fair Value	21,066
12 Months or Greater, Unrealized Losses	(86)	(102)
Total, Fair Value	164,569
Total, Unrealized Losses	$ (325)	(282)
Less than 12 Months, Fair Value		109,238
12 Months or Greater, Fair Value		17,124
Total, Fair Value		$ 126,362

Fair Value of Financial Instr37

Fair Value of Financial Instruments - Schedule of Fair Value of Cash Equivalents and Marketable Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Available for sale securities	$ 199,400	$ 129,738
Assets fair value disclosure	306,267	176,384
Corporate Bonds [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Available for sale securities	179,399	129,738
U.S. Government and Federal Agency Securities [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Available for sale securities	19,266
Certificates of Deposit [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Available for sale securities	735
Money Market Mutual Funds [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Cash equivalents	106,867	46,646
Significant Other Observable Inputs (Level 2) [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets fair value disclosure	306,267	176,384
Significant Other Observable Inputs (Level 2) [Member] | Corporate Bonds [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Available for sale securities	179,399	129,738
Significant Other Observable Inputs (Level 2) [Member] | U.S. Government and Federal Agency Securities [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Available for sale securities	19,266
Significant Other Observable Inputs (Level 2) [Member] | Certificates of Deposit [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Available for sale securities	735
Significant Other Observable Inputs (Level 2) [Member] | Money Market Mutual Funds [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Cash equivalents	$ 106,867	$ 46,646

Fair Value of Financial Instr38

Fair Value of Financial Instruments - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Fair value hierarchy level 1 to level 2 transfers amount	$ 0		$ 0
Fair value hierarchy level 2 to level 1 transfers amount	0		0
Non-marketable equity securities	400,000		400,000		$ 400,000
impairment losses on non-marketable equity securities	$ 0	$ 0	$ 0	$ 0

Property and Equipment Net - Sc

Property and Equipment Net - Schedule of Property and Equipment (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Total property and equipment	$ 21,685	$ 17,254
Accumulated depreciation and amortization	(4,987)	(3,277)
Property and equipment, net	16,698	13,977
Computer Equipment and Software [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	1,814	1,481
Lab Equipment [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	9,496	8,561
Furniture and Fixtures [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	1,695	1,384
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	$ 8,680	$ 5,828

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($)

1 Months Ended

6 Months Ended

12 Months Ended

44 Months Ended

112 Months Ended

Nov. 30, 2016

May 31, 2016

Jan. 31, 2016

Sep. 30, 2015

Mar. 31, 2015

Nov. 30, 2014

Mar. 31, 2009

Jun. 30, 2018

Dec. 31, 2017

Jun. 30, 2018

Jun. 30, 2018

Jun. 30, 2017

Other Commitments [Line Items]

Tenant improvement allowance received

$ 400,000

Restricted cash as collateral with financial institution

$ 225,000

$ 225,000

$ 225,000

$ 225,000

$ 225,000

Claims paid to date related to indemnification issues

0

Accruals or expenses related to indemnification issues

0

0

European Patent Office Proceeding [Member]

Other Commitments [Line Items]

Liability related to third party opposition

$ 0

0

0

The Trustees of the University of Pennsylvania [Member]

Other Commitments [Line Items]

Percentage of ownership interest issued as consideration for license agreement

24.50%

Accrued expenses

$ 100,000

100,000

100,000

The Trustees of the University of Pennsylvania [Member] | Maximum [Member]

Other Commitments [Line Items]

Accrued expenses

100,000

GlaxoSmithKline LLC [Member]

Other Commitments [Line Items]

Percentage of ownership interest issued as consideration for license agreement

19.90%

Accrued expenses

1,300,000

300,000

1,300,000

1,300,000

Milestone payment obligation

$ 1,500,000

GlaxoSmithKline LLC [Member] | Milestone Fees [Member]

Other Commitments [Line Items]

Related party transaction expense

500,000

Regents of the University of Minnesota [Member] | Maximum [Member]

Other Commitments [Line Items]

Up-front fee paid and patent maintenance expenses reimbursed

$ 100,000

Up-front fee paid

$ 100,000

Amounts due under agreement

$ 300,000

$ 100,000

300,000

$ 300,000

Regents of the University of Minnesota [Member] | Maximum [Member] | Milestone Fees [Member]

Other Commitments [Line Items]

Related party transaction expense

$ 100,000

Regents of the University of Minnesota [Member] | Maximum [Member] | Annual License Maintenance Fee [Member]

Other Commitments [Line Items]

Contractual obligation

100,000

Regents of the University of Minnesota [Member] | Maximum [Member] | Royalties And Sublicense Fees [Member]

Other Commitments [Line Items]

Contractual obligation

$ 100,000

Rockville, Maryland [Member]

Other Commitments [Line Items]

Lease term commenced date

2016-02

2015-04

Lease term

September 2015, November 2015 and July 2017: Ultimately extend the term of the lease to September 2021.

Options to extend the additional lease term

6 years

Tenant improvement allowance received

$ 800,000

Lease period

7 years 6 months

Lease expiration date

Sep. 30, 2023

Options to extend the lease

5 years

Monthly lease payments commencement period

2016-09

New York, New York [Member]

Other Commitments [Line Items]

Lease term commenced date

2016-07

Lease expiration date

Oct. 31, 2020

Monthly lease payments commencement period

2016-10

Lease expiration term

51 months

Letters of credit

$ 200,000

Commitments and Contingencies41

Commitments and Contingencies - Schedule of Future Minimum Lease Payments Under Non-Cancelable Operating Leases (Detail) $ in Thousands	Jun. 30, 2018USD ($)
Operating Leases Future Minimum Payments Due [Abstract]
2018 (remainder of year)	$ 1,193
2,019	2,395
2,020	2,411
2,021	1,797
2,022	621
Thereafter	479
Total minimum lease payments	$ 8,896

Commitments and Contingencies42

Commitments and Contingencies - Summary of Expenses Incurred by Company (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
GlaxoSmithKline LLC [Member]
Other Commitments [Line Items]
Total related party transaction expense	$ 4,095	$ 672	$ 6,520	$ 862
GlaxoSmithKline LLC [Member] | Sublicense Fees [Member]
Other Commitments [Line Items]
Total related party transaction expense	3,998	656	6,030	701
GlaxoSmithKline LLC [Member] | Royalties on Sales of Reagents [Member]
Other Commitments [Line Items]
Total related party transaction expense		2		2
GlaxoSmithKline LLC [Member] | Maintenance of Licensed Patents [Member]
Other Commitments [Line Items]
Total related party transaction expense	97	14	490	159
Regents of the University of Minnesota [Member]
Other Commitments [Line Items]
Total related party transaction expense	5	11	392	16
Regents of the University of Minnesota [Member] | Sublicense Fees [Member]
Other Commitments [Line Items]
Total related party transaction expense	(125)		250
Regents of the University of Minnesota [Member] | Maintenance of Licensed Patents [Member]
Other Commitments [Line Items]
Total related party transaction expense	130	11	142	16
License Fees [Member] | The Trustees of the University of Pennsylvania [Member]
Other Commitments [Line Items]
Total related party transaction expense	98	745	117	874
License Fees [Member] | The Trustees of the University of Pennsylvania [Member] | Sublicense Fees [Member]
Other Commitments [Line Items]
Total related party transaction expense		656		701
License Fees [Member] | The Trustees of the University of Pennsylvania [Member] | Royalties on Sales of Reagents [Member]
Other Commitments [Line Items]
Total related party transaction expense		4		4
License Fees [Member] | The Trustees of the University of Pennsylvania [Member] | Maintenance of Licensed Patents [Member]
Other Commitments [Line Items]
Total related party transaction expense	$ 98	$ 85	$ 117	$ 169

License Revenue - Additional In

License Revenue - Additional Information (Detail)	1 Months Ended				3 Months Ended		6 Months Ended
	May 31, 2018USD ($)	Jan. 31, 2018USD ($)	Jun. 30, 2017USD ($)	Mar. 31, 2014USD ($)	Jun. 30, 2018USD ($)ProductCandidate	Jun. 30, 2017USD ($)	Jun. 30, 2018USD ($)ProductCandidate	Jun. 30, 2017USD ($)	Mar. 31, 2018USD ($)	Dec. 31, 2017USD ($)
License Revenue [Line Items]
Milestone fee upon commencement of clinical trials in humans							$ 25,900,000
Milestone fee upon submission of regulatory approval filings							46,000,000
Milestone fee upon approval of commercial products by regulatory agencies							105,500,000
Milestone fee upon achievement of specified sales targets for licensed products							172,000,000
Accounts receivable					$ 5,224,000		5,224,000		$ 58,621,000	$ 5,850,000
Deferred revenue					600,000		600,000
License revenue					40,031,000	$ 6,562,000	172,422,000	$ 7,017,000
Interest income from licensing					6,898,000		8,253,000
Impirment losses on receivables or contract assets							$ 0
Transaction price description							Upon its execution, the transaction price of the January 2018 Amendment was $132.1 million, which was fully recognized as license revenue upon the delivery of the modified license in January 2018. In May 2018, as a result of the acquisition of AveXis by Novartis, the transaction price was increased by $40.0 million to account for the acceleration of the sale-based milestone which was previously excluded from the transaction price. The $40.0 million increase in the transaction price was recognized as license revenue upon the completion of the change of control in May 2018 since the amended license had been fully delivered to AveXis. Additionally, due to the acceleration of the two $30.0 million payments originally due in January 2019 and January 2020, the Company recognized $6.1 million of interest income from licensing upon the completion of the change of control of AveXis, which represents the remaining present value discount on such payments as of the date of the change of control of AveXis. The transaction price of $172.1 million as of June 30, 2018 includes the following fixed consideration: (i) the $80.0 million payment in January 2018, (ii) the present value, as of the date of the January 2018 Amendment, of the two $30.0 million payments originally due in January 2019 and January 2020 and (iii) the $40.0 million sales-based milestone which was accelerated upon the change of control in May 2018. Variable consideration under the January 2018 Amendment, which has been excluded from the transaction price, includes the remaining sales-based milestone payment of $80.0 million, as well as any potential sublicense fees or royalties on sales of licensed products, which will be recognized in the period of the underlying sales or sublicenses, if any.
Accounts receivable, current					739,000		$ 739,000			473,000
Accounts receivable, non -current					4,485,000		4,485,000
AveXis, Inc. [Member]
License Revenue [Line Items]
License fees received					100,000,000
Accounts receivable									$ 53,300,000
License [Member]
License Revenue [Line Items]
License revenue					$ 40,031,000	6,555,000	$ 172,422,000	7,010,000
NAV Technology Licensees [Member] | NAV Technology Platform [Member] | Minimum [Member]
License Revenue [Line Items]
Number of development partnered product candidates | ProductCandidate					20		20
Licence Agreement [Member]
License Revenue [Line Items]
Interest income from licensing					$ 6,900,000		$ 300,000
Licence Agreement [Member] | License [Member]
License Revenue [Line Items]
License revenue					39,900,000		200,000
March 2014 License Agreement [Member]
License Revenue [Line Items]
Up-front fee paid							2,000,000
Transaction price of license					3,500,000		3,500,000
Accounts receivable										0
March 2014 License Agreement [Member] | AveXis, Inc. [Member]
License Revenue [Line Items]
Interest income from licensing					6,800,000		8,000,000
Up-front fee paid				$ 2,000,000
Milestone fee payments upon achievement of various development and commercialization				$ 12,300,000
License agreement amendment date				2018-01
Accounts receivable					200,000		200,000
Accounts receivable, non -current					200,000		200,000
March 2014 License Agreement [Member] | Maximum [Member] | AveXis, Inc. [Member]
License Revenue [Line Items]
Accounts receivable, current					100,000		100,000
March 2014 License Agreement [Member] | License [Member]
License Revenue [Line Items]
License revenue						0		100,000
March 2014 License Agreement [Member] | License [Member] | AveXis, Inc. [Member]
License Revenue [Line Items]
License revenue					40,000,000		172,100,000
January 2018 Amendment Agreement [Member]
License Revenue [Line Items]
Interest income from licensing	$ 6,100,000
License fee		$ 80,000,000
Sales-based milestone payment unpaid in the event of change of control		40,000,000
Transaction price of license		132,100,000			172,100,000		172,100,000
January 2018 Amendment Agreement [Member] | First Anniversary [Member]
License Revenue [Line Items]
License fee		30,000,000
January 2018 Amendment Agreement [Member] | Second Anniversary [Member]
License Revenue [Line Items]
License fee		$ 30,000,000
January 2018 Amendment Agreement [Member] | AveXis, Inc. [Member]
License Revenue [Line Items]
License fees received	100,000,000
License agreement period		14 years
January 2018 Amendment Agreement [Member] | Minimum [Member]
License Revenue [Line Items]
Milestone payment		$ 120,000,000
January 2018 Amendment Agreement [Member] | License [Member]
License Revenue [Line Items]
Increase in transaction price of license	$ 40,000,000
June 2017 License Agreement [Member] | AveXis, Inc. [Member]
License Revenue [Line Items]
Up-front fee paid			$ 6,000,000
Milestone fee payments upon achievement of various development and commercialization			$ 36,000,000
Accounts receivable					700,000		700,000			$ 0
Accounts receivable, current					100,000		100,000
Accounts receivable, non -current					600,000		600,000
June 2017 License Agreement [Member] | Maximum [Member] | AveXis, Inc. [Member]
License Revenue [Line Items]
Interest income from licensing					100,000		100,000
June 2017 License Agreement [Member] | License [Member] | AveXis, Inc. [Member]
License Revenue [Line Items]
License revenue					$ 0	$ 6,000,000	$ 0	$ 6,000,000

License Revenue - Summary of Ch

License Revenue - Summary of Changes in Balances of Receivables, Contract Assets and Contract Liabilities (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018
Receivables and contract assets:
Accounts receivable, current and non-current, Balance at Beginning of Period	$ 58,621	$ 5,850
Accounts receivable, current and non-current, Net Additions (Deductions)	(53,397)	(626)
Accounts receivable, current and non-current, Balance at End of Period	5,224	5,224
Contract assets, Balance at Beginning of Period	250	350
Contract assets, Net Additions (Deductions)	(250)	(350)
Contract liabilities:
Deferred revenue, Net Additions (Deductions)	600	600
Deferred revenue, Balance at End of Period	$ 600	$ 600

License Revenue - Additional 45

License Revenue - Additional Information (Detail1) - January 2018 Amendment Agreement [Member] - USD ($) $ in Millions	1 Months Ended	6 Months Ended
	May 31, 2018	Jun. 30, 2018
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2018-01-01
License Revenue [Line Items]
Revenue, remaining performance obligation, expected timing of satisfaction, period		1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: (nil)
License Revenue [Line Items]
Sales-based milestone payment exclude transaction price		$ 80
Payments due from related party		$ 30
Consideration payment due period one		2019-01
Consideration payment due period two		2020-01
Share-based Milestone [Member] | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: (nil)
License Revenue [Line Items]
Increase in transaction price of license	$ 40

Stock-based Compensation - Addi

Stock-based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	6 Months Ended
	Jan. 31, 2018	Jun. 30, 2018	Jun. 30, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock-based compensation expense		$ 39,700
Unrecognized stock-based compensation, weighted-average period		2 years 8 months 12 days
Proceeds from exercise of stock options		$ 6,999	$ 329
2015 Equity Incentive Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Additional shares to be issued	1,251,810
Common stock shares authorized for issuance		9,488,413
Shares available for future grants		2,236,302
Weighted-average fair values of options granted		$ 22.88
Exercise of stock options, Shares		960,838
Total intrinsic value of options exercised		$ 28,100
2015 Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock shares authorized for issuance		254,000
Shares available for future grants		186,752
Common stock shares issued to participants		19,528

Stock-based Compensation - Stoc

Stock-based Compensation - Stock-Based Compensation Expense by Award Type (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 3,982	$ 2,483	$ 7,272	$ 5,074
Stock Option [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	3,818	2,332	6,939	4,771
Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	69	69	136	138
Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 95	$ 82	$ 197	$ 165

Stock-based Compensation - St48

Stock-based Compensation - Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 3,982	$ 2,483	$ 7,272	$ 5,074
Research and Development Costs [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	1,895	1,137	3,424	2,374
General and Administrative [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 2,087	$ 1,346	$ 3,848	$ 2,700

Stock-based Compensation - Summ

Stock-based Compensation - Summary of Stock Option Activity (Detail) - 2014 and 2015 Equity Incentive Plan [Member] - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Number of Shares [Roll Forward]
Shares Outstanding, Beginning Balance	5,468
Shares, Granted	1,137
Shares, Exercised	(961)
Shares, Cancelled or forfeited	(199)
Shares Outstanding, Ending Balance	5,445	5,468
Shares, Exercisable	2,748
Shares, Vested and expected to vest	5,424
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted-average Exercise Price Outstanding, Beginning Balance	$ 10.25
Weighted-average Exercise Price, Granted	34.39
Weighted-average Exercise Price, Exercised	7.32
Weighted-average Exercise Price, Cancelled or forfeited	15.88
Weighted-average Exercise Price, Outstanding, Ending Balance	15.61	$ 10.25
Weighted-average Exercise Price, Exercisable	7.88
Weighted-average Exercise Price, Vested and expected to vest	$ 15.66
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted-average Remaining Contractual Life (Years) Outstanding	7 years 10 months 24 days	7 years 10 months 24 days
Weighted-average Remaining Contractual Life (Years), Exercisable	7 years 1 month 6 days
Weighted-average Remaining Contractual Life (Years), Vested and expected to vest	7 years 10 months 24 days
Aggregate Intrinsic Value Outstanding	$ 305,721	$ 125,738
Aggregate Intrinsic Value, Exercisable	175,499
Aggregate Intrinsic Value, Vested and expected to vest	$ 304,215

Stock-based Compensation - Su50

Stock-based Compensation - Summary of Unvested RSUs Activity Under 2015 Plan (Detail) - Restricted Stock Units [Member] shares in Thousands	6 Months Ended
	Jun. 30, 2018$ / sharesshares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares Unvested, Beginning Balance	40
Shares, Granted	0
Shares, Vested	0
Shares, Forfeited	0
Shares Unvested, Ending Balance	40
Weighted-average Grant Date Fair Value, Unvested Beginning Balance | $ / shares	$ 20.90
Weighted-average Grant Date Fair Value, Unvested Ending Balance | $ / shares	$ 20.90

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail)	3 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018
Income Tax Disclosure [Abstract]
U.S. federal statutory income tax rate, percent	21.00%	21.00%

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Detail) - Research and Development Costs [Member] - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2018	Jun. 30, 2017
FOXKISER,LLP [Member] | Service Agreements [Member]
Related Party Transaction [Line Items]
Related party transaction expense	$ 0.5	$ 0.4		$ 1.1	$ 0.8
Scientific Founder and Special Advisor [Member]
Related Party Transaction [Line Items]
Related party transaction expenses from transactions with related party advisory fees			$ 0.1

Net Income (Loss) Per Share - C

Net Income (Loss) Per Share - Computations of Basic and Diluted Net Income (Loss) Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Basic net income (loss) per share:
Net income (loss) applicable to common stockholders	$ 10,594	$ (14,473)	$ 114,833	$ (36,466)
Shares used in computation:
Weighted-average common shares outstanding	32,082	30,662	31,858	28,678
Basic net income (loss) per share	$ 0.33	$ (0.47)	$ 3.60	$ (1.27)
Diluted net income (loss) per share:
Net income (loss) applicable to common stockholders	$ 10,594	$ (14,473)	$ 114,833	$ (36,466)
Shares used in computation:
Weighted-average diluted common shares	35,272	30,662	34,884	28,678
Diluted	$ 0.30	$ (0.47)	$ 3.29	$ (1.27)
Common Shares [Member]
Shares used in computation:
Weighted-average diluted common shares	32,082	30,662	31,858	28,678
Stock Option [Member]
Shares used in computation:
Weighted-average diluted common shares	3,153		2,995
Restricted Stock Units [Member]
Shares used in computation:
Weighted-average diluted common shares	30		27
Employee Stock Purchase Plan [Member]
Shares used in computation:
Weighted-average diluted common shares	7		4

Net Income (Loss) Per Share - S

Net Income (Loss) Per Share - Schedules for Computation of Diluted Weighted-Average Shares Outstanding (Detail) - shares shares in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted weighted average shares outstanding	804	5,901	1,236	5,901
Stock Options Issued and Outstanding [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted weighted average shares outstanding	804	5,835	1,236	5,835
Unvested Restricted Stock Units Outstanding [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted weighted average shares outstanding		40		40
Employee Stock Purchase Plan [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted weighted average shares outstanding		26		26

Supplemental Disclosures - Sche

Supplemental Disclosures - Schedules of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Accrued personnel costs	$ 4,902	$ 5,789
Accrued external research and development expenses	2,602	2,072
Accrued income taxes payable	2,439
Accrued external general and administrative expenses	1,203	1,078
Accrued licensing costs	350
Accrued purchases of property and equipment	139	430
Other accrued expenses and current liabilities	388	236
Accrued expenses and other current liabilities	$ 12,023	$ 9,605