REGENXBIO (RGNX) 10-Q 8 Aug 17 2017 Q2 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2017	Aug. 04, 2017
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2017
Document Fiscal Year Focus	2,017
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		30,894,443

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Current assets
Cash and cash equivalents	$ 57,649	$ 24,840
Marketable securities	104,434	64,714
Accounts receivable	50	1,032
Prepaid expenses	2,432	1,775
Other current assets	1,252	1,010
Total current assets	165,817	93,371
Marketable securities	46,417	69,412
Property and equipment, net	11,524	9,324
Restricted cash	225	225
Other assets	393	400
Total assets	224,376	172,732
Current liabilities
Accounts payable	3,948	1,543
Accrued expenses and other current liabilities	7,514	8,126
Total current liabilities	11,462	9,669
Deferred rent, net of current portion	1,217	1,326
Total liabilities	12,679	10,995
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, 2017 and December 31, 2016
Common stock; $0.0001 par value; 100,000 shares authorized at June 30, 2017 and December 31, 2016; 30,853 and 26,477 shares issued and outstanding at June 30, 2017 and December 31, 2016, respectively	3	3
Additional paid-in capital	363,393	276,354
Accumulated other comprehensive loss	(646)	(33)
Accumulated deficit	(151,053)	(114,587)
Total stockholders’ equity	211,697	161,737
Total liabilities and stockholders’ equity	$ 224,376	$ 172,732

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Jun. 30, 2017	Dec. 31, 2016
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	30,853,000	26,477,000
Common stock, shares outstanding	30,853,000	26,477,000

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Revenues
License revenue	$ 6,555	$ 2,245	$ 7,010	$ 2,573
Reagent sales		107		166
Grant revenue	7	23	7	29
Total revenues	6,562	2,375	7,017	2,768
Costs of revenues
Licensing costs	1,311	449	1,402	515
Costs of reagent sales	6	49	6	79
Research and development	13,917	10,680	30,536	16,863
General and administrative	6,355	6,169	12,977	11,648
Other operating expenses (income)	29	(20)	74	(134)
Total operating expenses	21,618	17,327	44,995	28,971
Loss from operations	(15,056)	(14,952)	(37,978)	(26,203)
Other Income
Investment income	583	515	1,512	998
Total other income	583	515	1,512	998
Net loss	(14,473)	(14,437)	(36,466)	(25,205)
Other Comprehensive Income (Loss)
Unrealized gain (loss) on available-for-sale securities,net of reclassifications of $480 for the six months ended June 30, 2017	(74)	246	(613)	1,240
Total other comprehensive income (loss)	(74)	246	(613)	1,240
Comprehensive loss	(14,547)	(14,191)	(37,079)	(23,965)
Net loss applicable to common stockholders	$ (14,473)	$ (14,437)	$ (36,466)	$ (25,205)
Basic and diluted net loss per common share	$ (0.47)	$ (0.55)	$ (1.27)	$ (0.96)
Weighted-average basic and diluted common shares	30,662	26,362	28,678	26,344

Consolidated Statements of Ope5

Consolidated Statements of Operations and Comprehensive Loss (Parenthetical) $ in Thousands	6 Months Ended
	Jun. 30, 2017USD ($)
Income Statement [Abstract]
Unrealized gain (loss) on available-for-sale securities, net of reclassifications	$ 480

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Cash flows from operating activities
Net loss	$ (36,466)	$ (25,205)
Adjustments to reconcile net loss to net cash used in operating activities
Stock-based compensation expense	5,074	3,202
Net amortization of premiums and accretion of discounts on marketable debt securities	945	987
Depreciation and amortization	1,257	104
Realized gains on sales of marketable securities	(480)
Other non-cash adjustments	40
Changes in operating assets and liabilities
Accounts receivable	982	1,353
Prepaid expenses	(657)	(460)
Other current assets	(242)	(586)
Other assets	(86)	(98)
Accounts payable	2,723	930
Accrued expenses and other current liabilities	(31)	2,982
Advance payments		(127)
Deferred rent	(89)	601
Net cash used in operating activities	(27,030)	(16,317)
Cash flows from investing activities
Purchases of marketable securities	(46,593)	(32,261)
Maturities of marketable securities	28,010	26,131
Sales of marketable securities	780
Purchases of property and equipment	(4,609)	(1,491)
Restricted cash		(225)
Net cash used in investing activities	(22,412)	(7,846)
Cash flows from financing activities
Proceeds from exercise of stock options	329	130
Proceeds from issuance of common stock under employee stock purchase plan	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	82,251	130
Net increase (decrease) in cash and cash equivalents	32,809	(24,033)
Cash and cash equivalents
Beginning of period	24,840	54,116
End of period	57,649	30,083
Supplemental disclosures of non-cash investing and financing activities
Purchases of property and equipment in accounts payable and accrued expenses		$ 939
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, 2017
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, 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. Follow-on Public Offering On March 27, 2017, the Company completed a follow-on public offering whereby the Company sold 3,700,000 shares of common stock at a price of $20.50 per share. In connection with the offering, the Company granted the underwriters an option to purchase up to 555,000 additional shares of common stock at the public offering price. The underwriters exercised the option in full and purchased the additional shares on April 26, 2017. The aggregate net proceeds received by the Company from the offering, inclusive of the underwriters’ option exercise, were $81.5 million, net of underwriting discounts and commissions and offering expenses payable by the Company. Liquidity and Risks As of June 30, 2017, the Company had generated an accumulated deficit of $151.1 million since inception. As the Company continues to incur losses, transition to 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 profitability, and unless and until it does, the Company will continue to need to raise additional capital. As of June 30, 2017, the Company had cash, cash equivalents and marketable securities of $208.5 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 commercial production of products.

Summary of Significant Accounti

Summary of Significant Accounting Policies	6 Months Ended
	Jun. 30, 2017
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, 2016 included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2016, which was filed with the SEC on March 7, 2017. 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, 2017, and the results of its operations and its cash flows for the interim periods ended June 30, 2017 and 2016. 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, 2016, 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 subsidiary, REGENXBIO EU Ltd., which was formed in June 2017. 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 materially differ 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: stock-based compensation expense, accrued research and development expenses and the 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. Net Loss Per Share Basic net loss per share is calculated by dividing net loss applicable to holders of common stock by the weighted-average shares outstanding during the period, without consideration for common stock equivalents. Diluted net loss per share is calculated by adjusting weighted-average shares outstanding for the dilutive effect of common stock equivalents outstanding for the period, determined using the treasury-stock method. For purposes of the diluted net loss per share calculation, outstanding stock options, outstanding restricted stock units and withholdings under the employee stock purchase plan are considered to be common stock equivalents but have been excluded from the calculation of diluted net loss per share, as their effect would be anti-dilutive. Contingently convertible shares in which conversion is based on non-market-priced contingencies are excluded from the calculations of both basic and diluted net loss per share until the contingency has been fully met. Accordingly, basic and diluted net loss per share were the same for all periods presented. Recently Announced Accounting Pronouncements In March 2016, the FASB issued ASU 2016-09, Compensation—Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers (Topic 606), Revenue Recognition. Revenue from Contracts with Customers (Topic 606), Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Practical Expedients Technical Corrections and Improvements to Topic 606, Revenue from Contracts with Customers 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 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) In January 2016, the FASB issued ASU 2016-01, Financial Instruments—Overall (Topic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities,

Marketable Securities

Marketable Securities	6 Months Ended
	Jun. 30, 2017
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 securities (in thousands): Amortized Cost Unrealized Gains Unrealized Losses Fair Value June 30, 2017 Corporate bonds $ 146,069 $ 18 $ (229 ) $ 145,858 Commercial paper 4,993 — — 4,993 $ 151,062 $ 18 $ (229 ) $ 150,851 Amortized Cost Unrealized Gains Unrealized Losses Fair Value December 31, 2016 Corporate bonds $ 133,424 $ 82 $ (298 ) $ 133,208 Common equity securities 300 618 — 918 $ 133,724 $ 700 $ (298 ) $ 134,126 Common equity securities reported as of December 31, 2016 consisted of shares of common stock of Audentes Therapeutics, Inc. (Audentes), which became a publicly traded company in July 2016. The Company obtained these shares in connection with a license granted to Audentes in July 2013. The Company sold all of its shares of Audentes common stock during the first quarter of 2017. As of June 30, 2017 and December 31, 2016, 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, 2017 and December 31, 2016, 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 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 loss for the periods. The Company recognized realized gains of $0 and $0.5 million on the sale or maturity of available-for-sale securities during the three and six months ended June 30, 2017, respectively, which were reclassified out of accumulated other comprehensive loss during the periods and are included in investment income in the consolidated statements of operations and comprehensive loss. During the three and six months ended June 30, 2016, the Company recognized net unrealized gains on available-for-sale securities of $0.2 million and $1.2 million, respectively, and income tax expense of $0 in other comprehensive income for the periods. The Company did not recognize any realized gains or losses on the sale or maturity of marketable securities for the three or six months ended June 30, 2016. The Company did not hold any securities in an unrealized loss position for more than 12 months as of June 30, 2017 or December 31, 2016. The aggregate fair value of securities held by the Company in an unrealized loss position for less than 12 months as of June 30, 2017 and December 31, 2016 was $127.2 million and $102.5 million, respectively. As of June 30, 2017, securities held by the Company which were in an unrealized loss position consisted of 45 investment grade corporate bond 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, 2017 or December 31, 2016.

Fair Value of Financial Instrum

Fair Value of Financial Instruments	6 Months Ended
	Jun. 30, 2017
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 solely of money market mutual funds. Marketable securities consist of corporate debt securities, as well as common equity securities as disclosed in Note 3. 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, 2017 Money market mutual funds (cash equivalents) $ — $ 57,649 $ — $ 57,649 Corporate bonds (marketable securities) — 145,858 — 145,858 Commercial paper (marketable securities) — 4,993 — 4,993 $ — $ 208,500 $ — $ 208,500 Quoted Significant prices other Significant in active observable unobservable markets inputs inputs (Level 1) (Level 2) (Level 3) Total December 31, 2016 Money market mutual funds (cash equivalents) $ — $ 24,840 $ — $ 24,840 Corporate bonds (marketable securities) — 133,208 — 133,208 Common equity securities (marketable securities) 918 — — 918 $ 918 $ 158,048 $ — $ 158,966 There were no transfers of financial instruments between levels of the fair value hierarchy during the six months ended June 30, 2017. Management estimates that the carrying amounts of its accounts receivable, accounts payable and accrued expenses and other current liabilities approximate fair value due to the short-term nature of those instruments.

Property and Equipment, Net

Property and Equipment, Net	6 Months Ended
	Jun. 30, 2017
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, 2017 December 31, 2016 Computer equipment and software $ 1,276 $ 1,038 Lab equipment 5,896 2,780 Furniture and fixtures 1,271 1,213 Leasehold improvements 4,961 4,917 Total property and equipment 13,404 9,948 Accumulated depreciation and amortization (1,880 ) (624 ) Property and equipment, net $ 11,524 $ 9,324

Commitments and Contingencies

Commitments and Contingencies	6 Months Ended
	Jun. 30, 2017
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 which 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 5.5-year, non-cancelable operating lease for office space at 9712 Medical Center Drive in Rockville, Maryland. The lease commenced in April 2015, and expires in September 2020. The Company has options to extend the lease for up to 6 years. Monthly payments under the lease began in October 2015 and escalate annually in accordance with the lease agreement. In September 2015 and November 2015, the Company amended the 9712 Medical Center Drive lease to include additional office and laboratory space at 9714 Medical Center Drive and extend the term of the lease for its existing space at that facility to October 2020. The lease term for the additional space commenced in April 2016, and has a 5-year term expiring in March 2021. The Company has options to extend the lease for the additional space to be coterminous with the Company’s existing lease at that facility. Monthly payments under the lease escalate annually in accordance with the lease agreement. The Company received a $0.3 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 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 lease commenced in February 2016, and expires in September 2023. Monthly payments under the lease began in September 2016 and escalate annually in accordance with the lease agreement. The Company received a $0.7 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, 2017, 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, 2017, future minimum lease payments under non-cancelable operating leases are as follows (in thousands): Operating Leases 2017 (remainder of year) $ 802 2018 1,638 2019 1,687 2020 1,626 2021 665 Thereafter 952 Total minimum lease payments $ 7,370 In July 2017, the Company amended the 9712 and 9714 Medical Center Drive lease to include additional office and laboratory space and extend the term of the lease for its existing space at that facility to September 2021. The lease term for the additional space commences partially in August 2017 and partially in February 2018 and is coterminous with the existing space at that facility. 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 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 loss. Royalties on sales of licensed reagents for use in research and development are included in costs of reagent sales in the consolidated statements of operations and comprehensive 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 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, 2017 2016 2017 2016 Sublicense fees $ 656 $ 225 $ 701 $ 257 Royalties on sales of reagents 4 7 4 9 Maintenance of licensed patents 85 14 169 43 $ 745 $ 246 $ 874 $ 309 As of June 30, 2017 and December 31, 2016, the Company had accrued $0.7 million and $0.2 million, respectively, in expenses payable to Penn under the license agreement, which are included in accounts payable and accrued expenses 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 obligated to pay GSK up to $1.5 million upon the achievement of various milestones. As of June 30, 2017, no milestones have been achieved, or deemed probable of achievement, and accordingly no milestone payments were payable to GSK. 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, 2017 2016 2017 2016 Sublicense fees $ 656 $ 225 $ 701 $ 257 Royalties on sales of reagents 2 4 2 6 Maintenance of licensed patents 14 137 159 230 $ 672 $ 366 $ 862 $ 493 As of June 30, 2017 and December 31, 2016, the Company had accrued $0.7 million and $0.4 million, respectively, in expenses payable to GSK under the license agreement, which are included in accounts payable and accrued expenses 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. As of June 30, 2017, no milestones have been achieved, or deemed probable of achievement, and accordingly no milestone payments were payable to Minnesota. Additionally, the Company has not incurred any royalties or sublicense fees payable to Minnesota since the inception of the license agreement. As of June 30, 2017 and December 31, 2016, the Company had accrued less than $0.1 million in patent maintenance expenses and up-front fees payable to Minnesota under the license agreement. 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 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, 2017 and December 31, 2016, 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 challenging the validity of a European patent owned by Penn for the AAV8 vector, which the Company has exclusively licensed. This matter is in its very early stages and the Company is unable to estimate the timing or outcome of this matter but intends to assist Penn in vigorously defending this patent. As of June 30, 2017, the Company has not recorded any liabilities related to this matter.

Significant Agreements

Significant Agreements	6 Months Ended
	Jun. 30, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Significant Agreements	7. Significant Agreements See Note 6 for license agreements granted to the Company. Licenses Granted by the Company The Company has granted a number of intellectual property licenses 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 equal to the life of the underlying patents and are terminable only at the option of the licensee. License agreements may require licensees to pay non-refundable up-front fees, option fees and annual maintenance fees. Additional contingent consideration under the licenses may include sublicense fees, milestone fees and royalties on net sales of commercialized 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-teen percentage of net sales by licensees. Milestone fees are payable to the Company upon the achievement of specific clinical and regulatory developments by licensees. As of June 30, 2017, 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 $0.5 million upon the submission of preclinical regulatory filings, $29.2 million upon the commencement of various stages of clinical trials, $48.5 million upon the submission of regulatory approval filings, $110.5 million upon the approval of commercial products by regulatory agencies and $92.0 million upon the achievement of specified sales targets for licensed products. License revenue consists of the following (in thousands): Three Months Ended June 30, Six Months Ended June 30, 2017 2016 2017 2016 Up-front fees and option fees for commercial licenses $ 6,000 $ 2,000 $ 6,000 $ 2,000 Maintenance fees for commercial licenses 140 80 390 330 Research and other license revenue 415 165 620 243 $ 6,555 $ 2,245 $ 7,010 $ 2,573 AveXis, Inc. In June 2017, the Company entered into an exclusive license agreement with AveXis, Inc. (AveXis) for the development and commercialization of products to treat Rett Syndrome and amyotrophic lateral sclerosis (ALS) caused by mutations in the gene that produces the copper zinc superoxide dismutase 1 (SOD1) enzyme using AAV9. Under the license agreement, the Company granted AveXis an exclusive, sublicensable worldwide license under the licensed intellectual property to make, have made, use, import, sell and offer for sale licensed products in the treatment of Rett Syndrome and ALS caused by mutations in the SOD1 gene using AAV9. In consideration for the license, AveXis paid the Company an up-front fee of $6.0 million, and is required to pay annual maintenance fees, milestone fees of up to $36.0 million, a low double digit royalty percentage on net sales of licensed products, subject to reduction in specified circumstances and a lower mid-double digit percentage of any sublicense fees AveXis receives from sublicensees for the licensed intellectual property rights. During the three and six months ended June 30, 2017, the Company recognized license revenue of $6.0 million from the license agreement with AveXis.

Stock-based Compensation

Stock-based Compensation	6 Months Ended
	Jun. 30, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-based Compensation	8. Stock-based Compensation In January 2017, an additional 1,059,065 shares became available for issuance under the 2015 Equity Incentive Plan (the 2015 Plan). As of June 30, 2017, the total number of shares of common stock authorized for issuance under the 2015 Plan and 2014 Stock Plan (the 2014 Plan) was 8,236,603, of which 1,972,348 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, 2017 2016 2017 2016 Stock options $ 2,332 $ 1,656 $ 4,771 $ 3,202 Restricted stock units 69 — 138 — Employee stock purchase plan 82 — 165 — $ 2,483 $ 1,656 $ 5,074 $ 3,202 As of June 30, 2017, the Company had $26.0 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 2.9 years. The Company has recorded aggregate stock-based compensation expense in the consolidated statement of operations and comprehensive loss as follows (in thousands): Three Months Ended June 30, Six Months Ended June 30, 2017 2016 2017 2016 Research and development $ 1,137 $ 619 $ 2,374 $ 1,011 General and administrative 1,346 1,037 2,700 2,191 $ 2,483 $ 1,656 $ 5,074 $ 3,202 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, 2016 5,118 $ 7.77 8.5 $ 57,400 Granted 996 $ 19.35 Exercised (100 ) $ 3.31 Cancelled or forfeited (179 ) $ 16.48 Outstanding at June 30, 2017 5,835 $ 9.55 8.3 $ 60,956 Exercisable at June 30, 2017 2,812 $ 5.43 7.7 $ 40,881 Vested and expected to vest at June 30, 2017 5,835 $ 9.55 8.3 $ 60,956 (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, 2017 was $13.17. During the six months ended June 30, 2017, the total number of stock options exercised was 99,594, resulting in total proceeds of $0.3 million. The total intrinsic value of options exercised during the six months ended June 30, 2017 was $1.7 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, 2016 40 $ 20.90 Granted — $ — Vested — $ — Forfeited — $ — Unvested balance at June 30, 2017 40 $ 20.90 Employee Stock Purchase Plan As of June 30, 2017, the total number of shares of common stock authorized for issuance under the 2015 ESPP was 254,000, of which 232,294 remain available for future issuance. During the six months ended June 30, 2017, 21,706 shares of common stock were issued under the 2015 ESPP.

Related Party Transactions

Related Party Transactions	6 Months Ended
	Jun. 30, 2017
Related Party Transactions [Abstract]
Related Party Transactions	9. Related Party Transactions FOXKISER LLP Effective January 2016, the Company entered into a Professional Services Agreement with FOXKISER LLP (FOXKISER), an affiliate of certain stockholders of the Company and an affiliate of one current and one former member of the Board of Directors, pursuant to which the Company incurs a fixed monthly fee in consideration for certain strategic planning, development and regulatory services provided by FOXKISER. The agreement expired in December 2016, and effective January 2017 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 months ended June 30, 2017 and 2016 were $0.4 million and $0.2 million, respectively. Costs incurred under the agreements with FOXKISER for the six months ended June 30, 2017 and 2016 were $0.8 million and $0.5 million, respectively. Costs incurred under the agreements with FOXKISER have been recorded as research and development expenses in the consolidated statements of operations and comprehensive loss. Special Scientific Advisor In September 2014, the Company entered into an advisory agreement, as amended, with James M. Wilson, M.D., Ph.D., the Company’s Scientific Founder and Special Advisor (formerly the Company’s Chief Scientific Advisor), who is also the Chairman of the Company’s Scientific Advisory Board. The agreement required a fixed monthly payment in consideration for scientific advisory services and expired in March 2017. The advisor will continue to provide services at no cost to the Company pursuant to a new advisory agreement entered into in March 2017. During the three months ended June 30, 2017 and 2016, the Company incurred advisory fees of $0 and $0.1 million, respectively, under the agreements. During the six months ended June 30, 2017 and 2016, the Company incurred advisory fees of $0.1 million and $0.1 million, respectively, under the agreements. Additionally, the Company has granted options to purchase 211,600 shares of common stock to the Scientific Founder and Special Advisor as compensation for services to be provided to the Company, which vest partially upon the completion of service conditions and partially upon the achievement of specified performance conditions as set forth in the award agreements.

Net Loss Per Share

Net Loss Per Share	6 Months Ended
	Jun. 30, 2017
Earnings Per Share [Abstract]
Net Loss Per Share	10. Net Loss Per Share The following potentially dilutive securities outstanding at the end of the period were excluded from the computations of diluted weighted-average shares outstanding for the periods indicated as they would be anti-dilutive (in thousands): Three and Six Months Ended June 30, 2017 2016 Stock options issued and outstanding 5,835 4,785 Unvested restricted stock units outstanding 40 — Employee stock purchase plan 26 — 5,901 4,785 Amounts in the table above reflect the common stock equivalents of the noted instruments.

Supplemental Disclosures

Supplemental Disclosures	6 Months Ended
	Jun. 30, 2017
Payables And Accruals [Abstract]
Supplemental Disclosures	11. Supplemental Disclosures Accrued expenses and other current liabilities consists of the following (in thousands): June 30, 2017 December 31, 2016 Accrued personnel costs $ 3,896 $ 3,338 Accrued external research and development expenses 2,133 3,005 Accrued external general and administrative expenses 943 672 Accrued purchases of property and equipment 50 844 Other accrued expenses and current liabilities 492 267 $ 7,514 $ 8,126

Summary of Significant Accoun18

Summary of Significant Accounting Policies (Policies)	6 Months Ended
	Jun. 30, 2017
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, 2016 included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2016, which was filed with the SEC on March 7, 2017. 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, 2017, and the results of its operations and its cash flows for the interim periods ended June 30, 2017 and 2016. 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, 2016, 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 subsidiary, REGENXBIO EU Ltd., which was formed in June 2017. 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 materially differ 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: stock-based compensation expense, accrued research and development expenses and the fair value of financial instruments.
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.
Net Loss Per Share	Net Loss Per Share Basic net loss per share is calculated by dividing net loss applicable to holders of common stock by the weighted-average shares outstanding during the period, without consideration for common stock equivalents. Diluted net loss per share is calculated by adjusting weighted-average shares outstanding for the dilutive effect of common stock equivalents outstanding for the period, determined using the treasury-stock method. For purposes of the diluted net loss per share calculation, outstanding stock options, outstanding restricted stock units and withholdings under the employee stock purchase plan are considered to be common stock equivalents but have been excluded from the calculation of diluted net loss per share, as their effect would be anti-dilutive. Contingently convertible shares in which conversion is based on non-market-priced contingencies are excluded from the calculations of both basic and diluted net loss per share until the contingency has been fully met. Accordingly, basic and diluted net loss per share were the same for all periods presented.
Recently Announced Accounting Pronouncements	Recently Announced Accounting Pronouncements In March 2016, the FASB issued ASU 2016-09, Compensation—Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers (Topic 606), Revenue Recognition. Revenue from Contracts with Customers (Topic 606), Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Practical Expedients Technical Corrections and Improvements to Topic 606, Revenue from Contracts with Customers 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 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) In January 2016, the FASB issued ASU 2016-01, Financial Instruments—Overall (Topic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities,

Marketable Securities (Tables)

Marketable Securities (Tables)	6 Months Ended
	Jun. 30, 2017
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 securities (in thousands): Amortized Cost Unrealized Gains Unrealized Losses Fair Value June 30, 2017 Corporate bonds $ 146,069 $ 18 $ (229 ) $ 145,858 Commercial paper 4,993 — — 4,993 $ 151,062 $ 18 $ (229 ) $ 150,851 Amortized Cost Unrealized Gains Unrealized Losses Fair Value December 31, 2016 Corporate bonds $ 133,424 $ 82 $ (298 ) $ 133,208 Common equity securities 300 618 — 918 $ 133,724 $ 700 $ (298 ) $ 134,126

Fair Value of Financial Instr20

Fair Value of Financial Instruments (Tables)	6 Months Ended
	Jun. 30, 2017
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, 2017 Money market mutual funds (cash equivalents) $ — $ 57,649 $ — $ 57,649 Corporate bonds (marketable securities) — 145,858 — 145,858 Commercial paper (marketable securities) — 4,993 — 4,993 $ — $ 208,500 $ — $ 208,500 Quoted Significant prices other Significant in active observable unobservable markets inputs inputs (Level 1) (Level 2) (Level 3) Total December 31, 2016 Money market mutual funds (cash equivalents) $ — $ 24,840 $ — $ 24,840 Corporate bonds (marketable securities) — 133,208 — 133,208 Common equity securities (marketable securities) 918 — — 918 $ 918 $ 158,048 $ — $ 158,966

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	6 Months Ended
	Jun. 30, 2017
Property Plant And Equipment [Abstract]
Schedule of Property and Equipment	Property and equipment, net consists of the following (in thousands): June 30, 2017 December 31, 2016 Computer equipment and software $ 1,276 $ 1,038 Lab equipment 5,896 2,780 Furniture and fixtures 1,271 1,213 Leasehold improvements 4,961 4,917 Total property and equipment 13,404 9,948 Accumulated depreciation and amortization (1,880 ) (624 ) Property and equipment, net $ 11,524 $ 9,324

Commitments and Contingencies (

Commitments and Contingencies (Tables)	6 Months Ended
	Jun. 30, 2017
Schedule of Future Minimum Lease Payments Under Non-Cancelable Operating Leases	As of June 30, 2017, future minimum lease payments under non-cancelable operating leases are as follows (in thousands): Operating Leases 2017 (remainder of year) $ 802 2018 1,638 2019 1,687 2020 1,626 2021 665 Thereafter 952 Total minimum lease payments $ 7,370
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, 2017 2016 2017 2016 Sublicense fees $ 656 $ 225 $ 701 $ 257 Royalties on sales of reagents 4 7 4 9 Maintenance of licensed patents 85 14 169 43 $ 745 $ 246 $ 874 $ 309
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, 2017 2016 2017 2016 Sublicense fees $ 656 $ 225 $ 701 $ 257 Royalties on sales of reagents 2 4 2 6 Maintenance of licensed patents 14 137 159 230 $ 672 $ 366 $ 862 $ 493

Significant Agreements (Tables)

Significant Agreements (Tables)	6 Months Ended
	Jun. 30, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Schedule of License Revenue	License revenue consists of the following (in thousands): Three Months Ended June 30, Six Months Ended June 30, 2017 2016 2017 2016 Up-front fees and option fees for commercial licenses $ 6,000 $ 2,000 $ 6,000 $ 2,000 Maintenance fees for commercial licenses 140 80 390 330 Research and other license revenue 415 165 620 243 $ 6,555 $ 2,245 $ 7,010 $ 2,573

Stock-based Compensation (Table

Stock-based Compensation (Tables)	6 Months Ended
	Jun. 30, 2017
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, 2017 2016 2017 2016 Stock options $ 2,332 $ 1,656 $ 4,771 $ 3,202 Restricted stock units 69 — 138 — Employee stock purchase plan 82 — 165 — $ 2,483 $ 1,656 $ 5,074 $ 3,202
Stock-Based Compensation Expense	The Company has recorded aggregate stock-based compensation expense in the consolidated statement of operations and comprehensive loss as follows (in thousands): Three Months Ended June 30, Six Months Ended June 30, 2017 2016 2017 2016 Research and development $ 1,137 $ 619 $ 2,374 $ 1,011 General and administrative 1,346 1,037 2,700 2,191 $ 2,483 $ 1,656 $ 5,074 $ 3,202
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, 2016 40 $ 20.90 Granted — $ — Vested — $ — Forfeited — $ — Unvested balance at June 30, 2017 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, 2016 5,118 $ 7.77 8.5 $ 57,400 Granted 996 $ 19.35 Exercised (100 ) $ 3.31 Cancelled or forfeited (179 ) $ 16.48 Outstanding at June 30, 2017 5,835 $ 9.55 8.3 $ 60,956 Exercisable at June 30, 2017 2,812 $ 5.43 7.7 $ 40,881 Vested and expected to vest at June 30, 2017 5,835 $ 9.55 8.3 $ 60,956 (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 Loss Per Share (Tables)

Net Loss Per Share (Tables)	6 Months Ended
	Jun. 30, 2017
Earnings Per Share [Abstract]
Schedules for Computation of Diluted Weighted-Average Shares Outstanding	The following potentially dilutive securities outstanding at the end of the period were excluded from the computations of diluted weighted-average shares outstanding for the periods indicated as they would be anti-dilutive (in thousands): Three and Six Months Ended June 30, 2017 2016 Stock options issued and outstanding 5,835 4,785 Unvested restricted stock units outstanding 40 — Employee stock purchase plan 26 — 5,901 4,785

Supplemental Disclosures (Table

Supplemental Disclosures (Tables)	6 Months Ended
	Jun. 30, 2017
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, 2017 December 31, 2016 Accrued personnel costs $ 3,896 $ 3,338 Accrued external research and development expenses 2,133 3,005 Accrued external general and administrative expenses 943 672 Accrued purchases of property and equipment 50 844 Other accrued expenses and current liabilities 492 267 $ 7,514 $ 8,126

Nature of Business - Additional

Nature of Business - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Mar. 27, 2017	Jun. 30, 2017	Dec. 31, 2016
Class of Stock [Line Items]
Net proceeds from issuance of common stock		$ 81,994
Accumulated deficit		(151,053)	$ (114,587)
Cash, cash equivalents and marketable securities		$ 208,500
Follow-on Public Offering [Member]
Class of Stock [Line Items]
Number of shares issued to during period	3,700,000
Stock issuance price per share	$ 20.50
Follow-on Public Offering [Member] | Common Stock [Member]
Class of Stock [Line Items]
Net proceeds from issuance of common stock	$ 81,500
Overallotment Option [Member] | Maximum [Member] | Common Stock [Member]
Class of Stock [Line Items]
Number of shares issued to during period	555,000

Summary of Significant Accoun28

Summary of Significant Accounting Policies - Additional Information (Detail)	6 Months Ended
	Jun. 30, 2017
Accounting Policies [Abstract]
Estimated forfeiture rate on share-based awards granted	0.00%

Marketable Securities - Summary

Marketable Securities - Summary of Company Marketable Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 151,062	$ 133,724
Unrealized Gains	18	700
Unrealized Losses	(229)	(298)
Fair Value	150,851	134,126
Common Equity Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost		300
Unrealized Gains		618
Fair Value		918
Corporate Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	146,069	133,424
Unrealized Gains	18	82
Unrealized Losses	(229)	(298)
Fair Value	145,858	$ 133,208
Commercial Paper [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	4,993
Fair Value	$ 4,993

Marketable Securities - Additio

Marketable Securities - Additional Information (Detail)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2017USD ($)Bond	Jun. 30, 2016USD ($)	Jun. 30, 2017USD ($)Bond	Jun. 30, 2016USD ($)	Dec. 31, 2016USD ($)
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)	$ 200,000	(100,000)	$ 1,200,000
Unrealized gains (losses) available-for-sale securities, income tax expense	0	0	0	0
Realized gains or losses available for sale securities			480,000
Aggregate fair value of securities in an unrealized loss position for less than twelve months	$ 127,200,000		127,200,000		102,500,000
Other-than-temporary impaired			$ 0		$ 0
Number of investment grade corporate bonds | Bond	45		45
Audentes Therapeutics, Inc. [Member] | Common Equity Securities [Member] | Investment Income [Member]
Schedule Of Available For Sale Securities [Line Items]
Realized gains or losses available for sale securities	$ 0	$ 0	$ 500,000	$ 0

Fair Value of Financial Instr31

Fair Value of Financial Instruments - Schedule of Fair Value of Cash Equivalents and Marketable Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Available for sale securities	$ 150,851	$ 134,126
Assets fair value disclosure	208,500	158,966
Common Equity Securities [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Available for sale securities		918
Corporate Bonds [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Available for sale securities	145,858	133,208
Commercial Paper [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Available for sale securities	4,993
Money Market Mutual Funds [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Money market mutual funds (cash equivalents)	57,649	24,840
Quoted Prices in Active Markets (Level 1) [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets fair value disclosure		918
Quoted Prices in Active Markets (Level 1) [Member] | Common Equity Securities [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Available for sale securities		918
Significant Other Observable Inputs (Level 2) [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Assets fair value disclosure	208,500	158,048
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	145,858	133,208
Significant Other Observable Inputs (Level 2) [Member] | Commercial Paper [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Available for sale securities	4,993
Significant Other Observable Inputs (Level 2) [Member] | Money Market Mutual Funds [Member]
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Money market mutual funds (cash equivalents)	$ 57,649	$ 24,840

Fair Value of Financial Instr32

Fair Value of Financial Instruments - Additional Information (Detail)	Jun. 30, 2017USD ($)
Fair Value Disclosures [Abstract]
Fair value hierarchy level 1 to level 2 transfers amount	$ 0
Fair value hierarchy level 2 to level 1 transfers amount	$ 0

Property and Equipment Net - Sc

Property and Equipment Net - Schedule of Property and Equipment (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Property Plant And Equipment [Line Items]
Total property and equipment	$ 13,404	$ 9,948
Accumulated depreciation and amortization	(1,880)	(624)
Property and equipment, net	11,524	9,324
Computer Equipment and Software [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	1,276	1,038
Lab Equipment [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	5,896	2,780
Furniture and Fixtures [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	1,271	1,213
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Total property and equipment	$ 4,961	$ 4,917

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail)	1 Months Ended							6 Months Ended	12 Months Ended
	Nov. 30, 2016USD ($)	May 31, 2016USD ($)	Jan. 31, 2016USD ($)	Sep. 30, 2015USD ($)	Mar. 31, 2015	Nov. 30, 2014USD ($)	Mar. 31, 2009USD ($)	Jun. 30, 2017USD ($)Milestone	Dec. 31, 2016USD ($)
Other Commitments [Line Items]
Tenant improvement allowance received				$ 300,000
Restricted cash as collateral with financial institution								$ 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]
Liabilities related to third party opposition								$ 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								$ 700,000	200,000
GlaxoSmithKline LLC [Member]
Other Commitments [Line Items]
Percentage of ownership interest issued as consideration for license agreement							19.90%
Accrued expenses								$ 700,000	400,000
Number of milestone | Milestone								0
Milestone payment obligation							$ 1,500,000	$ 0
Regents of the University of Minnesota [Member]
Other Commitments [Line Items]
Number of milestone | Milestone								0
Milestone payments								$ 0
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								$ 100,000	$ 100,000
Regents of the University of Minnesota [Member] | Annual License Maintenance Fee [Member] | Maximum [Member]
Other Commitments [Line Items]
Contractual obligation						100,000
Regents of the University of Minnesota [Member] | Royalties And Sublicense Fees [Member] | Maximum [Member]
Other Commitments [Line Items]
Contractual obligation						$ 100,000
Rockville, Maryland [Member]
Other Commitments [Line Items]
Lease period			7 years 6 months	5 years	5 years 6 months
Lease expiration date			Sep. 30, 2023	Mar. 31, 2021	Sep. 30, 2020
Options to extend the lease					6 years
Lease commenced date			2016-02		2015-04
Lease term								September 2015, and again in November 2015: Extend the term of the lease for its existing space at that facility to October 2020. July 2017: Extend the term of the lease for its existing space at that facility to September 2021.
Tenant improvement allowance received			$ 700,000
Monthly lease payments commencement period			2016-09
New York, New York [Member]
Other Commitments [Line Items]
Lease expiration date		Oct. 31, 2020
Lease commenced date		2016-07
Monthly lease payments commencement period		2016-10
Lease expiration term		51 months
Letters of credit		$ 200,000

Commitments and Contingencies35

Commitments and Contingencies - Schedule of Future Minimum Lease Payments Under Non-Cancelable Operating Leases (Detail) $ in Thousands	Jun. 30, 2017USD ($)
Operating Leases Future Minimum Payments Due [Abstract]
2017 (remainder of year)	$ 802
2,018	1,638
2,019	1,687
2,020	1,626
2,021	665
Thereafter	952
Total minimum lease payments	$ 7,370

Commitments and Contingencies36

Commitments and Contingencies - Summary of Expenses Incurred by Company (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
GlaxoSmithKline LLC [Member]
Other Commitments [Line Items]
Total related party transaction expense	$ 672	$ 366	$ 862	$ 493
GlaxoSmithKline LLC [Member] | Sublicense Fees [Member]
Other Commitments [Line Items]
Total related party transaction expense	656	225	701	257
GlaxoSmithKline LLC [Member] | Royalties on Sales of Reagents [Member]
Other Commitments [Line Items]
Total related party transaction expense	2	4	2	6
GlaxoSmithKline LLC [Member] | Maintenance of Licensed Patents [Member]
Other Commitments [Line Items]
Total related party transaction expense	14	137	159	230
License Fees [Member] | The Trustees of the University of Pennsylvania [Member]
Other Commitments [Line Items]
Total related party transaction expense	745	246	874	309
License Fees [Member] | The Trustees of the University of Pennsylvania [Member] | Sublicense Fees [Member]
Other Commitments [Line Items]
Total related party transaction expense	656	225	701	257
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	7	4	9
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	$ 85	$ 14	$ 169	$ 43

Significant Agreements - Additi

Significant Agreements - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Significant Agreements [Line Items]
Milestone fee upon submission of preclinical regulatory filings				$ 500
Milestone fee upon commencement of clinical trials in humans				29,200
Milestone fee upon submission of regulatory approval filings				48,500
Milestone fee upon approval of commercial products by regulatory agencies				110,500
Milestone fee upon achievement of specified sales targets for licensed products				92,000
Licenses revenue		$ 6,555	$ 2,245	7,010	$ 2,573
AveXis, Inc. [Member]
Significant Agreements [Line Items]
License up-front fee	$ 6,000
Milestone fee payments upon achievement of various development and commercialization	$ 36,000
Licenses revenue		$ 6,000		$ 6,000

Significant Agreements - Schedu

Significant Agreements - Schedule of License Revenue (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
License Agreement Revenue Recognition [Abstract]
Up-front fees and option fees for commercial licenses	$ 6,000	$ 2,000	$ 6,000	$ 2,000
Maintenance fees for commercial licenses	140	80	390	330
Research and other license revenue	415	165	620	243
Licenses revenue including amounts from related parties	$ 6,555	$ 2,245	$ 7,010	$ 2,573

Stock-based Compensation - Addi

Stock-based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	6 Months Ended
	Jan. 31, 2017	Jun. 30, 2017	Jun. 30, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock-based compensation expense		$ 26,000
Proceeds from exercise of stock options		329	$ 130
Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock-based compensation expense		$ 26,000
Unrecognized stock-based compensation, weighted-average period		2 years 10 months 24 days
Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock-based compensation expense		$ 26,000
Unrecognized stock-based compensation, weighted-average period		2 years 10 months 24 days
Stock Option [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock-based compensation, weighted-average period		2 years 10 months 24 days
2015 Equity Incentive Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Additional shares to be issued	1,059,065
Common stock shares authorized for issuance		8,236,603
Shares available for future grants		1,972,348
Weighted-average fair values of options granted		$ 13.17
Exercise of stock options, Shares		99,594
Total intrinsic value of options exercised		$ 1,700
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		232,294
Common stock shares issued to participants		21,706

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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 2,483	$ 1,656	$ 5,074	$ 3,202
Stock Option [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	2,332	$ 1,656	4,771	$ 3,202
Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	69		138
Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 82		$ 165

Stock-based Compensation - St41

Stock-based Compensation - Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 2,483	$ 1,656	$ 5,074	$ 3,202
Research and Development Costs [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	1,137	619	2,374	1,011
General and Administrative [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 1,346	$ 1,037	$ 2,700	$ 2,191

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, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Number of Shares [Roll Forward]
Shares Outstanding, Beginning Balance	5,118
Shares, Granted	996
Shares, Exercised	(100)
Shares, Cancelled or forfeited	(179)
Shares Outstanding, Ending Balance	5,835	5,118
Shares, Exercisable	2,812
Shares, Vested and expected to vest	5,835
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted-average Exercise Price Outstanding, Beginning Balance	$ 7.77
Weighted-average Exercise Price, Granted	19.35
Weighted-average Exercise Price, Exercised	3.31
Weighted-average Exercise Price, Cancelled or forfeited	16.48
Weighted-average Exercise Price, Outstanding, Ending Balance	9.55	$ 7.77
Weighted-average Exercise Price, Exercisable	5.43
Weighted-average Exercise Price, Vested and expected to vest	$ 9.55
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted-average Remaining Contractual Life (Years) Outstanding	8 years 3 months 19 days	8 years 6 months
Weighted-average Remaining Contractual Life (Years), Exercisable	7 years 8 months 12 days
Weighted-average Remaining Contractual Life (Years), Vested and expected to vest	8 years 3 months 19 days
Aggregate Intrinsic Value Outstanding	$ 60,956	$ 57,400
Aggregate Intrinsic Value, Exercisable	40,881
Aggregate Intrinsic Value, Vested and expected to vest	$ 60,956

Stock-based Compensation - Su43

Stock-based Compensation - Summary of Unvested RSUs Activity Under 2015 Plan (Detail) - Restricted Stock Units [Member] shares in Thousands	Jun. 30, 2017$ / sharesshares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares Unvested, Beginning Balance | shares	40
Shares Unvested, Ending Balance | shares	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

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
FOXKISER,LLP [Member] | Service Agreements [Member] | Research and Development Costs [Member]
Related Party Transaction [Line Items]
Related party transaction expense	$ 0.4	$ 0.2	$ 0.8	$ 0.5
Special Scientific Advisor [Member]
Related Party Transaction [Line Items]
Related party transaction expenses from transactions with related party advisory fees	$ 0	$ 0.1	$ 0.1	$ 0.1
Shares, Granted			211,600

Net Loss Per Share - Schedules

Net 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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted weighted average shares outstanding	5,901	4,785	5,901	4,785
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	5,835	4,785	5,835	4,785
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, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
Accrued personnel costs	$ 3,896	$ 3,338
Accrued external research and development expenses	2,133	3,005
Accrued external general and administrative expenses	943	672
Accrued purchases of property and equipment	50	844
Other accrued expenses and current liabilities	492	267
Accrued expenses and other current liabilities	$ 7,514	$ 8,126