REGENXBIO (RGNX) 10-Q 5 Nov 15 2015 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2015	Nov. 05, 2015
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2015
Document Fiscal Year Focus	2,015
Document Fiscal Period Focus	Q3
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	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		26,313,471

Balance Sheets

Balance Sheets - USD ($)	Sep. 30, 2015	Dec. 31, 2014
Current assets
Cash and cash equivalents	$ 205,324,000	$ 1,121,000
Marketable securities	8,025,000
Accounts receivable
Trade receivables	609,000	805,000
Related party receivables		750,000
Unbilled receivables		327,000
Prepaid expenses	1,400,000	28,000
Other current assets	127,000
Total current assets	215,485,000	3,031,000
Marketable securities	11,013,000
Property and equipment, net	385,000	0
Cost method investments	303,000	303,000
Deferred issuance costs		157,000
Other assets	128,000
Total assets	227,314,000	3,491,000
Current liabilities
Accounts payable	3,105,000	334,000
Accrued expenses	1,727,000	1,115,000
Due to related party under services agreement		1,423,000
Related party promissory notes		2,403,000
Other related party payables	349,000	3,761,000
Advance payments	127,000	153,000
Total current liabilities	5,308,000	9,189,000
Deferred rent	198,000
Total liabilities	$ 5,506,000	$ 9,189,000
Commitments and contingencies (Note 7)
Convertible preferred stock
Convertible preferred stock, value		$ 12,593,000
Stockholders' equity (deficit)
Preferred stock; $0.0001 par value; no shares authorized, issued and outstanding at December 31, 2014; 10,000 shares authorized, and no shares issued and outstanding at September 30, 2015
Common stock; $0.0001 par value; 9,500 and 100,000 shares authorized at December 31, 2014 and September 30, 2015, respectively; 2,645 and 26,306 shares issued and outstanding at December 31, 2014 and September 30, 2015, respectively	$ 3,000
Additional paid-in capital	268,275,000	$ 10,518,000
Accumulated other comprehensive loss	(26,000)
Accumulated deficit	(46,444,000)	(28,809,000)
Total stockholders' equity (deficit)	221,808,000	(18,291,000)
Total liabilities, convertible preferred stock and stockholders' equity (deficit)	$ 227,314,000	3,491,000
Series A Convertible Preferred Stock [Member]
Convertible preferred stock
Convertible preferred stock, value		3,963,000
Series B Convertible Preferred Stock [Member]
Convertible preferred stock
Convertible preferred stock, value		$ 8,630,000

Balance Sheets (Parenthetical)

Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2015	Dec. 31, 2014
Convertible preferred stock, par value		$ 0.0001
Convertible preferred stock, shares authorized		4,299,000
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	10,000,000	0
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	9,500,000
Common stock, shares issued	26,306,000	2,645,000
Common stock, shares outstanding	26,306,000	2,645,000
Series A Convertible Preferred Stock [Member]
Convertible preferred stock, par value		$ 0.0001
Convertible preferred stock, shares authorized	0	2,393,000
Convertible preferred stock, shares issued	0	2,393,000
Convertible preferred stock, shares outstanding	0	2,393,000
Series B Convertible Preferred Stock [Member]
Convertible preferred stock, par value		$ 0.0001
Convertible preferred stock, shares authorized	0	1,906,000
Convertible preferred stock, shares issued	0	1,906,000
Convertible preferred stock, shares outstanding	0	1,906,000

Statements of Operations and Co

Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Revenues
License revenue	$ 65		$ 635	$ 3,705
License revenue from related party	1,000	$ 150	2,000	150
Reagent sales	61	23	200	317
Grant revenue	14	310	305	800
Total revenues	1,140	483	3,140	4,972
Costs of revenues
Licensing costs to related parties	213		527	741
Costs of reagent sales (including amounts to related parties)	44	12	94	115
Research and development (including amounts to related parties)	5,664	1,085	12,471	2,869
General and administrative (including amounts to related parties)	2,567	1,070	7,671	2,732
Foreign currency transaction losses (gains)	4	25	41	12
Other operating income	(5)	(8)	(26)	(32)
Total operating expenses	8,487	2,184	20,778	6,437
Loss from operations	(7,347)	(1,701)	(17,638)	(1,465)
Other Income (Expense)
Investment income	15		23
Interest expense		(92)	(20)	(202)
Total other income (expense)	15	(92)	3	(202)
Net loss	(7,332)	(1,793)	(17,635)	(1,667)
Other Comprehensive Loss
Unrealized loss on available-for-sale securities	(26)		(26)
Total other comprehensive loss	(26)		(26)
Comprehensive loss	(7,358)	(1,793)	(17,661)	(1,667)
Reconciliation of net loss to net loss applicable to common stockholders
Net loss	(7,332)	(1,793)	(17,635)	(1,667)
Accretion and dividends on convertible preferred stock		(126)	(1,747)	(592)
Net gain on extinguishment of convertible preferred stock			759
Net loss applicable to common stockholders	$ (7,332)	$ (1,919)	$ (18,623)	$ (2,259)
Basic and diluted net loss per common share	$ (1.52)	$ (0.73)	$ (5.48)	$ (0.85)
Weighted-average basic and diluted common shares	4,809	2,643	3,397	2,643

Statements of Cash Flows

Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Cash flows from operating activities
Net loss	$ (17,635)	$ (1,667)
Adjustments to reconcile net loss to net cash used in operating activities
Unrealized foreign currency transaction gains	(4)	(25)
Stock-based compensation expense	2,059	220
Imputed interest on related party promissory notes	13	45
Other non-cash adjustments		1
Depreciation and amortization	43
Amortization of premiums on marketable debt securities	1
Changes in operating assets and liabilities
Trade receivables	200	(1,097)
Related party receivables	750	774
Unbilled receivables	327	30
Prepaid expenses	(1,372)	(131)
Other current assets	(127)
Other assets	(128)
Accounts payable	317	440
Accrued expenses	758	295
Due to related party under services agreement	(34)	(255)
Other related party payables	(3,412)	(1,335)
Advance payments	(26)	168
Deferred rent	198
Net cash used in operating activities	(18,072)	(2,537)
Cash flows from investing activities
Purchases of marketable securities	(19,065)
Purchases of property and equipment	(394)
Net cash used in investing activities	(19,459)
Cash flows from financing activities
Proceeds from related party promissory notes		2,400
Proceeds from exercise of stock options	100
Proceeds from initial public offering of common stock, net of underwriting discounts and commissions	148,233
Payments of initial public offering costs	(618)
Net cash provided by financing activities	241,734	2,400
Net increase (decrease) in cash and cash equivalents	204,203	(137)
Cash and cash equivalents
Beginning of period	1,121	1,119
End of period	205,324	982
Supplemental cash flow information
Cash paid for interest	7	$ 164
Supplemental disclosures of non-cash investing and financing activities
Conversion of accrued service fees into Series C convertible preferred stock	2,403
Conversion of related party promissory notes into Series C convertible preferred stock	1,389
Conversion of convertible preferred stock into common stock upon initial public offering	111,392
Purchases of property and equipment in accounts payable	34
Issuance costs for initial public offering in accounts payable and accrued expenses	2,431
Series C Convertible Preferred Stock [Member]
Cash flows from financing activities
Proceeds from issuance of convertible preferred stock, net of transaction costs	26,021
Series D Convertible Preferred Stock [Member]
Cash flows from financing activities
Proceeds from issuance of convertible preferred stock, net of transaction costs	$ 67,998

Nature of Business

Nature of Business	9 Months Ended
	Sep. 30, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of Business	1. Nature of Business REGENXBIO Inc. (the Company) was formed on July 16, 2008 in the state of Delaware as ReGenX, LLC, and on December 22, 2009, changed its name to ReGenX Biosciences, LLC. On September 16, 2014, the Company converted from a limited liability company (LLC) to a C-corporation, and changed its name to REGENXBIO Inc. The Company uses its proprietary NAV ® Initial Public Offering On September 22, 2015, the Company completed its initial public offering (IPO) whereby the Company sold 7,245 shares of common stock (inclusive of 945 shares of common stock sold by the Company pursuant to the full exercise of an option to purchase additional shares granted to the underwriters in connection with the offering) at a price of $22.00 per share. The shares began trading on the Nasdaq Global Select Market on September 17, 2015. The aggregate net proceeds received by the Company from the offering were $145,184, net of underwriting discounts and commissions and offering expenses payable by the Company. Upon the closing of the IPO, all outstanding shares of convertible preferred stock converted into 16,298 shares of common stock. Liquidity and Risks As of September 30, 2015, the Company has generated an accumulated deficit of $46,444 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. 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 preclinical manufacturing to commercial production of products.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2015
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation and Unaudited Interim Financial Information The accompanying 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 United States generally accepted accounting principles as found in the Accounting Standards Codification (ASC) and Accounting Standards Update (ASU) of the Financial Accounting Standards Board (FASB). Certain information and footnote disclosures normally included in the Company’s annual financial statements have been condensed or omitted. The interim unaudited financial statements have been prepared on the same basis as the annual audited financial statements as of and for the year ended December 31, 2014 included in the prospectus dated September 16, 2015, filed pursuant to Rule 424(b)(4) under the Securities Act of 1933, as amended, with the Securities and Exchange Commission on September 17, 2015 (the Prospectus). In the opinion of management, the accompanying financial statements reflect all adjustments, which include only normal recurring adjustments necessary for the fair statement of the Company’s financial position as of September 30, 2015, and the results of its operations and its cash flows for the interim periods ended September 30, 2014 and 2015. 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 financial statements should be read in conjunction with the audited financial statements as of and for the year ended December 31, 2014, and the notes thereto, which are included in the Prospectus. 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 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 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. Marketable Securities The Company’s marketable securities are classified as available-for-sale and are carried at fair value. Marketable securities with remaining maturity dates exceeding twelve months which are not intended to be sold prior to maturity for use in current operations are classified as non-current. Unrealized gains and losses, net of any related tax effects, are excluded from earnings and are included in other comprehensive loss and reported as a separate component of stockholders’ equity (deficit) until realized. A decline in the fair value below cost of available-for-sale securities that is deemed other-than-temporary is charged to results of operations, resulting in the establishment of a new cost basis for the security. Premiums and discounts are amortized or accreted into the cost basis over the life of the related security as adjustments to the yield using the effective-interest method. Interest income is recognized when earned. Realized gains and losses from the sale of marketable securities are based on the specific identification method and are included in results of operations. The Company regularly evaluates whether declines in the fair value of its investments below their cost are other-than-temporary. The evaluation includes consideration of the cause of the impairment, including the creditworthiness of the security issuers, the number of securities in an unrealized loss position, the severity and duration of the unrealized losses, whether the Company has the intent to sell the securities and whether it is more likely than not that the Company will be required to sell the securities before the recovery of their amortized cost basis. If the Company determines that the decline in fair value of an investment is below its accounting basis and this decline is other-than-temporary, the Company would reduce the carrying value of the security it holds and record a loss for the amount of such decline. The Company has not recorded any declines in value judged to be other-than-temporary on its investments. Concentrations of Credit Risk and Off-balance Sheet Risk Cash and cash equivalents, marketable securities and accounts receivable are financial instruments that are potentially subject to concentrations of credit risk. The Company’s cash and cash equivalents are deposited in accounts at multiple financial institutions, and amounts may exceed federally insured limits. The Company believes it is not exposed to significant credit risk due to the financial strength of the depository institutions in which the cash is held. The Company’s marketable securities consist of fixed income corporate debt securities and may subject the Company to concentrations of credit risk. The Company has adopted an investment policy, under which its marketable securities are purchased, which limits potential concentrations of investments and establishes minimum acceptable credit ratings, thereby reducing credit risk exposure. Management believes that it is not exposed to significant credit risk related to accounts receivable due to the credit quality and history of collections from its significant customers. The Company has no financial instruments with off-balance sheet risk of loss. 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. Please refer to Note 4 for further information on the fair value measurement of the Company’s financial instruments. Net Loss Per Share The Company computes net loss per share in conformity with the two-class method required for participating securities. The Company considers all series of convertible preferred stock outstanding prior to the IPO to be participating securities. The holders of convertible preferred stock outstanding prior to the IPO were entitled to receive preferential dividends in the event that a dividend was to be paid to the holders of common stock. The holders of convertible preferred stock outstanding prior to the IPO did not have a contractual obligation to share in the losses of the Company. As such, the Company’s net losses for the three months ended September 30, 2014 and 2015, and the nine months ended September 30, 2014 and 2015, were not allocated to these participating securities. In connection with the IPO, all outstanding shares of convertible preferred stock were automatically converted into shares of common stock. 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, convertible preferred stock, stock options and debt instruments containing share settlement options 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 and unit were the same for all periods presented. 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: Three and Nine Months Ended September 30, 2014 2015 Series A convertible preferred stock 2,393 — Series B convertible preferred stock 1,906 — Stock options issued and outstanding 1,885 3,464 Debt with share settlement option (Note 6) 3,295 — 9,479 3,464 Amounts in the table above reflect the common stock equivalents of the noted instruments. Comprehensive Loss The Company’s comprehensive loss includes its net loss as well as net unrealized losses on available-for-sale securities. Recently Announced Accounting Pronouncements In February 2015, the FASB issued ASU 2015-2, Consolidation (Topic 810): Amendments to the Consolidation Analysis In August 2014, the FASB issued ASU 2014-15, Presentation of Financial Statements—Going Concern (Subtopic 205-40): Disclosure of Uncertainties about an Entity’s Ability to Continue as a Going Concern In June 2014, the FASB issued ASU No. 2014-12, Compensation—Stock Compensation (Topic 718): Accounting for Share-Based Payments When the Terms of an Award Provide That a Performance Target Could be Achieved after the Requisite Service Period In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606) Revenue Recognition Revenue from Contracts with Customers (Topic 606)

Marketable Securities

Marketable Securities	9 Months Ended
	Sep. 30, 2015
Investments, Debt and Equity Securities [Abstract]
Marketable Securities	3. Marketable Securities The following table presents a summary of the Company’s marketable securities, which consist solely of available-for-sale securities: Amortized Unrealized Unrealized Fair September 30, 2015 Corporate debt securities $ 19,064 $ 2 $ (28 ) $ 19,038 $ 19,064 $ 2 $ (28 ) $ 19,038 The Company did not hold any available-for-sale securities as of December 31, 2014. As of September 30, 2015, 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 September 30, 2015, the balance in the Company’s accumulated other comprehensive loss consisted solely of net unrealized gains and losses on available-for-sale securities. For the nine months ended September 30, 2015, the Company recognized net unrealized losses on available-for-sale securities of $26, which is included in other comprehensive loss. There were no realized gains or losses recognized on the sale or maturity of available-for-sale securities during the nine months ended September 30, 2015, and as a result, the Company did not reclassify any amounts out of accumulated other comprehensive loss for the period. As of September 30, 2015, the Company did not hold any available-for-sale securities in an unrealized loss position for more than twelve months. The aggregate fair value of securities held by the Company in an unrealized loss position for less than twelve months as of September 30, 2015 was $18,012, which consisted of corporate debt securities with credit ratings of A-2/A or higher. The Company has the intent and ability to hold such securities until recovery. The Company determined that there have been no material changes in the credit risk of these investments since their initial purchases. As a result, the Company determined that none of these investments are other-than-temporarily impaired as of September 30, 2015.

Fair Value of Financial Instrum

Fair Value of Financial Instruments	9 Months Ended
	Sep. 30, 2015
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. The following tables present the fair value of cash equivalents and marketable securities in accordance with the hierarchy discussed in Note 2: Quoted Significant prices other Significant in active observable unobservable markets inputs inputs (Level 1) (Level 2) (Level 3) Total December 31, 2014 Money market mutual funds (cash equivalents) $ — $ 844 $ — $ 844 $ — $ 844 $ — $ 844 Quoted Significant prices other Significant in active observable unobservable markets inputs inputs (Level 1) (Level 2) (Level 3) Total September 30, 2015 Money market mutual funds (cash equivalents) $ — $ 205,324 $ — $ 205,324 Corporate debt securities (marketable securities) — 19,038 — 19,038 $ — $ 224,362 $ — $ 224,362 Management estimates that the carrying amounts of its accounts receivable, accounts payable, accrued expenses and related party payables approximate fair value due to the short-term nature of those instruments. Certain debt instruments (Note 6) outstanding at December 31, 2014 accrued interest at below market rates. The Company has recorded a discount to the face value of the instrument to account for the difference between the present value of the debt at an estimated market rate versus face value. Accordingly, management believes that the carrying values of all debt instruments approximate fair value. The Company has determined that it is not practicable to estimate the fair value of cost method investments. The Company has not identified any events or changes in circumstances that would have an adverse effect on the fair value of its cost method investments.

Property and Equipment, Net

Property and Equipment, Net	9 Months Ended
	Sep. 30, 2015
Property, Plant and Equipment [Abstract]
Property and Equipment, Net	5. Property and Equipment, Net Property and equipment, net consists of the following as of September 30, 2015: September 30, 2015 Computer equipment and software $ 332 Furniture and fixtures 94 Leasehold improvements 2 Total property and equipment 428 Accumulated depreciation and amortization (43 ) Property and equipment, net $ 385 As of December 31, 2014, the Company had recorded no property and equipment because the Company’s resources used in operations were provided by a related party (Note 12).

Related Party Debt Instruments

Related Party Debt Instruments	9 Months Ended
	Sep. 30, 2015
Debt Disclosure [Abstract]
Related Party Debt Instruments	6. Related Party Debt Instruments As of December 31, 2014, the Company had accrued $1,423 payable to FoxKiser, LLP (FoxKiser) under a services agreement (Note 12), which may be settled in preferred or common shares. Additionally, the Company had $2,403 of principal and interest due under related party promissory notes to FoxKiser, which also may be settled in preferred or common shares. On January 13, 2015, in conjunction with the Company’s issuance of Series C Preferred Stock (Note 8), FoxKiser elected its share settlement options and converted $1,389 of the amount due under the services agreement and $2,403 of principal and interest due under the promissory notes, for a total of $3,792, into 585 shares of Series C Preferred Stock at a per share price of $6.477. No amounts were outstanding under the services agreement or promissory notes as of September 30, 2015.

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2015
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	7. Commitments and Contingencies Lease Agreements The Company recognizes rent expense on a straight-line basis over the term of its operating leases. The Company has entered into an operating lease for laboratory space in Philadelphia, Pennsylvania for use in its research and development activities. The lease is renewed in six-month terms and as of September 30, 2015 had an expiration date of December 31, 2015. Monthly rent under the lease agreement is $4. Effective January 31, 2015, the services agreement with FoxKiser (Note 12) was terminated and the Company entered into an operating lease with FoxKiser for office space in Washington, D.C. The lease agreement, which had a month-to-month term, required monthly payments of $20. The lease was terminated in April 2015. In March 2015, the Company entered into a 5.5-year, non-cancelable operating lease for office space 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. Initial monthly payments required under the lease were $24 and escalate annually in accordance with the lease. In September 2015, the Company amended its operating lease in Rockville, Maryland to include additional office and laboratory space, and extend the term of the existing lease to October 2020. The lease for the additional space is expected to commence in March 2016, and has a 5-year term. The Company has options to extend the lease for the additional space to be coterminous with the Company’s existing lease at that facility. Initial monthly payments required under the lease for the additional space are $41 and escalate annually in accordance with the lease. As of September 30, 2015, future minimum lease payments under non-cancelable operating leases are as follows: Operating Leases 2015 (remainder of year) $ 73 2016 721 2017 825 2018 850 2019 875 2020 833 Thereafter 96 Total minimum lease payments $ 4,273 License Agreements Granted to the Company License agreements granted to the Company may require the Company to make future payments relating to sublicense fees, milestone fees for milestones not met as of December 31, 2014 and September 30, 2015 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. The Trustees of the University of Pennsylvania GlaxoSmithKline LLC ARIAD Pharmaceuticals, Inc. Regents of the University of Minnesota 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 December 31, 2014 and September 30, 2015, the Company did not have any material indemnification claims that were probable or reasonably possible and consequently has not recorded any related liabilities.

Capitalization

Capitalization	9 Months Ended
	Sep. 30, 2015
Equity [Abstract]
Capitalization	8. Capitalization As of September 30, 2015, the authorized capital stock of the Company included 100,000 shares of common stock, par value $0.0001 per share, and 10,000 shares of preferred stock, par value $0.0001 per share. The Company’s restated certificate of incorporation and bylaws, which were adopted in connection with the IPO, contain the rights, preferences and privileges of the Company’s stockholders and their respective shares. As of December 31, 2014, the authorized capital stock of the Company included 9,500 shares of common stock, par value $0.001 per share, and 4,299 shares of preferred stock, par value $0.0001 per share. The Company’s authorized preferred stock included 2,393 shares designated as Series A Preferred Stock and 1,906 shares designated as Series B Preferred Stock. Issuance of Series C Preferred Stock On January 13, 2015, the Company completed the issuance and sale of 4,632 shares of Series C Preferred Stock, par value $0.0001 per share, at a per share price of $6.477 for aggregate gross proceeds of $30,000. The aggregate purchase price of $30,000 included $26,021 of cash proceeds, net of issuance costs of $187, and the conversion of $3,792 of debt as discussed in Notes 6 and 12. Along with the issuance of the Series C Preferred Stock, the Company increased the number of shares reserved for future issuance under the 2014 Stock Plan to 2,800. Extinguishment of Preferred Stock In connection with the issuance of the Series C Preferred Stock in January 2015, the rights, preferences and privileges of Series A Preferred Stock and Series B Preferred Stock then outstanding were modified. More specifically, Series C Preferred Stock received preference in dividends and liquidation proceeds over Series A Preferred Stock and Series B Preferred Stock. Additionally, the dividend rights changed from cumulative dividend rights to non-cumulative dividend rights, and all accrued but unpaid cumulative dividends on the Series A Preferred Stock and Series B Preferred Stock as of January 13, 2015 were forfeited. As a result of this modification, the redemption value and liquidation preferences of Series A Preferred Stock and Series B Preferred Stock, which were previously equal to original issue price plus accrued but unpaid cumulative dividends, were reduced to original issue price plus non-cumulative dividends declared. Additionally, the redemption date of Series A Preferred Stock and Series B Preferred Stock was changed from October 30, 2018 to December 31, 2019. The Company has accounted for the amendment to the rights, preferences and privileges of the Series A Preferred Stock and Series B Preferred Stock as an extinguishment of the original convertible preferred stock and issuance of new convertible preferred stock due to the significance of the modifications to the substantive contractual terms of the convertible preferred stock and the associated fundamental changes to the nature of the convertible preferred stock. Accordingly, upon extinguishment the Company recorded a loss of $1,317 on the Series A Preferred Stock and a gain of $2,076 on the Series B Preferred Stock within stockholders’ equity (deficit) equal to the difference between the fair value of the new shares of preferred stock issued and the carrying amount of the old shares of preferred stock extinguished. The Company allocated the entire net gain on extinguishment of convertible preferred stock of $759 to additional paid-in capital. The net gain on extinguishment is reflected in the calculation of net loss available to common stockholders in accordance with FASB ASC Topic 260, Earnings per Share Issuance of Series D Preferred Stock On May 15, 2015, the Company completed the sale and issuance of 7,367 shares of Series D Preferred Stock, par value $0.0001 per share, at a per share price of $9.5699 for proceeds of $67,998, net of issuance costs of $2,502. Along with the issuance of the Series D Preferred Stock, the Company increased the number of shares reserved for future issuance under the 2014 Stock Plan to 4,100. In connection with the issuance of the Series D Preferred Stock, the Company amended and restated its certificate of incorporation. Series A Preferred Stock, Series B Preferred Stock, Series C Preferred Stock and Series D Preferred Stock are herein collectively referred to as Preferred Stock. Under the amended and restated certificate of incorporation in effect prior to the IPO, the rights, preferences and privileges of Preferred Stock consisted of the following: Dividends. Liquidation Preference. Redemption. Conversion. Each share of Preferred Stock was to automatically convert into shares of common stock at the applicable conversion rate upon (i) a qualified public offering, as defined in the Company’s amended and restated certificate of incorporation, of at least $40,000 in aggregate gross proceeds, prior to deduction of underwriting discounts and commissions, or (ii) the majority vote of the holders of Preferred Stock on a per share and as-converted to common stock basis. Voting. As long as shares of Series A Preferred Stock remained outstanding, the holders of Series A Preferred Stock, voting as a separate class, were entitled to elect three directors to the Board of Directors. As long as shares of Series B Preferred Stock remained outstanding, the holders of Series B Preferred Stock, voting as a separate class, were entitled to elect one director to the Board of Directors. As long as shares of Series C Preferred Stock remained outstanding, the holders of Series C Preferred Stock, voting as a separate class, were entitled to elect one director to the Board of Directors. As long as shares of Series D Preferred Stock remained outstanding, the holders of Series D Preferred Stock, voting as a separate class, were entitled to elect one director to the Board of Directors. The holders of outstanding common stock, voting as a separate class, were entitled to elect one director to the Board of Directors. The holders of Preferred Stock and common stock, voting together as a single class on an as-converted basis for Preferred Stock, were entitled to elect any remaining directors of the Company. Prior to the closing of the IPO on September 22, 2015, all of the outstanding shares of the Company’s Preferred Stock were converted into 16,298 shares of its common stock. As of September 30, 2015, the Company did not have any convertible preferred stock issued or outstanding. Common stock Prior to the closing of the Company’s IPO on September 22, 2015, the dividend, liquidation and voting rights of the Company’s common stockholders were subject to, and qualified by, the rights, preferences and privileges of the holders of the outstanding Preferred Stock. The Company’s reserved shares of common stock for future issuance are as follows: December 31, September 30, 2014 2015 Series A convertible preferred stock 2,393 — Series B convertible preferred stock 1,906 — Reserved for issuance under equity incentive plans 2,500 6,005 Reserved for issuance under employee stock purchase plan — 254 Debt with share settlement option (Note 6) 3,715 — 10,514 6,259

Significant Agreements

Significant Agreements	9 Months Ended
	Sep. 30, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Significant Agreements	9. Significant Agreements See Note 12 for significant agreements with related parties and Note 7 for license agreements granted to the Company. License Agreements 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. 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 specific clinical and regulatory developments by licensees. As of September 30, 2015, 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 $500 upon the submission of preclinical regulatory filings, $11,300 upon the commencement of various stages of clinical trials, $20,000 upon the submission of regulatory approval filings, $51,500 upon the approval of commercial products by regulatory agencies and $2,000 upon the achievement of specified sales targets for licensed products. License revenue, including amounts from related parties, recognized for the three months ended September 30, 2014 and 2015, and the nine months ended September 30, 2014 and 2015, is as follows: Three Months Ended Nine Months Ended September 30, September 30, 2014 2015 2014 2015 Up-front fees and option fees for commercial licenses $ — $ 1,000 $ 3,600 $ 2,000 Milestone fees for commercial licenses — — — 250 Maintenance fees for commercial licenses — 65 65 245 Research and other license revenue 150 — 190 140 $ 150 $ 1,065 $ 3,855 $ 2,635

Stock-based Compensation

Stock-based Compensation	9 Months Ended
	Sep. 30, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock-based Compensation	10. Stock-based Compensation Equity Incentive Plans In June 2015, the Company’s Board of Directors adopted the 2015 Equity Incentive Plan (2015 Plan). The 2015 Plan replaces the 2014 Stock Plan (2014 Plan). Any options or awards outstanding under the 2014 Plan at the effective date of the 2015 Plan remain outstanding and effective. As of September 30, 2015, the total number of shares of common stock authorized for issuance under the 2015 Plan and 2014 Plan is 6,005, of which 2,541 remain available for future grants. The 2015 Plan provides for the issuance of stock options, stock appreciation rights, restricted and unrestricted stock awards and performance cash awards to employees, members of the Board of Directors and consultants of the Company. No stock appreciation rights, restricted or unrestricted stock awards or performance cash awards have been granted under the 2014 Plan or the 2015 Plan, since the inception of the plans. The initial amount of shares authorized for issuance under the 2015 Plan was 2,952. The number of authorized shares under the 2015 Plan automatically increases annually on January 1, beginning January 1, 2016, by the lesser of (i) 4% of the total number of shares of common stock outstanding on December 31 of the prior year, or (ii) a number of common shares determined by the Board of Directors. Stock options under the 2015 Plan generally expire ten years following the date of grant. Options typically vest over a four year period, but vesting provisions can vary by award based on the discretion of the Board of Directors. Certain awards issued by the Company include performance conditions that must be achieved in order for vesting to occur. Stock options under the 2015 Plan carry an exercise price at least equal to the estimated fair value of the Company’s common stock on the date of grant. The following table summarizes stock option activity under the Company’s equity incentive plans: Weighted- average Weighted- Remaining average Contractual Aggregate Exercise Life Intrinsic Shares Price (Years) Value (a) Outstanding at December 31, 2014 2,107 $ 0.85 9.8 $ 379 Granted 1,498 $ 9.05 Exercised (118 ) $ 0.85 Cancelled or forfeited (23 ) $ 0.85 Outstanding at September 30, 2015 3,464 $ 4.39 9.3 $ 61,097 Exercisable at September 30, 2015 1,182 $ 1.20 9.1 $ 24,629 Vested and expected to vest at September 30, 2015 3,464 $ 4.39 9.3 $ 61,097 (a) The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the estimated fair value of the common stock for the options that were in the money at September 30, 2015. The weighted-average grant date fair value of options granted during the nine months ended September 30, 2015 was $5.88. During the nine months ended September 30, 2015, the total number of stock options exercised was 118, resulting in total proceeds of $100. The total intrinsic value of options exercised during the nine months ended September 30, 2015 was $354. Stock-based compensation expense for the three months ended September 30, 2014 and 2015, and the nine months ended September 30, 2014 and 2015, relates solely to stock options granted under the 2014 Plan and the 2015 Plan. The Company has recorded aggregate stock-based compensation expense related to the issuance of stock option awards to employees and non-employees in the statement of operations and comprehensive loss as follows: Three Months Ended Nine Months Ended September 30, September 30, 2014 2015 2014 2015 Research and development $ 16 $ 1,101 $ 16 $ 1,413 General and administrative 204 248 204 646 $ 220 $ 1,349 $ 220 $ 2,059 Stock Options Granted to Employees. Three Months Ended Nine Months Ended September 30, September 30, 2014 2015 2014 2015 Expected volatility 65 % 75 % 65 % 67 % Expected term (in years) 5.9 6.2 5.9 6.1 Risk-free interest rate 2.0 % 1.8 % 2.0 % 1.7 % Expected dividend yield 0.0 % 0.0 % 0.0 % 0.0 % As of September 30, 2015, there was $4,999 of unrecognized stock-based compensation expense related to employees’ awards that is expected to be recognized over a weighted-average period of 3.2 years. Stock Options Granted to Non-employees. Three Months Ended Nine Months Ended September 30, September 30, 2014 2015 2014 2015 Expected volatility 65 % 81 % 65 % 71 % Expected term (in years) 10.0 9.3 10.0 9.7 Risk-free interest rate 2.6 % 2.1 % 2.6 % 2.1 % Expected dividend yield 0.0 % 0.0 % 0.0 % 0.0 % Employee Stock Purchase Plan In June 2015, the Company’s Board of Directors adopted the 2015 Employee Stock Purchase Plan (2015 ESPP), which became effective on September 16, 2015, the date on which the registration statement for the IPO was declared effective. The 2015 ESPP authorizes the initial issuance of up to a total of 254 shares of the Company’s common stock to participating employees. The number of shares reserved for issuance under the 2015 ESPP automatically increases on the first business day of each fiscal year, commencing in 2016, by a number equal to the lesser of (i) 1% of the shares of common stock outstanding on the last business day of the prior fiscal year; or (ii) the number of shares determined by the Company’s Board of Directors. Unless otherwise determined by the administrator of the 2015 ESPP, two offering periods of six months’ duration will begin each year on January 1 and July 1. As of September 30, 2015 there was no activity under the 2015 ESPP.

Retirement Plan

Retirement Plan	9 Months Ended
	Sep. 30, 2015
Postemployment Benefits [Abstract]
Retirement Plan	11. Retirement Plan As of December 31, 2014, the Company did not sponsor any retirement plans. In February 2015, the Company established a defined-contribution retirement plan under Section 401(k) of the Internal Revenue Code (the 401(k) Plan). The 401(k) Plan covers all employees who meet defined minimum age and service requirements, and allows participants to defer a portion of their annual compensation on a pre-tax basis. The Company matches employee deferrals up to 5.75 percent of eligible compensation. For the three months and the nine months ended September 30, 2015, the Company contributed $56 and $120, respectively, in matching contributions to the 401(k) Plan.

Related Party Transactions

Related Party Transactions	9 Months Ended
	Sep. 30, 2015
Related Party Transactions [Abstract]
Related Party Transactions	12. Related Party Transactions The Trustees of the University of Pennsylvania On February 20, 2009, the Company entered into a license agreement, as amended, with Penn for exclusive, worldwide rights to certain patents owned by Penn. Under the terms of the agreement, in consideration for the license, the Company issued to Penn 24.5 percent of the then outstanding membership interest in the LLC 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. Penn also provides manufacturing services and research and development services to the Company. Expenses incurred by the Company under its license from Penn, as well as for manufacturing and research and development for the three months ended September 30, 2014 and 2015, and the nine months ended September 30, 2014 and 2015, are as follows: Three Months Ended Nine Months Ended September 30, September 30, 2014 2015 2014 2015 Sublicense fees $ — $ 107 $ 371 $ 264 Royalties on sales of reagents 5 2 16 8 Maintenance of licensed patents 33 29 191 118 Manufacturing of reagents for sale 5 40 87 80 Research and development services 142 2,493 624 5,136 $ 185 $ 2,671 $ 1,289 $ 5,606 Sublicense fees are included in licensing costs to related parties in the statements of operations. Royalties on sales of reagents and manufacturing of reagents for sale are included in costs of reagent sales in the statements of operations. Maintenance of licensed patents is included in general and administrative expenses in the statements of operations. Research and development services are included in research and development expenses in the statements of operations. As of December 31, 2014 and September 30, 2015, the Company had accrued $1,732 and $163, respectively, in expenses payable to Penn which are included in other related party payables on the balance sheets. As a result of various factors, including the dilution to the Company’s stockholders resulting from the sale and issuance of Series C Preferred Stock and Series D Preferred Stock, and the closing of the Company’s IPO on September 22, 2015, the Company no longer considers Penn to be a related party subsequent to the period ended September 30, 2015. GlaxoSmithKline LLC On March 6, 2009, the Company entered into a license agreement, as amended, with GSK for exclusive, worldwide rights to certain patents owned by Penn and exclusively licensed to GSK. Under the terms of the agreement, in consideration for the license the Company issued to GSK 19.9 percent of then outstanding membership interest in the LLC 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 $1,650 upon the achievement of various milestones. As of September 30, 2015, no milestones have been achieved and accordingly no milestone payments were payable to GSK which are included in other related party payables on the balance sheets. Expenses incurred by the Company under its license from GSK for the three months ended September 30, 2014 and 2015, and the nine months ended September 30, 2014 and 2015, are as follows: Three Months Ended Nine Months Ended September 30, September 30, 2014 2015 2014 2015 Sublicense fees $ — $ 107 $ 371 $ 264 Royalties on sales of reagents 3 1 10 5 Maintenance of licensed patents 108 57 324 474 $ 111 $ 165 $ 705 $ 743 Sublicense fees are included in licensing costs to related parties in the statements of operations. Royalties on sales of reagents are included in costs of reagent sales in the statements of operations. Maintenance of licensed patents is included in general and administrative expenses in the statements of operations. As of December 31, 2014 and September 30, 2015, the Company had accrued $2,028 and $186, respectively, payable to GSK under the license agreement. As a result of various factors, including the dilution to the Company’s stockholders resulting from the sale and issuance of Series C Preferred Stock and Series D Preferred Stock, and the closing of the Company’s IPO on September 22, 2015, the Company no longer considers GSK to be a related party subsequent to the period ended September 30, 2015. Dimension Therapeutics, Inc. On October 30, 2013, the Company granted an exclusive, sublicensable, worldwide commercial license to Dimension Therapeutics, Inc. (Dimension) for preclinical and clinical research and development and commercialization of drug therapies using the Company’s licensed patents for the treatment of hemophilia A and hemophilia B, as well as a one year option to obtain exclusive licenses for the commercialization of two other diseases to be elected by Dimension in the future. The agreement requires on-going annual maintenance fees of $35, for each indication elected by Dimension, beginning in October 2014. The agreement also requires Dimension to pay royalties on net sales, if any, to the Company at an amount intended to approximate the royalties that will be due by the Company to Penn and GSK on such sales. In consideration for the license granted, Dimension issued the Company, and various members, directors and executives of the Company, an aggregate total of 10,000 shares of its common stock. Of the 10,000 shares, a total of 10 shares were issued to the Company, with an estimated fair value of $3, which is included in cost method investments on the balance sheets. In addition to related parties of the Company holding common stock in Dimension as a result of the license agreement noted above, three of the Company’s board members served on the board of directors of Dimension on the effective the date of the license. Management has evaluated consolidation guidance under FASB ASC Topic 810, Consolidation In connection with the license agreement granted to Dimension, the Company entered into an arrangement with Penn and Dimension in which the Company helped coordinate and manage research and development activities performed by Penn on behalf of Dimension. Under the arrangement, Dimension reimbursed the Company at an amount equal to costs incurred and paid to Penn, and the Company retains rights to certain intellectual property discovered under the contracted research and development. Due to the uncertainty of any future intellectual property rights that may be discovered and retained by the Company, and because such intellectual property would have no future alternative use due to the stage of development of the drug therapies under development, the Company has not recognized any benefit from the arrangement as consideration paid by Dimension to the Company as a result of the license agreement. Management has evaluated the facts and circumstances of the arrangements with regards to ASC 605-45 Revenue Recognition-Principal Agent Considerations In September 2014, Dimension elected its third indication under the license agreement, and the license was amended to extend the term of the option to elect the fourth and final disease indication for an additional six months. In consideration for the extension of the option, Dimension paid an extension fee of $150. In January 2015, Dimension elected its fourth and final indication under the license. In March 2015, the Company entered into an option and license agreement with Dimension that grants Dimension options to commercial exclusive licenses for four new disease indications to be elected by Dimension in the future. If elected, each option carries an option fee of $1,000 payable to the Company upon exercise, and annual maintenance fees of $50. Additionally, for each option exercised, Dimension is obligated to pay the Company up to $9,000 upon achievement of various substantive milestones, as well as mid to upper-single digit percentage royalties on net sales of licensed products and mid-single digit to low-double digit percentage sublicense fees, if any. In May 2015, Dimension exercised its first option under the agreement and paid $1,000 to the Company. In August 2015, Dimension exercised its second option under the agreement and paid $1,000 to the Company. During the three months ended September 30, 2014 and 2015, the Company recognized $150 and $1,000, respectively, in license revenue from license agreements with Dimension. During the nine months ended September 30, 2014 and 2015 the Company recognized $150 and $2,000, respectively, in license revenue from license agreements with Dimension. During the year ended December 31, 2014, the Company received $200 from Dimension for the purchase of materials owned by the Company and used in the Company’s manufacturing process for research and development and clinical trials. The $200 is recognized as a gain on disposal of the material as the material is delivered to Dimension. For the three months ended September 30, 2014 and 2015, the Company recognized gains of $8 and $5, respectively, related to the delivery of purchased material. For the nine months ended September 30, 2014 and 2015, the Company recognized gains of $32 and $26, respectively, related to the delivery of purchased material. Gains resulting from the delivery of the purchased material to Dimension is included in other operating income in the statements of operations. As of December 31, 2014 and September 30, 2015, the Company recorded an advance payment liability of $153 and $127, respectively, for proceeds received for undelivered material. As of September 30, 2015, the Company does not have any overlapping board members with Dimension. Additionally, the Company’s directors and management do not have significant influence over Dimension. Accordingly, the Company no longer considers Dimension to be a related party subsequent to the period ended September 30, 2015. FoxKiser The Company was party to a services agreement, as amended from time to time, with FoxKiser, an affiliate of certain stockholders of the Company and affiliate of one current and one former member of the Board of Directors, which was terminated in January 2015. Under the agreement, the Company paid a fixed monthly fee plus an additional support fee, as determined by FoxKiser on a monthly basis, as consideration for office facilities, equipment, supplies, general management, accounting, financial management, human resources, legal, secretarial, regulatory compliance and other services provided to the Company. The amounts outstanding to FoxKiser under the services agreement in excess of 30 days from their due date accrued interest at 1.5 percent per month, compounding monthly. The Company allocates the service fees from FoxKiser between research and development and general and administrative expense. For the three months ended September 30, 2014 and 2015, and the nine months ended September 30, 2014 and 2015, costs incurred under the services agreement with FoxKiser were as follows: Three Months Ended Nine Months Ended September 30, September 30, 2014 2015 2014 2015 Research and development $ 300 $ — $ 855 $ 168 General and administrative 399 — 1,134 237 $ 699 $ — $ 1,989 $ 405 As of December 31, 2014, the Company had recorded $1,423 payable to FoxKiser under the services agreement. As discussed in Note 6 and Note 8, amounts owed under the services agreement were converted into Series C Preferred Stock on January 13, 2015. In January 2015, the services agreement was terminated and the remaining amounts due to the FoxKiser under the agreement were paid in full in cash. The Company also entered into promissory notes with FoxKiser as discussed in Note 6.

Summary of Significant Accoun18

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2015
Accounting Policies [Abstract]
Basis of Presentation and Unaudited Interim Financial Information	Basis of Presentation and Unaudited Interim Financial Information The accompanying 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 United States generally accepted accounting principles as found in the Accounting Standards Codification (ASC) and Accounting Standards Update (ASU) of the Financial Accounting Standards Board (FASB). Certain information and footnote disclosures normally included in the Company’s annual financial statements have been condensed or omitted. The interim unaudited financial statements have been prepared on the same basis as the annual audited financial statements as of and for the year ended December 31, 2014 included in the prospectus dated September 16, 2015, filed pursuant to Rule 424(b)(4) under the Securities Act of 1933, as amended, with the Securities and Exchange Commission on September 17, 2015 (the Prospectus). In the opinion of management, the accompanying financial statements reflect all adjustments, which include only normal recurring adjustments necessary for the fair statement of the Company’s financial position as of September 30, 2015, and the results of its operations and its cash flows for the interim periods ended September 30, 2014 and 2015. 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 financial statements should be read in conjunction with the audited financial statements as of and for the year ended December 31, 2014, and the notes thereto, which are included in the Prospectus.
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 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 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.
Marketable Securities	Marketable Securities The Company’s marketable securities are classified as available-for-sale and are carried at fair value. Marketable securities with remaining maturity dates exceeding twelve months which are not intended to be sold prior to maturity for use in current operations are classified as non-current. Unrealized gains and losses, net of any related tax effects, are excluded from earnings and are included in other comprehensive loss and reported as a separate component of stockholders’ equity (deficit) until realized. A decline in the fair value below cost of available-for-sale securities that is deemed other-than-temporary is charged to results of operations, resulting in the establishment of a new cost basis for the security. Premiums and discounts are amortized or accreted into the cost basis over the life of the related security as adjustments to the yield using the effective-interest method. Interest income is recognized when earned. Realized gains and losses from the sale of marketable securities are based on the specific identification method and are included in results of operations. The Company regularly evaluates whether declines in the fair value of its investments below their cost are other-than-temporary. The evaluation includes consideration of the cause of the impairment, including the creditworthiness of the security issuers, the number of securities in an unrealized loss position, the severity and duration of the unrealized losses, whether the Company has the intent to sell the securities and whether it is more likely than not that the Company will be required to sell the securities before the recovery of their amortized cost basis. If the Company determines that the decline in fair value of an investment is below its accounting basis and this decline is other-than-temporary, the Company would reduce the carrying value of the security it holds and record a loss for the amount of such decline. The Company has not recorded any declines in value judged to be other-than-temporary on its investments.
Concentrations of Credit Risk and Off-balance Sheet Risk	Concentrations of Credit Risk and Off-balance Sheet Risk Cash and cash equivalents, marketable securities and accounts receivable are financial instruments that are potentially subject to concentrations of credit risk. The Company’s cash and cash equivalents are deposited in accounts at multiple financial institutions, and amounts may exceed federally insured limits. The Company believes it is not exposed to significant credit risk due to the financial strength of the depository institutions in which the cash is held. The Company’s marketable securities consist of fixed income corporate debt securities and may subject the Company to concentrations of credit risk. The Company has adopted an investment policy, under which its marketable securities are purchased, which limits potential concentrations of investments and establishes minimum acceptable credit ratings, thereby reducing credit risk exposure. Management believes that it is not exposed to significant credit risk related to accounts receivable due to the credit quality and history of collections from its significant customers. The Company has no financial instruments with off-balance sheet risk of loss.
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. 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 The Company computes net loss per share in conformity with the two-class method required for participating securities. The Company considers all series of convertible preferred stock outstanding prior to the IPO to be participating securities. The holders of convertible preferred stock outstanding prior to the IPO were entitled to receive preferential dividends in the event that a dividend was to be paid to the holders of common stock. The holders of convertible preferred stock outstanding prior to the IPO did not have a contractual obligation to share in the losses of the Company. As such, the Company’s net losses for the three months ended September 30, 2014 and 2015, and the nine months ended September 30, 2014 and 2015, were not allocated to these participating securities. In connection with the IPO, all outstanding shares of convertible preferred stock were automatically converted into shares of common stock. 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, convertible preferred stock, stock options and debt instruments containing share settlement options 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 and unit were the same for all periods presented. 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: Three and Nine Months Ended September 30, 2014 2015 Series A convertible preferred stock 2,393 — Series B convertible preferred stock 1,906 — Stock options issued and outstanding 1,885 3,464 Debt with share settlement option (Note 6) 3,295 — 9,479 3,464 Amounts in the table above reflect the common stock equivalents of the noted instruments.
Comprehensive Loss	Comprehensive Loss The Company’s comprehensive loss includes its net loss as well as net unrealized losses on available-for-sale securities.
Recently Announced Accounting Pronouncements	Recently Announced Accounting Pronouncements In February 2015, the FASB issued ASU 2015-2, Consolidation (Topic 810): Amendments to the Consolidation Analysis In August 2014, the FASB issued ASU 2014-15, Presentation of Financial Statements—Going Concern (Subtopic 205-40): Disclosure of Uncertainties about an Entity’s Ability to Continue as a Going Concern In June 2014, the FASB issued ASU No. 2014-12, Compensation—Stock Compensation (Topic 718): Accounting for Share-Based Payments When the Terms of an Award Provide That a Performance Target Could be Achieved after the Requisite Service Period In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606) , Revenue Recognition Revenue from Contracts with Customers (Topic 606)

Summary of Significant Accoun19

Summary of Significant Accounting Policies (Tables)	9 Months Ended
	Sep. 30, 2015
Accounting Policies [Abstract]
Schedule of Anti-dilutive Securities Excluded from 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: Three and Nine Months Ended September 30, 2014 2015 Series A convertible preferred stock 2,393 — Series B convertible preferred stock 1,906 — Stock options issued and outstanding 1,885 3,464 Debt with share settlement option (Note 6) 3,295 — 9,479 3,464

Marketable Securities (Tables)

Marketable Securities (Tables)	9 Months Ended
	Sep. 30, 2015
Investments, Debt and Equity Securities [Abstract]
Summary of Company Marketable Securities	The following table presents a summary of the Company’s marketable securities, which consist solely of available-for-sale securities: Amortized Unrealized Unrealized Fair September 30, 2015 Corporate debt securities $ 19,064 $ 2 $ (28 ) $ 19,038 $ 19,064 $ 2 $ (28 ) $ 19,038

Fair Value of Financial Instr21

Fair Value of Financial Instruments (Tables)	9 Months Ended
	Sep. 30, 2015
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: Quoted Significant prices other Significant in active observable unobservable markets inputs inputs (Level 1) (Level 2) (Level 3) Total December 31, 2014 Money market mutual funds (cash equivalents) $ — $ 844 $ — $ 844 $ — $ 844 $ — $ 844 Quoted Significant prices other Significant in active observable unobservable markets inputs inputs (Level 1) (Level 2) (Level 3) Total September 30, 2015 Money market mutual funds (cash equivalents) $ — $ 205,324 $ — $ 205,324 Corporate debt securities (marketable securities) — 19,038 — 19,038 $ — $ 224,362 $ — $ 224,362

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	9 Months Ended
	Sep. 30, 2015
Property, Plant and Equipment [Abstract]
Schedule of Property and Equipment	Property and equipment, net consists of the following as of September 30, 2015: September 30, 2015 Computer equipment and software $ 332 Furniture and fixtures 94 Leasehold improvements 2 Total property and equipment 428 Accumulated depreciation and amortization (43 ) Property and equipment, net $ 385

Commitments and Contingencies (

Commitments and Contingencies (Tables)	9 Months Ended
	Sep. 30, 2015
Commitments and Contingencies Disclosure [Abstract]
Schedule of Future Minimum Lease Payments Under Non-Cancelable Operating Leases	As of September 30, 2015, future minimum lease payments under non-cancelable operating leases are as follows: Operating Leases 2015 (remainder of year) $ 73 2016 721 2017 825 2018 850 2019 875 2020 833 Thereafter 96 Total minimum lease payments $ 4,273

Capitalization (Tables)

Capitalization (Tables)	9 Months Ended
	Sep. 30, 2015
Equity [Abstract]
Schedule of Reserved Shares of Common Stock for Future Issuance	The Company’s reserved shares of common stock for future issuance are as follows: December 31, September 30, 2014 2015 Series A convertible preferred stock 2,393 — Series B convertible preferred stock 1,906 — Reserved for issuance under equity incentive plans 2,500 6,005 Reserved for issuance under employee stock purchase plan — 254 Debt with share settlement option (Note 6) 3,715 — 10,514 6,259

Significant Agreements (Tables)

Significant Agreements (Tables)	9 Months Ended
	Sep. 30, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Schedule of License Revenue	License revenue, including amounts from related parties, recognized for the three months ended September 30, 2014 and 2015, and the nine months ended September 30, 2014 and 2015, is as follows: Three Months Ended Nine Months Ended September 30, September 30, 2014 2015 2014 2015 Up-front fees and option fees for commercial licenses $ — $ 1,000 $ 3,600 $ 2,000 Milestone fees for commercial licenses — — — 250 Maintenance fees for commercial licenses — 65 65 245 Research and other license revenue 150 — 190 140 $ 150 $ 1,065 $ 3,855 $ 2,635

Stock-based Compensation (Table

Stock-based Compensation (Tables)	9 Months Ended
	Sep. 30, 2015
Stock-Based Compensation Expense Related to Issuance of Stock Option Awards to Employees and Non-employees	The Company has recorded aggregate stock-based compensation expense related to the issuance of stock option awards to employees and non-employees in the statement of operations and comprehensive loss as follows: Three Months Ended Nine Months Ended September 30, September 30, 2014 2015 2014 2015 Research and development $ 16 $ 1,101 $ 16 $ 1,413 General and administrative 204 248 204 646 $ 220 $ 1,349 $ 220 $ 2,059
Schedule of Fair Value of Options Granted to Employees Valuation Assumptions	The fair value of options granted to employees was estimated at the date of grant using the Black-Scholes valuation model with the following weighted-average assumptions: Three Months Ended Nine Months Ended September 30, September 30, 2014 2015 2014 2015 Expected volatility 65 % 75 % 65 % 67 % Expected term (in years) 5.9 6.2 5.9 6.1 Risk-free interest rate 2.0 % 1.8 % 2.0 % 1.7 % Expected dividend yield 0.0 % 0.0 % 0.0 % 0.0 %
Non Employee Stock Option [Member]
Schedule of Non-Employees Stock Purchase Plan Valuation Assumptions	The Company used the following weighted-average assumptions in estimating non-employees stock-based compensation expense: Three Months Ended Nine Months Ended September 30, September 30, 2014 2015 2014 2015 Expected volatility 65 % 81 % 65 % 71 % Expected term (in years) 10.0 9.3 10.0 9.7 Risk-free interest rate 2.6 % 2.1 % 2.6 % 2.1 % Expected dividend yield 0.0 % 0.0 % 0.0 % 0.0 %
2015 Equity Incentive Plan [Member]
Summary of Stock Option Activity	The following table summarizes stock option activity under the Company’s equity incentive plans: Weighted- average Weighted- Remaining average Contractual Aggregate Exercise Life Intrinsic Shares Price (Years) Value (a) Outstanding at December 31, 2014 2,107 $ 0.85 9.8 $ 379 Granted 1,498 $ 9.05 Exercised (118 ) $ 0.85 Cancelled or forfeited (23 ) $ 0.85 Outstanding at September 30, 2015 3,464 $ 4.39 9.3 $ 61,097 Exercisable at September 30, 2015 1,182 $ 1.20 9.1 $ 24,629 Vested and expected to vest at September 30, 2015 3,464 $ 4.39 9.3 $ 61,097 (a) The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the estimated fair value of the common stock for the options that were in the money at September 30, 2015.

Related Party Transactions (Tab

Related Party Transactions (Tables)	9 Months Ended
	Sep. 30, 2015
The Trustees of the University of Pennsylvania [Member]
Summary of Expenses Incurred by Company	Expenses incurred by the Company under its license from Penn, as well as for manufacturing and research and development for the three months ended September 30, 2014 and 2015, and the nine months ended September 30, 2014 and 2015, are as follows: Three Months Ended Nine Months Ended September 30, September 30, 2014 2015 2014 2015 Sublicense fees $ — $ 107 $ 371 $ 264 Royalties on sales of reagents 5 2 16 8 Maintenance of licensed patents 33 29 191 118 Manufacturing of reagents for sale 5 40 87 80 Research and development services 142 2,493 624 5,136 $ 185 $ 2,671 $ 1,289 $ 5,606
GlaxoSmithKline LLC [Member]
Summary of Expenses Incurred by Company	Expenses incurred by the Company under its license from GSK for the three months ended September 30, 2014 and 2015, and the nine months ended September 30, 2014 and 2015, are as follows: Three Months Ended Nine Months Ended September 30, September 30, 2014 2015 2014 2015 Sublicense fees $ — $ 107 $ 371 $ 264 Royalties on sales of reagents 3 1 10 5 Maintenance of licensed patents 108 57 324 474 $ 111 $ 165 $ 705 $ 743
FoxKiser,LLP [Member]
Summary of Expenses Incurred by Company	For the three months ended September 30, 2014 and 2015, and the nine months ended September 30, 2014 and 2015, costs incurred under the services agreement with FoxKiser were as follows: Three Months Ended Nine Months Ended September 30, September 30, 2014 2015 2014 2015 Research and development $ 300 $ — $ 855 $ 168 General and administrative 399 — 1,134 237 $ 699 $ — $ 1,989 $ 405

Nature of Business - Additional

Nature of Business - Additional Information (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	Sep. 22, 2015	Sep. 30, 2015	Dec. 31, 2014
Class of Stock [Line Items]
Accumulated deficit		$ (46,444)	$ (28,809)
Common Stock [Member]
Class of Stock [Line Items]
Conversion of preferred stock, common stock issued upon conversion	16,298
Initial Public Offering [Member]
Class of Stock [Line Items]
Issuance of common stock	7,245
Stock issuance price per share	$ 22
Initial Public Offering [Member] | Common Stock [Member]
Class of Stock [Line Items]
Net proceeds from issuance of common stock	$ 145,184
Overallotment Option [Member]
Class of Stock [Line Items]
Issuance of common stock	945

Summary of Significant Accoun29

Summary of Significant Accounting Policies - Schedule of Anti-dilutive Securities Excluded from Computation of Diluted Weighted-average Shares Outstanding (Detail) - shares shares in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted weighted-average shares outstanding	3,464	9,479	3,464	9,479
Series A Convertible Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted weighted-average shares outstanding		2,393		2,393
Series B Convertible Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted weighted-average shares outstanding		1,906		1,906
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	3,464	1,885	3,464	1,885
Debt with Share Settlement Option [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted weighted-average shares outstanding		3,295		3,295

Marketable Securities - Summary

Marketable Securities - Summary of Company Marketable Securities (Detail) - USD ($)	Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 19,064,000
Unrealized Gains	2,000
Unrealized Losses	(28,000)
Fair Value	19,038,000	$ 0
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	19,064,000
Unrealized Gains	2,000
Unrealized Losses	(28,000)
Fair Value	$ 19,038,000

Marketable Securities - Additio

Marketable Securities - Additional Information (Detail) - USD ($)	3 Months Ended	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2015	Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract]
Available for sale securities	$ 19,038,000	$ 19,038,000	$ 0
Unrealized loss on available-for-sale securities	26,000	26,000
Realized gains or losses available for sale securities		0
Aggregate fair value of securities in an unrealized loss position for less than twelve months	$ 18,012,000	$ 18,012,000

Fair Value of Financial Instr32

Fair Value of Financial Instruments - Schedule of Fair Value of Cash Equivalents and Marketable Securities (Detail) - USD ($)	Sep. 30, 2015	Dec. 31, 2014
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Corporate debt securities (marketable securities)	$ 19,038,000	$ 0
Assets fair value disclosure	224,362,000	844,000
Corporate Debt Securities [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Corporate debt securities (marketable securities)	19,038,000
Money Market Mutual Funds [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Money market mutual funds (cash equivalents)	205,324,000	844,000
Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets fair value disclosure	224,362,000	844,000
Significant Other Observable Inputs (Level 2) [Member] | Corporate Debt Securities [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Corporate debt securities (marketable securities)	19,038,000
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)	$ 205,324,000	$ 844,000

Property and Equipment Net - Sc

Property and Equipment Net - Schedule of Property and Equipment (Detail) - USD ($)	Sep. 30, 2015	Dec. 31, 2014
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 428,000
Accumulated depreciation and amortization	(43,000)
Property and equipment, net	385,000	$ 0
Computer Equipment and Software [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	332,000
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	94,000
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 2,000

Property and Equipment Net - Ad

Property and Equipment Net - Additional Information (Detail) - USD ($)	Sep. 30, 2015	Dec. 31, 2014
Property, Plant and Equipment [Abstract]
Property and equipment, net	$ 385,000	$ 0

Related Party Debt Instruments

Related Party Debt Instruments - Additional Information (Detail) - USD ($)	Jan. 13, 2015	Sep. 30, 2015	Dec. 31, 2014
Related Party Transaction [Line Items]
Due to related party under services agreement			$ 1,423,000
Principal and interest due under related party promissory notes			$ 2,403,000
Related party settlement, amount converted to stock		$ 2,403,000
Preferred stock, shares issued		0	0
Preferred stock, par value		$ 0.0001	$ 0.0001
FoxKiser,LLP [Member]
Related Party Transaction [Line Items]
Due to related party under services agreement			$ 1,423,000
FoxKiser,LLP [Member] | Promissory Notes [Member]
Related Party Transaction [Line Items]
Due to related party under services agreement		$ 0
Principal and interest due under related party promissory notes			$ 2,403,000
FoxKiser,LLP [Member] | Series C Preferred Stock [Member]
Related Party Transaction [Line Items]
Convertible preferred stock, conversion of other debt	$ 3,792,000
Preferred stock, shares issued	585,000
Preferred stock, par value	$ 6.477
FoxKiser,LLP [Member] | Series C Preferred Stock [Member] | Promissory Notes [Member]
Related Party Transaction [Line Items]
Related party settlement, amount converted to stock	$ 2,403,000
FoxKiser,LLP [Member] | Service Agreements [Member]
Related Party Transaction [Line Items]
Due to related party under services agreement		$ 0
FoxKiser,LLP [Member] | Service Agreements [Member] | Series C Preferred Stock [Member]
Related Party Transaction [Line Items]
Related party settlement, amount converted to stock	$ 1,389,000

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail)	Jan. 31, 2015USD ($)	Nov. 10, 2014USD ($)	Nov. 19, 2010USD ($)	Sep. 30, 2015USD ($)	Mar. 31, 2015USD ($)	Sep. 30, 2015USD ($)Milestone	Dec. 31, 2014USD ($)Milestone
Other Commitments [Line Items]
Claims paid to date related to indemnification issues						$ 0
Accruals or expenses related to indemnification issues						$ 0	$ 0
FoxKiser,LLP [Member]
Other Commitments [Line Items]
Lease expiration date						Apr. 30, 2015
Monthly lease payments	$ 20,000
ARIAD Pharmaceuticals, Inc [Member]
Other Commitments [Line Items]
Annual maintenance fees			$ 50,000			$ 0	$ 0
Number of milestone | Milestone						0	0
Milestone payments						$ 0	$ 0
Royalty expense incurred						0
Sublicense fees						$ 0
ARIAD Pharmaceuticals, Inc [Member] | Common Class A [Member]
Other Commitments [Line Items]
Fair value of stock issued for license			726,000
ARIAD Pharmaceuticals, Inc [Member] | Maximum [Member]
Other Commitments [Line Items]
Contractual obligation			$ 2,300,000
Regents of the University of Minnesota [Member]
Other Commitments [Line Items]
Annual maintenance fees		$ 9,000
Number of milestone | Milestone						0	0
Milestone payments						$ 0	$ 0
Royalty expense incurred						0
Sublicense fees						$ 0
Up-front fee paid		25,000
Regents of the University of Minnesota [Member] | Maximum [Member] | Annual License Maintenance Fee [Member]
Other Commitments [Line Items]
Contractual obligation		15,000
Regents of the University of Minnesota [Member] | Maximum [Member] | Royalties And Sublicense Fees [Member]
Other Commitments [Line Items]
Contractual obligation		125,000
Regents of the University of Minnesota [Member] | Minimum [Member] | Annual License Maintenance Fee [Member]
Other Commitments [Line Items]
Contractual obligation		$ 5,000
Philadelphia, Pennsylvania [Member]
Other Commitments [Line Items]
Operating lease renewal period						6 years
Lease expiration date						Dec. 31, 2015
Monthly lease payments						$ 4,000
Rockville, Maryland [Member]
Other Commitments [Line Items]
Lease expiration date					Sep. 30, 2020
Monthly lease payments				$ 41,000	$ 24,000
Lease period				5 years	5 years 6 months
Options to extend the lease					6 years
Lease commenced date					2015-04
Lease term						Extend the term of the existing lease to October 2020

Commitments and Contingencies37

Commitments and Contingencies - Schedule of Future Minimum Lease Payments Under Non-Cancelable Operating Leases (Detail) $ in Thousands	Sep. 30, 2015USD ($)
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2015 (remainder of year)	$ 73
2,016	721
2,017	825
2,018	850
2,019	875
2,020	833
Thereafter	96
Total minimum lease payments	$ 4,273

Capitalization - Additional Inf

Capitalization - Additional Information (Detail)	May. 15, 2015USD ($)$ / sharesshares	Jan. 13, 2015USD ($)$ / sharesshares	Sep. 30, 2015USD ($)Directors$ / sharesshares	Dec. 31, 2014$ / sharesshares
Schedule of Capitalization [Line Items]
Common stock, shares authorized | shares			100,000,000	9,500,000
Common stock, par value			$ 0.0001	$ 0.0001
Preferred stock, shares authorized | shares			10,000,000	0
Preferred stock, par value			$ 0.0001	$ 0.0001
Convertible preferred stock, shares authorized | shares				4,299,000
Convertible preferred stock, par value				$ 0.0001
Convertible preferred stock, net of issuance cost | $			$ 618,000
Gain (loss) on extinguishment of preferred stock | $			$ 759,000
Voting right of the preferred stock holders			One vote for each share of common stock
Number of directors entitled to elect by outstanding common stock holders | Directors			1
Closing date of IPO			Sep. 22, 2015
Outstanding convertible preferred stock into common stock | shares			16,298,000
Minimum [Member]
Schedule of Capitalization [Line Items]
Gross proceeds from issuance and sale of convertible preferred stock | $			$ 40,000,000
Percentage of voting power held by stockholders			50.00%
Series A Convertible Preferred Stock [Member]
Schedule of Capitalization [Line Items]
Convertible preferred stock, shares authorized | shares			0	2,393,000
Convertible preferred stock, par value				$ 0.0001
Convertible preferred stock, shares issuance and sale | shares			0	2,393,000
Redemption date of preferred stock			Dec. 31, 2019
Gain (loss) on extinguishment of preferred stock | $			$ (1,317,000)
Preferred stock dividend rate per annum for each share			$ 0.1003
Original issuance price of liquidation preference per share			1.255
Original issuance redemption price per share			1.255
Original issuance price of conversion rate per share			$ 1.255
Number of directors entitled to elect by temporary equity holders | Directors			3
Convertible preferred stock, shares outstanding | shares			0	2,393,000
Series B Convertible Preferred Stock [Member]
Schedule of Capitalization [Line Items]
Convertible preferred stock, shares authorized | shares			0	1,906,000
Convertible preferred stock, par value				$ 0.0001
Convertible preferred stock, shares issuance and sale | shares			0	1,906,000
Redemption date of preferred stock			Dec. 31, 2019
Gain (loss) on extinguishment of preferred stock | $			$ 2,076,000
Preferred stock dividend rate per annum for each share			$ 0.331192
Original issuance price of liquidation preference per share			4.1399
Original issuance redemption price per share			4.1399
Original issuance price of conversion rate per share			$ 4.1399
Number of directors entitled to elect by temporary equity holders | Directors			1
Convertible preferred stock, shares outstanding | shares			0	1,906,000
Series C Convertible Preferred Stock [Member]
Schedule of Capitalization [Line Items]
Convertible preferred stock, par value		$ 0.0001
Convertible preferred stock, shares issuance and sale | shares		4,632,000
Convertible preferred stock, per share price		$ 6.477
Gross proceeds from issuance and sale of convertible preferred stock | $		$ 30,000,000	$ 26,021,000
Proceeds from exercise of stock options | $		26,021,000
Convertible preferred stock, net of issuance cost | $		187,000
Convertible preferred stock, conversion of other debt | $		$ 3,792,000
Convertible preferred stock, shares reserved for future issuance | shares		2,800,000
Preferred stock dividend rate per annum for each share			$ 0.51816
Original issuance price of liquidation preference per share			6.477
Original issuance redemption price per share			6.477
Original issuance price of conversion rate per share			$ 6.477
Number of directors entitled to elect by temporary equity holders | Directors			1
Series D Convertible Preferred Stock [Member]
Schedule of Capitalization [Line Items]
Convertible preferred stock, par value	$ 0.0001
Convertible preferred stock, shares issuance and sale | shares	7,367,000
Convertible preferred stock, per share price	$ 9.5699
Gross proceeds from issuance and sale of convertible preferred stock | $	$ 67,998,000		$ 67,998,000
Convertible preferred stock, net of issuance cost | $	$ 2,502,000
Convertible preferred stock, shares reserved for future issuance | shares	4,100,000
Preferred stock dividend rate per annum for each share			$ 0.765592
Original issuance price of liquidation preference per share			9.5699
Original issuance redemption price per share			9.5699
Original issuance price of conversion rate per share			$ 9.5699
Number of directors entitled to elect by temporary equity holders | Directors			1
Convertible Preferred Stock [Member]
Schedule of Capitalization [Line Items]
Convertible preferred stock, shares issuance and sale | shares			0
Convertible preferred stock, shares outstanding | shares			0

Capitalization - Schedule of Re

Capitalization - Schedule of Reserved Shares of Common Stock for Future Issuance (Detail) - shares shares in Thousands	Sep. 30, 2015	Dec. 31, 2014
Schedule of Capitalization [Line Items]
Reserved common shares	6,259	10,514
Series A Convertible Preferred Stock [Member]
Schedule of Capitalization [Line Items]
Reserved common shares		2,393
Series B Convertible Preferred Stock [Member]
Schedule of Capitalization [Line Items]
Reserved common shares		1,906
Equity Incentive Plans [Member]
Schedule of Capitalization [Line Items]
Reserved common shares	6,005	2,500
Employee Stock Purchase Plan [Member]
Schedule of Capitalization [Line Items]
Reserved common shares	254
Debt with Share Settlement Option [Member]
Schedule of Capitalization [Line Items]
Reserved common shares		3,715

Significant Agreements - Additi

Significant Agreements - Additional Information (Detail) $ in Thousands	9 Months Ended
	Sep. 30, 2015USD ($)
License Agreements [Abstract]
Milestone fee upon submission of preclinical regulatory filings	$ 500
Milestone fee upon commencement of clinical trials in humans	20,000
Milestone fee upon submission of regulatory approval filings	11,300
Milestone fee upon approval of commercial products by regulatory agencies	51,500
Milestone fee upon achievement of specified sales targets for licensed products	$ 2,000

Significant Agreements - Schedu

Significant Agreements - Schedule of License Revenue (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
License Agreement Revenue Recognition [Abstract]
Up-front fees and option fees for commercial licenses	$ 1,000		$ 2,000	$ 3,600
Milestone fees for commercial licenses			250
Maintenance fees for commercial licenses	65		245	65
Research and other license revenue		$ 150	140	190
Licenses revenue	$ 1,065	$ 150	$ 2,635	$ 3,855

Stock-based Compensation - Addi

Stock-based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Sep. 16, 2015	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted period		9 years 3 months 18 days	10 years	9 years 8 months 12 days	10 years
Proceeds from exercise of stock options				$ 100
Unrecognized stock-based compensation expense related to employees' awards		$ 4,999		$ 4,999
Unrecognized stock-based compensation, weighted-average period				3 years 2 months 12 days
2015 Equity Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock shares authorized for issuance		6,005,000		6,005,000
Shares available for future grants		2,541,000		2,541,000
Amount of shares authorized for issuance				$ 2,952
Percentage of annual increase in number of shares available for future issuance				4.00%
Stock options granted period				10 years
Stock options vesting period				4 years
Weighted-average fair values of options granted				$ 5.88
Total number of stock options exercised				(118,000)
Proceeds from exercise of stock options				$ 100
Total intrinsic value of options exercised				$ 354
2015 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, description				The number of shares reserved for issuance under the 2015 ESPP automatically increases on the first business day of each fiscal year, commencing in 2016.
Share-based compensation arrangement by share-based payment award, percentage of outstanding stock minimum	1.00%
2015 Employee Stock Purchase Plan [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock shares authorized for issuance	254,000

Stock-based Compensation - Summ

Stock-based Compensation - Summary of Stock Option Activity (Detail) - 2015 Equity Incentive Plan [Member] - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Number of Shares [Roll Forward]
Shares Outstanding, Beginning Balance	2,107
Shares, Granted	1,498
Shares, Exercised	(118)
Shares, Cancelled or forfeited	(23)
Shares Outstanding, Ending Balance	3,464	2,107
Shares, Exercisable	1,182
Shares, Vested and expected to vest	3,464
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted-average Exercise Price Outstanding, Beginning Balance	$ 0.85
Weighted-average Exercise Price, Granted	9.05
Weighted-average Exercise Price, Exercised	0.85
Weighted-average Exercise Price, Outstanding, Ending Balance	4.39	$ 0.85
Weighted-average Exercise Price, Exercisable	1.20
Weighted-average Exercise Price, Vested and expected to vest	$ 4.39
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted-average Remaining Contractual Life (Years) Outstanding	9 years 3 months 18 days	9 years 9 months 18 days
Weighted-average Remaining Contractual Life (Years), Exercisable	9 years 1 month 6 days
Weighted-average Remaining Contractual Life (Years), Vested and expected to vest	9 years 3 months 18 days
Aggregate Intrinsic Value Outstanding, Beginning Balance	$ 379
Aggregate Intrinsic Value Outstanding, Ending Balance	61,097	$ 379
Aggregate Intrinsic Value, Exercisable	24,629
Aggregate Intrinsic Value, Vested and expected to vest	$ 61,097

Stock-based Compensation - Stoc

Stock-based Compensation - Stock-Based Compensation Expense Related to Issuance of Stock Option Awards to Employees and Non-employees (Detail) - 2014 and 2015 Equity Incentive Plan [Member] - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total share-based compensation	$ 1,349	$ 220	$ 2,059	$ 220
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total share-based compensation	1,101	16	1,413	16
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total share-based compensation	$ 248	$ 204	$ 646	$ 204

Stock-based Compensation - Sche

Stock-based Compensation - Schedule of Employees Stock Purchase Plan Valuation Assumptions (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility	81.00%	65.00%	71.00%	65.00%
Expected term (in years)	9 years 3 months 18 days	10 years	9 years 8 months 12 days	10 years
Risk-free interest rate	2.10%	2.60%	2.10%	2.60%
Expected dividend yield	0.00%	0.00%	0.00%	0.00%
Non Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility	75.00%	65.00%	67.00%	65.00%
Expected term (in years)	6 years 2 months 12 days	5 years 10 months 24 days	6 years 1 month 6 days	5 years 10 months 24 days
Risk-free interest rate	1.80%	2.00%	1.70%	2.00%
Expected dividend yield	0.00%	0.00%	0.00%	0.00%

Retirement Plan - Additional In

Retirement Plan - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2015
Defined Benefit Plan Disclosure [Line Items]
Contribution by company	$ 56	$ 120
Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Percentage of employee deferrals		5.75%

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Detail) - USD ($) shares in Thousands	1 Months Ended				3 Months Ended		9 Months Ended		12 Months Ended
	Aug. 31, 2015	May. 31, 2015	Mar. 31, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014	Mar. 06, 2009
Related Party Transaction [Line Items]
Accrued expenses payable					$ 1,727,000		$ 1,727,000		$ 1,115,000
Closing date of IPO							Sep. 22, 2015
Related party receivables									750,000
Research and development (including amounts to related parties)					5,664,000	$ 1,085,000	$ 12,471,000	$ 2,869,000
Related party receivables							(750,000)	(774,000)
Due to related party under services agreement							$ (34,000)	(255,000)
Agreement expiry date							Dec. 31, 2014
License revenue from license agreements					65,000		$ 635,000	3,705,000
Advance payment liability for proceeds received for undelivered material							$ (26,000)	168,000
Due to related party under services agreement									1,423,000
Commercial License Agreement [Member]
Related Party Transaction [Line Items]
Consideration in shares issued by Dimension for license granted							10,000
Commercial License Agreement [Member] | Parent Company [Member]
Related Party Transaction [Line Items]
Consideration in shares issued by Dimension for license granted							10
License agreement, estimated fair value of stock issued							$ 3,000
The Trustees of the University of Pennsylvania [Member]
Related Party Transaction [Line Items]
Percentage of ownership interest issued as consideration for license agreement							24.50%
Accrued expenses payable					163,000		$ 163,000		1,732,000
The Trustees of the University of Pennsylvania [Member] | Initial Public Offering [Member]
Related Party Transaction [Line Items]
Closing date of IPO							Sep. 22, 2015
GlaxoSmithKline LLC [Member]
Related Party Transaction [Line Items]
Percentage of ownership interest issued as consideration for license agreement							19.90%
Accrued expenses payable					186,000		$ 186,000		2,028,000
Milestone payment obligation					0		0			$ 1,650,000
Dimension Therapeutics, Inc. [Member]
Related Party Transaction [Line Items]
Annual maintenance fees			$ 50,000
Carrying amount of investment					3,000		3,000		3,000
Related party receivables					0		0		750,000
Research and development expense reimbursements earned						1,455,000		3,115,000
Related party receivables									750,000
Extension fee paid by Dimension for extension of option				$ 150,000
Option fee payable upon exercise			1,000,000
Milestone payment received from option and license agreement	$ 1,000,000	$ 1,000,000	$ 9,000,000
License revenue from license agreements					1,000,000	150,000	2,000,000	150,000
Amount received for purchase of materials from related party									200,000
Gain on disposal of the material to related party									200,000
Gains related to delivery of purchased material					$ 5,000	8,000	26,000	32,000
Advance payment liability for proceeds received for undelivered material							$ 127,000		153,000
Dimension Therapeutics, Inc. [Member] | Commercial License Agreement [Member]
Related Party Transaction [Line Items]
License agreement initiation date							Oct. 30, 2013
Annual maintenance fees							$ 35,000
Penn [Member]
Related Party Transaction [Line Items]
Research and development expense to Penn						$ 1,455,000		$ 3,115,000
Due to related party under services agreement									750,000
FoxKiser,LLP [Member]
Related Party Transaction [Line Items]
Related party transaction, description							The amounts outstanding to FoxKiser under the services agreement in excess of 30 days from their due date accrue interest at 1.5 percent per month, compounding monthly.
Accrued interest compounding monthly on amount outstanding							1.50%
Due to related party under services agreement									$ 1,423,000

Related Party Transactions - Su

Related Party Transactions - Summary of Expenses Incurred by Company (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
The Trustees of the University of Pennsylvania [Member]
Related Party Transaction [Line Items]
Total related party transaction expense	$ 2,671	$ 185	$ 5,606	$ 1,289
The Trustees of the University of Pennsylvania [Member] | Sublicense Fees [Member]
Related Party Transaction [Line Items]
Total related party transaction expense	107		264	371
The Trustees of the University of Pennsylvania [Member] | Royalties on Sales of Reagents [Member]
Related Party Transaction [Line Items]
Total related party transaction expense	2	5	8	16
The Trustees of the University of Pennsylvania [Member] | Maintenance of Licensed Patents [Member]
Related Party Transaction [Line Items]
Total related party transaction expense	29	33	118	191
The Trustees of the University of Pennsylvania [Member] | Manufacturing of Reagents for Sale [Member]
Related Party Transaction [Line Items]
Total related party transaction expense	40	5	80	87
The Trustees of the University of Pennsylvania [Member] | Research and Development Services [Member]
Related Party Transaction [Line Items]
Total related party transaction expense	2,493	142	5,136	624
GlaxoSmithKline LLC [Member]
Related Party Transaction [Line Items]
Total related party transaction expense	165	111	743	705
GlaxoSmithKline LLC [Member] | Sublicense Fees [Member]
Related Party Transaction [Line Items]
Total related party transaction expense	107		264	371
GlaxoSmithKline LLC [Member] | Royalties on Sales of Reagents [Member]
Related Party Transaction [Line Items]
Total related party transaction expense	1	3	5	10
GlaxoSmithKline LLC [Member] | Maintenance of Licensed Patents [Member]
Related Party Transaction [Line Items]
Total related party transaction expense	$ 57	108	474	324
FoxKiser,LLP [Member]
Related Party Transaction [Line Items]
Total related party transaction expense		699	405	1,989
FoxKiser,LLP [Member] | Research and Development [Member]
Related Party Transaction [Line Items]
Total related party transaction expense		300	168	855
FoxKiser,LLP [Member] | General and Administrative [Member]
Related Party Transaction [Line Items]
Total related party transaction expense		$ 399	$ 237	$ 1,134