aTyr Pharma (ATYR) 10-K 30 Mar 16 2015 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2015	Mar. 24, 2016	Jun. 30, 2015
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2015
Document Fiscal Year Focus	2,015
Document Fiscal Period Focus	FY
Trading Symbol	LIFE
Entity Registrant Name	ATYR PHARMA INC
Entity Central Index Key	1,339,970
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		23,677,303
Entity Public Float			$ 282,395,534

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Current assets:
Cash and cash equivalents	$ 53,025	$ 13,899
Short-term investments	42,510	1,954
Prepaid expenses and other assets	2,415	656
Total current assets	97,950	16,509
Long-term investments	29,814
Property and equipment, net	1,793	1,925
Other assets	118	2,210
Total assets	129,675	20,644
Current liabilities:
Accounts payable	3,872	1,433
Accrued expenses	4,595	2,932
Current portion of deferred rent	315	295
Current portion of commercial bank debt	3,366	3,134
Convertible promissory note		2,000
Preferred stock warrant liabilities		319
Total current liabilities	12,148	10,113
Deferred rent, net of current portion	130	445
Commercial bank debt, net of current portion	1,776	5,142
Other long-term liabilities	$ 571	$ 335
Commitments and contingencies (Note 5)
Redeemable convertible preferred stock, $0.001 par value; authorized shares – 7,285,456 and 75,772,871 at December 31, 2015 and 2014, respectively; issued and outstanding shares – none and 73,487,415 at December 31, 2015 and 2014, respectively; liquidation preference of $0 and $95,619 at December 31, 2015 and 2014, respectively		$ 95,619
Stockholders’ equity (deficit):
Undesignated preferred stock, $0.001 par value; authorized shares – 5,000,000 at December 31, 2015 and none at December 31, 2014; issued and outstanding shares – none at December 31, 2015 and 2014
Common stock, $0.001 par value; authorized shares – 150,000,000 and 95,500,000 at December 31, 2015 and 2014, respectively; issued and outstanding shares – 23,670,079 and 909,880 at December 31, 2015 and 2014, respectively	$ 24	$ 1
Additional paid-in capital	273,321	19,209
Stockholder note receivable		(69)
Accumulated other comprehensive loss	(171)
Accumulated deficit	(158,124)	(110,151)
Total stockholders’ equity (deficit)	115,050	(91,010)
Total liabilities, redeemable convertible preferred stock and stockholders’ equity (deficit)	$ 129,675	$ 20,644

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Preferred stock, shares authorized		75,772,871
Redeemable convertible preferred stock, liquidation preference	$ 0	$ 95,619
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	150,000,000	95,500,000
Common stock, shares issued	23,670,079	909,880
Common stock, shares outstanding	23,670,079	909,880
Undesignated Preferred Stock [Member]
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	5,000,000	0
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Redeemable Convertible Preferred Stock [Member]
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	7,285,456	75,772,871
Preferred stock, shares issued	0	73,487,415
Preferred stock, shares outstanding	0	73,487,415

Consolidated Statements of Oper

Consolidated Statements of Operations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating expenses:
Research and development	$ 34,504	$ 16,777	$ 13,832
General and administrative	13,112	6,777	5,710
Total operating expenses	47,616	23,554	19,542
Loss from operations	(47,616)	(23,554)	(19,542)
Other income (expense):
Interest income (expense), net	(386)	(832)	(444)
Change in fair value of warrant liabilities	29	36	(28)
Total other income (expense)	(357)	(796)	(472)
Net loss	(47,973)	(24,350)	(20,014)
Accretion to redemption value of redeemable convertible preferred stock	(15)	(416)	(1,637)
Net loss attributable to common stockholders	$ (47,988)	$ (24,766)	$ (21,651)
Net loss per share attributable to common stockholders, basic and diluted	$ (3.03)	$ (29.69)	$ (28.39)
Weighted average common stock shares outstanding, basic and diluted	15,838,353	834,221	762,761

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (47,973)	$ (24,350)	$ (20,014)
Other comprehensive loss:
Change in unrealized loss on available for sale investments	(171)
Comprehensive loss	$ (48,144)	$ (24,350)	$ (20,014)

Consolidated Statements of Rede

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Stockholder Note Receivable [Member]	Non-Controlling Interest [Member]	Other Comprehensive Loss [Member]	Accumulated Deficit [Member]	Redeemable Convertible Preferred Stock [Member]
Redeemable Convertible Preferred Stock, Ending balance at Dec. 31, 2012								$ 63,225
Redeemable Convertible Preferred Stock, Ending balance, shares at Dec. 31, 2012								55,211,585
Beginning balance at Dec. 31, 2012	$ (64,554)	$ 1	$ 5	$ (69)	$ 25		$ (64,516)
Beginning balance, Shares at Dec. 31, 2012		788,946
Issuance of Series D and E redeemable convertible preferred stock for cash	2,431				2,431			$ 36,683
Issuance of Series D and E redeemable convertible preferred stock for cash, Shares								14,504,841
Issuance of Series D redeemable convertible preferred stock for conversion of debt and interest								$ 9,537
Issuance of Series D redeemable convertible preferred stock for conversion of debt and interest, Shares								3,770,989
Series D and E redeemable convertible preferred stock issuance costs	(65)				(65)			$ (389)
Exercise of common stock options	77		54		23
Exercise of common stock options, Shares		67,645
Changes in share repurchase liability	(3)		(3)
Stock-based compensation	155		155
Accretion to redemption value of redeemable convertible preferred stock	(1,637)		(366)				(1,271)	1,637
Capital contribution related to reversal of historical accretion of redeemable convertible preferred stock	17,528		17,528					(17,528)
Net loss	(20,014)						(20,014)
Redeemable Convertible Preferred Stock, Ending balance at Dec. 31, 2013								$ 93,165
Redeemable Convertible Preferred Stock, Ending balance, shares at Dec. 31, 2013								73,487,415
Ending balance at Dec. 31, 2013	(66,082)	$ 1	17,373	(69)	2,414		(85,801)
Ending balance, Shares at Dec. 31, 2013		856,591
Exercise of common stock options	72		43		29
Exercise of common stock options, Shares		53,289
Changes in share repurchase liability	13		13
Stock-based compensation	1,791		1,791
Dissolution of Affiliates	(2,038)		405		$ (2,443)			$ 2,038
Accretion to redemption value of redeemable convertible preferred stock	(416)		(416)					416
Net loss	(24,350)						(24,350)
Redeemable Convertible Preferred Stock, Ending balance at Dec. 31, 2014	95,619							$ 95,619
Redeemable Convertible Preferred Stock, Ending balance, shares at Dec. 31, 2014								73,487,415
Ending balance at Dec. 31, 2014	(91,010)	$ 1	19,209	(69)			(110,151)
Ending balance, Shares at Dec. 31, 2014		909,880
Issuance of Series D and E redeemable convertible preferred stock for cash								$ 75,650
Issuance of Series D and E redeemable convertible preferred stock for cash, Shares								68,166,894
Conversion of redeemable convertible preferred stock in connection with IPO	171,284	$ 16	171,268					$ (171,284)
Conversion of redeemable convertible preferred stock in connection with IPO, Shares		16,279,859						(141,654,309)
Issuance of common stock through initial public offering, net	75,903	$ 6	75,897
Issuance of common stock through initial public offering, net, Shares		6,164,000
Repayment of stockholder note receivable	60		(9)	$ 69
Exercise of common stock options	$ 535	$ 1	534
Exercise of common stock options, Shares	197,253	196,500
Reclassification of preferred stock warrant liability to additional paid-in-capital	$ 290		290
Issuance of common stock to The Scripps Research Institute	1,411		1,411
Issuance of common stock to The Scripps Research Institute, Share		119,840
Changes in share repurchase liability	(120)		(120)
Stock-based compensation	4,856		4,856
Accretion to redemption value of redeemable convertible preferred stock	(15)		(15)					$ 15
Change in unrealized loss on available for sale investments	(171)					$ (171)
Net loss	(47,973)						(47,973)
Redeemable Convertible Preferred Stock, Ending balance, shares at Dec. 31, 2015								0
Ending balance at Dec. 31, 2015	$ 115,050	$ 24	$ 273,321			$ (171)	$ (158,124)
Ending balance, Shares at Dec. 31, 2015		23,670,079

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Cash flows from operating activities:
Net loss	$ (47,973)	$ (24,350)	$ (20,014)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	869	829	714
Issuance of common stock for technology	1,411
Stock-based compensation	4,856	1,791	155
Amortization of debt discount	297	426	211
Change in fair value of preferred stock warrant liability	(29)	(36)	28
Amortization of investment premium	789	43
Deferred rent	(295)	(277)	(254)
Changes in operating assets and liabilities
Prepaid expenses and other assets	(666)	(1,043)	323
Accounts payable and accrued expenses	3,944	(207)	1,526
Net cash used in operating activities	(36,797)	(22,824)	(17,311)
Cash flows from investing activities:
Purchase of property and equipment	(664)	(249)	(644)
Purchases of investment securities	(109,445)	(5,397)
Maturities of investment securities	38,115	3,400
Net cash used in investing activities	(71,994)	(2,246)	(644)
Cash flows from financing activities:
Issuance of preferred stock for cash, net of issuance costs	75,648		38,660
Issuance of common stock through initial public offering, net of offering costs	76,902
Costs paid in connection with initial public offering		(999)
Proceeds from issuance of common stock through option exercises	604	72	77
Proceeds from notes payable to bank		5,000	5,000
Repayments on notes payable to bank	(3,237)	(1,561)	(2,500)
Proceeds from convertible debt			10,000
Repayment of convertible debt	(2,000)		(500)
Net cash provided by financing activities	147,917	2,512	50,737
Net change in cash and cash equivalents	39,126	(22,558)	32,782
Cash and cash equivalents at beginning of the period	13,899	36,457	3,675
Cash and cash equivalents at the end of the period	53,025	13,899	36,457
Supplemental disclosure of cash flow information:
Interest paid	925	415	254
Supplemental schedule of noncash investing and financing activities:
Issuance of warrants in connection with long-term debt		148	137
Changes in share repurchase liability	$ (120)	$ 13	(3)
Capital contribution related to reversal of historical accretion of redeemable convertible preferred stock			17,528
Conversion of convertible debt and accrued interest			$ 9,537

Organization, Business and Basi

Organization, Business and Basis of Presentation	12 Months Ended
	Dec. 31, 2015
Accounting Policies [Abstract]
Organization, Business and Basis of Presentation	1. Organization, Business and Basis of Presentation Organization and Business We were incorporated in the state of Delaware on September 8, 2005. We are focused on the discovery and clinical development of innovative medicines for patients suffering from severe rare diseases. Initial Public Offering On May 12, 2015, we completed our initial public offering (IPO) of 6,164,000 shares of common stock at $14.00 per share, resulting in gross proceeds of approximately $86.3 million and net proceeds of $75.9 million, after underwriting and other expenses of approximately $10.4 million (consisting of approximately $6.0 million in underwriting discounts and commissions and approximately $4.4 million in other offering expenses). In connection with the IPO, all outstanding shares of redeemable convertible preferred stock were converted into 16,279,859 shares of our common stock and warrants to purchase 206,581 shares of redeemable convertible preferred stock were converted into warrants to purchase 25,970 shares of our common stock with a resultant reclassification of the warrant liabilities to additional paid-in capital. In addition, we filed an amended and restated certificate of incorporation on May 12, 2015, authorizing 150,000,000 shares of common stock and 7,285,456 shares of preferred stock, 5,000,000 of which is undesignated preferred stock. Upon the closing of the IPO, 1,574,566 shares of common stock were reserved for future issuance under the 2015 Stock Option and Incentive Plan (the 2015 Plan) and 227,623 shares of common stock were reserved for future issuance under the 2015 Employee Stock Purchase Plan (the 2015 ESPP). Principles of Consolidation Our consolidated financial statements include our accounts, our 98% majority-owned subsidiary in Hong Kong, Pangu BioPharma Limited (Pangu BioPharma). All intercompany transactions and balances are eliminated in consolidation. Reverse Stock Split On May 5, 2015, we filed an amendment to our amended and restated certificate of incorporation to effect a one-for-7.95413 reverse stock split of our common stock (the Reverse Stock Split). The par value and the authorized shares of the common and convertible preferred stock were not adjusted as a result of the Reverse Stock Split. All share information has been retroactively restated to reflect the Reverse Stock Split. Use of Estimates Our consolidated financial statements are prepared in accordance with generally accepted accounting principles (GAAP). The preparation of our consolidated financial statements requires us to make estimates and assumptions that impact the reported amounts of assets, liabilities and expenses and the disclosure of contingent assets and liabilities in our consolidated financial statements and accompanying notes. The most significant estimates in our consolidated financial statements relate to the fair value of equity issuances and awards, and clinical trial and research and development expenses. Although these estimates are based on our knowledge of current events and actions we may undertake in the future, actual results may ultimately differ materially from these estimates and assumptions. Segment Reporting Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision-maker in making decisions regarding resource allocation and assessing performance. We view our operations and manage our business in one operating segment.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2015
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Cash and Cash Equivalents Cash and cash equivalents consist primarily of readily available checking, money market accounts and money market funds. We consider all highly liquid investments that mature in three months or less when purchased to be cash equivalents. Investment Securities Investment securities primarily consist of investment grade corporate debt securities, asset-backed securities, commercial paper and United States Treasury securities. We classify all investment securities as available-for-sale. Investment securities are carried at fair value, with the unrealized gains and losses, if any, reported as a component of other comprehensive income (loss) in stockholders’ equity (deficit) until realized. Realized gains and losses from the sale of investment securities, if any, are determined on a specific identification basis. A decline in the market value of any investment security below cost that is determined to be other than temporary will result in an impairment charge to earnings and a new cost basis for the security is established. No such impairment charges were recorded for any period presented. Premiums and discounts are amortized or accreted over the life of the related security as an adjustment to yield using the straight-line method and are included in interest income. Interest income is recognized when earned. As of December 31, 2015, we held an aggregate total of $72.3 million of investment securities which consisted of corporate debt securities, asset-backed securities, commercial paper and United States Treasury securities, all of which will mature in less than two years and there was $0.2 million difference between the amortized cost and fair value of these investment securities. As of December 31, 2014, we held $2.0 million of corporate debt securities, all of which mature in less than three months, and there was no difference between the amortized cost and fair value of these investment securities. Concentration of Credit Risk Financial instruments that potentially subject us to significant concentration of credit risk consist primarily of cash, cash equivalents and investment securities. We have established guidelines regarding diversification of investments and their maturities, which are designed to maintain principal and maximize liquidity. We maintain deposits in federally insured financial institutions in excess of federally insured limits. We have not experienced any losses in such accounts and we believe that we are not exposed to significant credit risk due to the financial position of the depository institutions in which those deposits are held. Property and Equipment Property and equipment are stated at cost and depreciated on a straight-line basis over the estimated useful life of the related assets (generally three to seven years). Leasehold improvements are stated at cost and amortized on a straight-line basis over the lesser of the remaining term of the related lease or the estimated useful life of the leasehold improvements. Repairs and maintenance costs are charged to expense as incurred . Impairment of Long-Lived Assets Long-lived assets consist primarily of property and equipment. An impairment loss is recorded if and when events and circumstances indicate that assets might be impaired and the undiscounted cash flows estimated to be generated by those assets are less than the carrying amount of those assets. While our current and historical operating losses are indicators of impairment, we believe that future cash flows to be received support the carrying value of our long-lived assets and, accordingly, have not recognized any impairment losses since inception. Accrued Expenses As part of the process of preparing our consolidated financial statements, we are required to estimate accrued expenses, including accrued research and development expenses for fees paid to investigative sites and CROs in connection with clinical trials; service providers in connection with preclinical development activities; service providers related to product manufacturing; and other professional services. The accrual process involves reviewing open contracts and purchase orders, communicating with our personnel to identify services that have been performed on our behalf and estimating the level of service performed and the associated cost incurred for the service when we have not yet been invoiced or otherwise notified of the actual cost. We make estimates of accrued expenses as of each balance sheet date in our consolidated financial statements based on facts and circumstances known to us at that time. Although we do not expect the estimates to be materially different from amounts actually incurred, if the estimates of the status and timing of services performed differs from the actual status and timing of services performed, we may report amounts that are too high or too low in any particular period. Deferred Rent Rent expense, including the value of tenant improvement allowances received, is recorded on a straight-line basis over the term of the lease. The difference between rent expense and amounts paid under the lease agreements is recorded as deferred rent in the accompanying consolidated balance sheets. Research and Development Costs Research and development costs are expensed as incurred. Research and development costs include: salaries and employee-related expenses, including stock-based compensation and benefits for personnel in research and product development functions; costs associated with conducting our preclinical, development and regulatory activities, including fees paid to third-party professional consultants, service providers and our scientific, therapeutic and clinical advisory boards; costs to acquire, develop and manufacture preclinical study and clinical trial materials; costs incurred under clinical trial agreements with clinical research organizations and investigative sites; costs for laboratory supplies; payments related to licensed products and technologies; allocated facilities and information technology costs; and depreciation. Patent Costs Costs related to filing and pursuing patent applications are recorded as general and administrative expense and expensed as incurred since recoverability of such expenditures is uncertain. Stock-Based Compensation Stock-based compensation expense represents the cost of the grant date fair value of employee stock option grants recognized over the requisite service period of the awards (usually the vesting period) on a straight-line basis, net of estimated forfeitures. For stock option grants with performance-based milestones, the expense is recorded over the service period after the achievement of the milestone is probable or the performance condition is achieved. For stock option grants with market-based conditions, the expense is recorded using the accelerated attribution method over the requisite service period for each vesting tranche. We account for stock options granted to non-employees using the fair value approach. These option grants are subject to periodic revaluation over their vesting terms. We estimate the fair value of employee and non-employee stock option grants using the Black-Scholes option pricing model. We estimate the fair value of the market-based stock option grants using a Monte Carlo simulation. Income Taxes We account for income taxes under the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the financial statements. Under this method, deferred tax assets and liabilities are determined on the basis of the differences between the financial statements and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. The effect of a change in tax rates on deferred tax assets and liabilities is recognized as income in the period that includes the enactment date. We recognize net deferred tax assets to the extent that we believe these assets are more likely than not to be realized. In making such a determination, we consider all available positive and negative evidence, including future reversals of existing taxable temporary differences, projected future taxable income, tax-planning strategies and results of recent operations. If we determine that we would be able to realize the deferred tax assets in the future in excess of their net recorded amount, we would make an adjustment to the deferred tax asset valuation allowance, which would reduce the provision for income taxes. We record uncertain tax positions on the basis of a two-step process whereby (1) we determine whether it is more likely than not that the tax positions will be sustained on the basis of the technical merits of the position and (2) for those tax positions that meet the more-likely-than-not recognition threshold, we recognize the largest amount of tax benefit that is more than 50% likely to be realized upon ultimate settlement with the related tax authority. We recognize interest and penalties related to unrecognized tax benefits within income tax expense. Any accrued interest and penalties are included within the related tax liability. In November 2015, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) No. 2015-17, “Balance Sheet Classification of Deferred Taxes,” which requires all deferred tax assets and liabilities to be classified as noncurrent on the balance sheet. The new accounting guidance is effective for annual reporting periods beginning after December 15, 2016 and interim periods therein. Early adoption is permitted as of the beginning of interim or annual reporting periods. We elected to early adopt this guidance prospectively beginning in the year ended December 31, 2015 and prior periods were not retrospectively adjusted. There was no material impact on the financial statements upon adoption. Net Loss Per Share Basic net loss per share is calculated by dividing the net loss attributable to common stockholders by the weighted average number of common shares outstanding for the period, without consideration for common stock equivalents and adjusted for the weighted average number of common shares outstanding that are subject to repurchase. We have excluded 61,814, 61,457 and 76,587 shares subject to repurchase from the weighted average number of common shares outstanding for the years ended December 31, 2015, 2014 and 2013, respectively. Diluted net loss per share attributable to common stockholders is calculated by dividing the net loss attributable to common stockholders by the weighted average number of common stock equivalents outstanding for the period determined using the treasury-stock method. Dilutive common stock equivalents are comprised of redeemable convertible preferred stock, redeemable convertible preferred stock issuable upon conversion of convertible promissory note, warrants for the purchase of redeemable convertible preferred stock, warrants for common stock and options outstanding under our stock option plan. For all periods presented, there is no difference in the number of shares used to calculate basic and diluted shares outstanding due to our net loss position. Potentially dilutive securities not included in the calculation of diluted net loss per share because to do so would be anti-dilutive are as follows (in common share equivalents): Years Ended December 31, 2015 2014 2013 Redeemable convertible preferred stock outstanding — 9,238,868 9,238,868 Redeemable convertible preferred stock issuable upon conversion of convertible promissory note — 94,455 94,455 Warrants for redeemable convertible preferred stock — 25,970 18,514 Warrants for common stock 25,970 — — Common stock options 2,625,280 1,514,471 821,057 Employee stock purchase plan 17,363 — — 2,668,613 10,873,764 10,172,894 The following table summarizes our net loss per share (in thousands, except per share data): Years Ended December 31, 2015 2014 2013 Numerator: Consolidated net loss (47,973 ) (24,350 ) (20,014 ) Accretion to redemption value (15 ) (416 ) (1,637 ) Net loss attributable to common stockholders (47,988 ) (24,766 ) (21,651 ) Denominator: Weighted average common shares outstanding 15,900,167 895,678 839,348 Weighted average common shares subject to repurchase (61,814 ) (61,457 ) (76,587 ) Weighted average common shares outstanding - basic and diluted 15,838,353 834,221 762,761 Net loss per share - basic and diluted $ (3.03 ) $ (29.69 ) $ (28.39 ) Recent Accounting Pronouncements In August 2014, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) No. 2014-15, Presentation of Financial Statements — Going Concern. ASU 2014-15 provides that in connection with preparing financial statements for each annual and interim reporting period, an entity’s management should evaluate whether there are conditions or events, considered in the aggregate, that raise substantial doubt about the entity’s ability to continue as a going concern within one year after the date that the financial statements are issued (or within one year after the date that the financial statements are available to be issued when applicable). ASU 2014-15 is effective for the annual reporting period ending after December 15, 2016, and for annual and interim periods thereafter. Early adoption is permitted. The adoption of ASU 2014-15 is not expected to have a material impact on our consolidated financial position or results of operations. In April 2015, the FASB issued ASU No. 2015-03, Interest - Imputation of Interest (Subtopic 835-30): Simplifying the Presentation of Debt Issuance Costs. ASU 2015-03 requires that debt issuance costs related to a recognized debt liability be presented in the balance sheet as a direct deduction from that debt liability, consistent with the presentation of a debt discount. The recognition and measurement guidance for debt issuance costs is not affected by ASU 2015-03. ASU 2015-03 is effective for fiscal years beginning after December 15, 2015, and interim periods within those fiscal years. Early application is permitted. The adoption of ASU 2015-03 is not expected to have a material impact on our consolidated financial position or results of operations. In November 2015, the FASB issued ASU No. 2015-17, “Balance Sheet Classification of Deferred Taxes,” which requires all deferred tax assets and liabilities to be classified as noncurrent on the balance sheet. The new accounting guidance is effective for annual reporting periods beginning after December 15, 2016 and interim periods therein. Early adoption is permitted as of the beginning of interim or annual reporting periods. We elected to early adopt this guidance prospectively beginning in the year ended December 31, 2015 and prior periods were not retrospectively adjusted. There was no material impact on our consolidated financial statements upon adoption. In January 2016, the FASB issued ASU No. 2016-01, “Financial Instruments-Overall: Recognition and Measurement of Financial Assets and Financial Liabilities,” which requires that (i) all equity investments, other than equity-method investments, in unconsolidated entities generally be measured at fair value through earnings and (ii) when the fair value option has been elected for financial liabilities, changes in fair value due to instrument-specific credit risk will be recognized separately in other comprehensive income. Additionally, the ASU changes the disclosure requirements for financial instruments. The new standard will be effective for fiscal years beginning after December 15, 2018, and interim periods within fiscal years beginning after December 15, 2019. Early adoption is permitted for certain provisions. The adoption of ASU 2015-03 is not expected to have a material impact on our consolidated financial position or results of operations. In February 2016, the FASB issued ASU 2016-02, Leases, to increase transparency and comparability among organizations by requiring recognition of lease assets and lease liabilities on the balance sheet and disclosure of key information about leasing arrangements. The standard will become effective for interim and annual periods beginning after December 15, 2018, with early adoption permitted. The guidance is required to be adopted at the earliest period presented using a modified retrospective approach. We are currently evaluating the impact the provisions will have on our consolidated financial statements and whether we will adopt the guidance early.

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2015
Fair Value Disclosures [Abstract]
Fair Value Measurements	3. Fair Value Measurements The carrying amounts of cash equivalents, prepaid and other assets, accounts payable and accrued liabilities are considered to be representative of their respective fair values because of the short-term nature of those instruments. Based on the borrowing rates currently available to us for loans with similar terms, which is considered a Level 2 input, we believe that the fair value of our commercial bank debt and convertible promissory notes approximate their carrying values. Investment securities and preferred stock warrant liabilities are recorded at fair value. The accounting guidance defines fair value, establishes a consistent framework for measuring fair value and expands disclosure for each major asset and liability category measured at fair value on either a recurring or nonrecurring basis. Fair value is defined as an exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As such, fair value is a market-based measurement that should be determined based on assumptions that market participants would use in pricing an asset or liability. As a basis for considering such assumptions, the accounting guidance establishes a three-tier fair value hierarchy, which prioritizes the inputs used in measuring fair value as follows: Level 1: Observable inputs such as quoted prices in active markets. Level 2: Inputs, other than the quoted prices in active markets that are observable either directly or indirectly. Level 3: Unobservable inputs in which there is little or no market data, which require the reporting entity to develop its own assumptions. Financial assets measured at fair value on a recurring basis consist of investment securities. Investment securities are recorded at fair value, defined as the exit price in the principal market in which we would transact, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. Level 2 securities are valued using quoted market prices for similar instruments, non-binding market prices that are corroborated by observable market data, or discounted cash flow techniques and include our investments in corporate debt securities, commercial paper, asset-backed securities and United States Treasury securities. Financial liabilities measured at fair value on a recurring basis include our preferred stock warrant liabilities. None of our non-financial assets and liabilities is recorded at fair value on a non-recurring basis. No transfers between levels have occurred during the periods presented. Assets and liabilities measured at fair value on a recurring basis are as follows (in thousands): Fair Value Measurements Using Quoted Active for Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) As of December 31, 2015: Assets: Current: Cash and cash equivalents $ 53,025 $ 53,025 $ — $ — Short-term investments: Commercial paper 2,996 — 2,996 — Corporate debt securities 39,514 — 39,514 — Sub-total short-term investments 42,510 — 42,510 — Long-term investments: United States Treasury securities 1,999 1,999 — — Asset-backed securities 10,912 — 10,912 — Corporate debt securities 16,903 — 16,903 — Sub-total long-term investments 29,814 1,999 27,815 — Total assets measured at fair value $ 125,349 $ 55,024 $ 70,325 $ — As of December 31, 2014: Assets: Cash and cash equivalents $ 13,899 $ 13,899 $ — $ — Short-term investments - Corporate debt securities 1,954 — 1,954 — Total assets measured at fair value $ 15,853 $ 13,899 $ 1,954 $ — Liabilities: Preferred stock warrant liabilities $ 319 $ — $ — $ 319 As of December 31, 2015 and 2014, available-for-sale investments are detailed as follows (in thousands): December 31, 2015 Gross Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Market Value Short-term investments: Commercial paper $ 2,996 $ — $ — $ 2,996 Corporate debt securities 39,575 — (61 ) 39,514 $ 42,571 $ — $ (61 ) $ 42,510 Long-term investments: United States Treasury securities $ 2,006 $ — $ (7 ) $ 1,999 Asset-backed securities 10,928 — (16 ) 10,912 Corporate debt securities 16,990 — (87 ) 16,903 $ 29,924 $ — $ (110 ) $ 29,814 December 31, 2014 Gross Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Market Value Short-term: Corporate debt securities $ 1,954 $ — $ — $ 1,954 Available-for-sale investments that are in an unrealized loss position as of December 31, 2015 are as follows (in thousands): Estimated Fair Value Gross Unrealized Losses United States Treasury securities $ 1,999 $ (7 ) Asset-backed securities 10,912 (16 ) Corporate debt securities 56,416 (148 ) $ 69,327 $ (171 ) As of December 31, 2015, all available-for-sale investments have contractual maturity dates within two years. As of December 31, 2015, there are 39 available-for-sale investments in gross unrealized loss position, all of which have been in such position for less than twelve months. At each reporting date, we perform an evaluation of impairment to determine if the unrealized losses are other-than-temporary. Factors considered in determining whether a loss is other-than-temporary include the length of time and extent to which fair value has been less than the cost basis, the financial condition of the issuer, and our intent and ability to hold the investment until recovery of its amortized cost basis. We intend, and have the ability, to hold our investments in unrealized loss positions until their amortized cost basis has been recovered. Based on our evaluation, we determined that the unrealized losses were not other-than-temporary as of December 31, 2015. All warrant liabilities were recorded at fair value utilizing the Black-Scholes option pricing model using significant unobservable inputs consistent with the inputs used for our stock-based compensation expense adjusted for the warrants’ expected life. The following table provides a reconciliation of all liabilities measured at fair value using Level 3 significant unobservable inputs (in thousands): Warrant Liabilities Balance as of December 31, 2014 $ 319 Change in fair value (29 ) Balance as of May 5, 2015 $ 290 Reclassification to additional paid-in capital as of IPO on May 6, 2015 (290 ) Balance as of December 31, 2015 $ —

Balance Sheet Details

Balance Sheet Details	12 Months Ended
	Dec. 31, 2015
Balance Sheet Details [Abstract]
Balance Sheet Details	4. Balance Sheet Details Property and equipment consist of the following (in thousands): December 31, 2015 2014 Computer and office equipment $ 336 $ 372 Scientific and laboratory equipment 3,518 2,848 Tenant improvements 1,687 1,668 5,541 4,888 Less accumulated depreciation and amortization (3,748 ) (2,963 ) $ 1,793 $ 1,925 Accrued expenses consist of the following (in thousands): December 31, 2015 2014 Accrued salaries, wages and benefits 1,710 684 Other accrued expenses 2,885 2,248 $ 4,595 $ 2,932

Debt, Commitments and Contingen

Debt, Commitments and Contingencies	12 Months Ended
	Dec. 31, 2015
Debt Instruments [Abstract]
Debt, Commitments and Contingencies	5. Debt, Commitments and Contingencies Commercial Bank Debt Commercial bank debt and unamortized discount balances are as follows (in thousands): December 31, 2015 2014 Commercial bank debt $ 5,202 $ 8,439 Less debt discount, net of current portion (6 ) (60 ) Commercial bank debt, net of debt discount 5,196 8,379 Less current portion of commercial bank debt (3,420 ) (3,237 ) Commercial bank debt, net of current portion $ 1,776 $ 5,142 Current portion of commercial bank debt $ 3,420 $ 3,237 Current portion of debt discount (54 ) (103 ) Current portion of commercial bank debt $ 3,366 $ 3,134 In each of April 2012 and August 2012, we borrowed $1.25 million under a loan and security agreement with Silicon Valley Bank (SVB Loan), at fixed interest rates of 4.89% and 4.85%, respectively. We were obligated to make interest-only payments through December 2012 and, beginning in December 2013, monthly payments of principal and interest through the maturity date in December 2015. The SVB Loan was amended in July 2013 to increase the available credit under the agreement to $10.0 million. In July 2013, we borrowed $5.0 million under the SVB Loan at a fixed interest rate of 5.0% and received $2.9 million of cash proceeds after repayment of the existing principal balance and related accrued interest and fees. In June 2014, we borrowed the remaining $5.0 million of available credit at a fixed interest rate of 5.88% and, subsequent to June 2014, had no available credit under the SVB Loan. We were obligated to make interest-only payments on each $5.0 million borrowing through June 2014 and, beginning in July 2014, monthly payments of principal and interest through the maturity date in June 2017. The final payment due in June 2017 includes an additional fee of $0.5 million, which is being accreted over the term of the debt using the effective interest method and is included in interest expense. The loan is collateralized by all of our assets, other than intellectual property, and contains customary affirmative and negative covenants, reporting requirements and events of default. In July 2013, in connection with the SVB Loan, we issued a warrant to purchase 59,312 shares of Series D redeemable convertible preferred stock at an exercise price of $2.529 per share. In June 2014, the warrant became exercisable for a total of 118,624 shares of Series D redeemable convertible preferred stock when we borrowed the remaining $5.0 million of available credit under the SVB Loan. In May 2015, upon the effectiveness of our IPO, the Series D redeemable convertible preferred stock warrants with Silicon Valley Bank (SVB) were converted to purchase 14,913 shares common stock. Future minimum principal and interest payments under our loan and security agreement with SVB, including the final payment, are as follows (in thousands): As of December 31, 2015 2016 $ 3,622 2017 2,311 $ 5,933 Less interest and final payment (731 ) Commercial bank debt $ 5,202 Facility Lease In December 2011, we entered into a noncancelable operating lease that included certain tenant improvement allowances and is subject to base lease payments, which escalate over the term of the lease, additional charges for common area maintenance and other costs. The lease expires in May 2017 and we have an option to extend the lease for a period of five years. Rent expense for the years ended December 31, 2015, 2014 and 2013 was $0.4 million, $0.2 million and $0.2 million, respectively. In conjunction with this lease, we borrowed $2.0 million under a subordinated unsecured convertible promissory note issued to the venture arm of our landlord. The convertible promissory note carried an annual interest rate of 8.0%. In May 2015, the $2.0 million outstanding principal balance of the convertible promissory note and the $0.5 million accrued interest on the convertible promissory note was repaid in full in connection with our IPO. Future minimum payments under the non-cancelable operating lease as of December 31, 2015 are as follows (in thousands): Operating Lease 2016 $ 610 2017 231 $ 841 Research Agreements and Funding Obligations In October 2007, we entered into a research funding and option agreement for certain technologies from The Scripps Research Institute (TSRI). Under the agreement, we provide funding to TSRI to conduct certain research activities. The agreement renews automatically for successive 12 month periods starting on May 31st of each year unless we provide 30 days’ prior written notice to terminate the agreement. TSRI has the right to terminate the agreement if we fail to make any payment under the agreement or for breach or insolvency. Under the research funding and option agreement, TSRI has granted us options to enter into license agreements to acquire rights and exclusive licenses to develop, make, have made, use, have used, import, have imported, offer to sell, sell, and have sold certain licensed products, processes and services based on certain technology arising from the sponsored research activities. Pursuant to the terms of these license agreements, TSRI is entitled to receive tiered royalties as a percentage of net sales and a percentage of nonroyalty revenue we may receive from our sublicensees or partners, with the amount owed decreasing if we enter into the applicable sublicense or partnering agreement after meeting a specified clinical milestone. In addition, we are obligated to pay TSRI up to an aggregate of $2.75 million under each license agreement upon the achievement of specific clinical and regulatory milestone events. In January 2015, we and TSRI entered into an amended and restated research funding and option agreement pursuant to which we agreed to issue 119,840 shares of our common stock to TSRI in consideration for the adjustment of sublicense payments and the assignment of certain intellectual property rights by TSRI to us. The $1.4 million fair value of the common stock issued to TSRI was recorded to research and development expense. We issued the shares of common stock to TSRI on March 31, 2015. During the years ended December 31, 2015, 2014 and 2013, excluding the fair value of the common stock issued to TSRI described above, we recognized expense under the agreement in the amount of $0.7 million, $0.7 million and $0.6 million, respectively. A member of our board of directors is a faculty member at TSRI and such payments fund a portion of his research activities conducted at TSRI. During the years ended December 31, 2015, 2014 and 2013, we provided charitable donations to the National Foundation for Cancer Research of $0.4 million. We have requested that the donations be restricted to certain basic research in cancer biology and therapeutics, a portion of which funds research activities conducted at TSRI in the laboratory of a member of our board of directors. FUJIFILM Diosynth Biotechnologies U.S.A., Inc. Agreement On June 16, 2015, we entered into a Master Services Agreement (the MSA) with FUJIFILM Diosynth Biotechnologies U.S.A., Inc. (Fujifilm) to complete the development of the manufacturing process and for the production of the drug substance for Resolaris, our drug in clinical development. Pursuant to the MSA, Fujifilm will provide the drug substance for Resolaris to support future clinical trials, including potential pivotal trials. Under the initial scope of work executed pursuant to the MSA, Fujifilm will conduct process optimization, scale-up and demonstration, and cGMP manufacturing of the drug substance of Resolaris, and we are required to pay Fujifilm based on development and production milestones up to the mid seven figures. In addition, we are billed for consumables on a pass-through basis. During the year ended December 31, 2015, expenses associated with this agreement were $5.1 million.

Stockholders' Equity

Stockholders' Equity	12 Months Ended
	Dec. 31, 2015
Equity [Abstract]
Stockholders' Equity	6. Stockholders’ Equity Sale of Series E Redeemable Convertible Preferred Stock On March 31, 2015, pursuant to a Series E stock purchase agreement, we issued an aggregate of 68,166,894 shares of our Series E redeemable convertible preferred stock at a purchase price of $1.119 per share, for aggregate cash consideration of $76.3 million and incurred $0.6 million of issuance costs. Each share of Series E redeemable convertible preferred stock was convertible into 0.12572 shares of our common stock. The purchase agreement also included an automatic conversion into approximately 0.10329 shares of common stock for each share of Series E redeemable convertible preferred stock upon completion of a qualified public offering on or before March 1, 2016. On May 12, 2015, all outstanding shares of Series E redeemable convertible preferred stock was converted into 7,040,991 shares of our common stock in connection with our IPO. Common Stock In March 2015, we amended and restated our certificate of incorporation to, among other things , (1) increase its authorized shares of common stock from 95,500,000 to 185,000,000 shares, (2) increase its authorized shares of preferred stock from 75,772,871 to 143,939,765 shares, of which 68,166,894 shares are designated as Series E preferred stock, and (3) set forth the rights, preferences and privileges of the Series E preferred stock. In May 2015, in connection with our IPO, we filed an amended and restated certificate of incorporation, authorizing 150,000,000 shares of common stock and 7,285,456 shares of preferred stock, 5,000,000 of which is undesignated preferred stock In addition, all outstanding shares of redeemable convertible preferred stock, including Series E, were converted into 16,279,859 shares of our common stock and warrants to purchase 206,581 shares of redeemable convertible preferred stock were converted into warrants to purchase 25,970 shares of our common stock with a resultant reclassification of the warrant liabilities to additional paid-in capital. 2014 Stock Plan We adopted a stock option plan in 2007 (the 2007 Plan), which was subsequently amended, restated and renamed in July 2014 (the 2014 Plan) to provide for the incentive stock options, nonstatutory stock options, stock and rights to purchase restricted stock to eligible recipients. Recipients of incentive stock options are eligible to purchase shares of our common stock at an exercise price equal to no less than the estimated fair market value of such stock on the date of grant. The maximum term of options under the 2014 Plan is ten years. Options granted generally vest over four years. 2015 Stock Plan In April 2015, our board of directors adopted, and our stockholders approved, the 2015 Plan. The 2015 Plan became effective on May 6, 2015 and we ceased granting any new awards under our 2014 Plan. Awards granted under the 2014 Plan prior to our IPO that are forfeited, canceled, reacquired by us prior to vesting satisfied without the issuance of stock or otherwise terminated (other than by exercise) will be added to shares available for issuance under the 2015 Plan. A total of 1,574,566 shares of our common stock were initially reserved for issuance under the 2015 Plan. In addition, the number of shares reserved and available for issuance under the 2015 Plan will automatically increase each January 1, beginning on January 1, 2016 and thereafter until January 1, 2019, by the lesser of (i) 1,840,000 shares, (ii) 4% of the outstanding number of shares of our common stock on the immediately preceding December 31 or (iii) an amount determined by our board of directors. Shares underlying any awards under the 2015 Plan that are forfeited, canceled, reacquired by us prior to vesting, satisfied without the issuance of stock or otherwise terminated (other than by exercise) will be added to shares available for issuance under the 2015 Plan. Employee Stock Purchase Plan In April 2015, our board of directors adopted, and our stockholders approved, our 2015 Employee Stock Purchase Plan (the 2015 ESPP). The 2015 ESPP became effective on May 6, 2015. A total of 227,623 shares of our common stock were initially reserved for issuance under the 2015 ESPP. In addition, the number of shares reserved and available for purchase under the 2015 ESPP will automatically increase each January 1, beginning on January 1, 2016 and thereafter until January 1, 2019, by 1% of the outstanding number of shares of our common stock on the immediately preceding December 31 or such lesser number of shares as determined by the administrator of the 2015 ESPP. Stock option activity is summarized as follows: Number of Outstanding Options Weighted Average Price Weighted Remaining Contractual Term Aggregate Intrinsic Value (in 000s) Outstanding as of December 31, 2014 1,514,471 $ 4.60 Granted 1,715,028 $ 11.29 Exercised (197,253 ) $ 2.78 Canceled (406,966 ) $ 6.37 Outstanding as of December 31, 2015 2,625,280 $ 8.83 8.02 $ 6,684 Options vested and expected to vest as of December 31, 2015 2,625,280 $ 8.83 8.02 $ 6,684 Options exercisable as of December 31, 2015 899,931 $ 4.91 6.64 $ 5,063 The assumptions used in the Black-Scholes option pricing model to determine the fair value of the employee stock option grants were as follows: Years Ended December 31, 2015 2014 2013 Expected term (in years) 5.50 – 6.08 5.77 – 6.56 6.52 – 6.56 Risk-free interest rate 1.5% – 1.9% 1.7% – 2.7% 2.0% – 2.2% Expected volatility 79.2% – 100.9% 111.0 % 109.0 % Expected dividend yield 0.0 % 0.0 % 0.0 % The assumptions used in the Black-Scholes option pricing model to determine the fair value of the ESPP offering were as follows: Year Ended December 31, 2015 Expected term (in years) 0.50 Risk-free interest rate 0.33 % Expected volatility 67.3 % Expected dividend yield 0.0 % Expected term . The expected term represents the period of time that options are expected to be outstanding. Because we do not have sufficient history of exercise behavior, we determine the expected life assumption using the simplified method, which is an average of the contractual term of the option and its vesting period. Risk-free interest rate. We base that risk-free interest rate assumption on the U.S. Treasury’s rates for U.S. Treasury zero-coupon bonds with maturities similar to those of the expected term of the award being valued. Expected volatility. The expected volatility assumption is based on volatilities of a peer group of similar companies whose share prices are publicly available. The peer group was developed based on companies in the biotechnology industry. Expected dividend yield. We base the expected dividend yield assumption on the fact that we have never paid cash dividends and have no present intention to pay cash dividends. The allocation of stock-based compensation for all options, including performance options,with market condition is as follows (in thousands): Years Ended December 31, 2015 2014 2013 Research and development $ 2,524 $ 527 $ 96 General and administrative 2,332 1,264 59 $ 4,856 $ 1,791 $ 155 The weighted–average grant date fair value per share of stock options granted by us during the years ended December 31, 2015, 2014 and 2013 was $11.29 per share, $10.18 per share and $3.42 per share, respectively. The aggregate intrinsic value of stock options exercised during the years ended December 31, 2015, 2014 and 2013 was $1.9 million, $0.4 million and $47,000, respectively. As of December 31, 2015, total unrecognized share-based compensation expense related to unvested stock options was approximately $12.7 million. This unrecognized cost is expected to be recognized over a weighted-average period of approximately 2.9 years ratably on a straight-line basis. During the fourth quarter of 2014, we modified certain vesting conditions of performance-based equity awards for our Chief Executive Officer which resulted in incremental share-based compensation costs of $0.7 million, of which $0.6 million was recognized as expense during the year ended December 31, 2014. In October 2015, our Compensation Committee of the Board of Directors approved an amendment to accelerate the vesting schedule of certain outstanding stock options representing 931,749 shares granted to active employees and certain consultants under the 2014 Plan to change the vesting schedule of such options from six-years to four-years retroactive to the original vesting commencement dates. We recorded $0.8 million of stock compensation expense in connection with the modification during the year ended December 31, 2015. In October 2015, we granted our employees and certain consultants performance options with a market condition to purchase up to an aggregate 169,402 shares of common stock at an exercise price of $10.24. Upon achievement of specified performance goals by October 2017, such performance-based options shall begin to vest over four years in equal monthly installments, otherwise the options will be subject to forfeiture. The fair value of the stock options awarded that include market-based performance conditions is estimated on the date of the grant using a Monte Carlo simulation, based on the market price of the underlying common stock, expected performance measurement period, expected peer group stock price volatility and expected risk-free interest rate. The weighted average grant date fair value was $4.23.The performance options with market conditions grants are expensed using the accelerated attribution method over the requisite service period of 5.0 years regardless of whether the market condition is achieved or earned and vest. The assumptions used to determine the fair value of the performance options with market condition were as follows: Year Ended December 31, 2015 Expected term (in years) 4.8 Risk-free interest rate 2.05 % Expected volatility 80.6 % Expected dividend yield 0.0 % 401(k) Plan We maintain a defined contribution 401(k) plan available to eligible employees. Employee contributions are voluntary and are determined on an individual basis, limited to the maximum amount allowable under federal tax regulations. In April 2015, our Board of Directors approved a policy, beginning on June 1, 2015, to match employee contributions equal to 50% of the participant’s contribution of up to a maximum of 6% of the participants’s annual salary. We made discretionary contributions totaling $0.1 million during the year ended December 31, 2015. Warrants Warrants outstanding as of December 31, 2015: Number Exercise Price Expiration Outstanding Per Share Date 9,051 $ 6.63 September 2017 2,006 $ 7.48 March 2021 14,913 $ 20.12 July 2023 25,970 Common Stock Reserved for Future Issuance Common stock reserved for future issuance is as follows: As of December 31, 2015 2014 Conversion of redeemable convertible preferred stock — 9,238,868 Conversion of redeemable convertible preferred stock issuable upon conversion of promissory note — 94,455 Redeemable convertible preferred stock warrants — 25,970 Common stock warrants 25,970 — Common stock options outstanding 2,625,280 1,514,471 Shares available under the 2014 Plan 984,357 180,190 Shares available under the 2015 Plan 903,350 — Shares available under the 2015 ESPP Plan 227,623 — 4,766,580 11,053,954

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Income Taxes	7. Income Taxes Pretax earnings (loss) were generated by both domestic and foreign operations as follows (in thousands): Years Ended December 31, 2015 2014 2013 United States $ (47,490 ) $ (34,885 ) $ (11,085 ) Foreign (483 ) 10,535 (8,929 ) $ (47,973 ) $ (24,350 ) $ (20,014 ) A reconciliation of the expected statutory federal income tax provision to the actual income tax provision is summarized as follows (in thousands): Years Ended December 31, 2015 2014 2013 Expected income taxes benefit at federal statutory rate $ (16,311 ) $ (8,279 ) $ (6,804 ) State income taxes, net of federal benefit — (2,023 ) (634 ) Permanent items and other 19 (321 ) 2 Stock-based compensation 734 396 — Research credits (2,674 ) (372 ) (397 ) Unrecognized tax benefits 1,070 144 159 Foreign rate differential 84 (3,391 ) 2,978 Change in tax rate 3,551 — — Other, net 112 293 (26 ) Change in valuation allowance 13,415 13,553 4,722 Income tax (benefit) expense $ — $ — $ — Deferred income taxes are provided for temporary differences in recognizing certain income and expense items for financial and tax reporting purposes. The deferred tax assets consisted primarily of the income tax benefits from net operating loss (NOL) carryforwards, research and development credits and capitalized research and development expenses, along with other accruals and reserves. Valuation allowances of $48.3 million, $34.8 million and $21.3 million as of December 31, 2015, 2014 and 2013, respectively, have been recorded to offset deferred tax assets as realization of such assets does not meet the more-likely-than-not threshold under ASC 740, Accounting for Income Taxes Significant components of our deferred tax assets are summarized as follows (in thousands): December 31, 2015 2014 2013 Net operating loss carryforwards $ 24,869 $ 20,066 $ 13,093 Capitalized research and development expenses 14,181 7,855 6,684 Research credits and other state credits 3,565 1,368 1,171 Intangible assets 4,176 4,926 28 Depreciation and amortization (120 ) (260 ) (360 ) Reserve and accruals 1,648 891 677 Valuation allowance (48,319 ) (34,846 ) (21,293 ) Net deferred tax assets $ — $ — $ — The consolidated financial statements in the prior years included six variable interest entities which we referred to as the Affiliates. In the fourth quarter of 2014, we dissolved all of the Affiliates and, as a result, acquired intellectual property originally developed by the Affiliates. For book purposes, as this was a transaction between consolidated entities, no intangible asset was recognized. For tax purposes, the intellectual property will be amortized over 15 years resulting in an increase to deferred tax assets as of December 31, 2014. The increase in deferred tax assets was offset by a corresponding adjustment to the valuation allowance. As a result of the dissolution, we forgave intercompany loans and recorded a corresponding tax deduction; whereas the Affiliates recognized cancellation of debt income which was offset by net operating losses. As of December 31, 2015, we had approximately $65.4 million, $48.9 million, and $5.8 million of net operating loss carryforwards for federal, state, and foreign purposes, respectively, net of Section 382 limitations, available to offset future taxable income. The federal and state net operating loss carryforwards begin to expire in 2025 and 2016, respectively. California net operating loss carryforwards of $0.2 million and $1.4 million will expire in 2016 and 2017, respectively. California net operating loss carryforwards of $47.3 million will expire from 2028 through 2034. The foreign net operating losses carry over indefinitely. As of December 31, 2015, we had federal and state research and development credit carryforwards of approximately $1.8 million and $1.9 million, respectively, net of Section 382 limitations, which begin to expire in 2026 for federal purposes and carry over indefinitely for state purposes. We had $2.9 million of federal Orphan Drug Credits as of December 31, 2015, which will begin to expire in 2035. In November 2015, the FASB issued ASU No. 2015-17, “Balance Sheet Classification of Deferred Taxes,” which requires all deferred tax assets and liabilities to be classified as noncurrent on the balance sheet. The new accounting guidance is effective for annual reporting periods beginning after December 15, 2016 and interim periods therein. Early adoption is permitted as of the beginning of interim or annual reporting periods. We elected to early adopt this guidance prospectively beginning in the year ended December 31, 2015 and prior periods were not retrospectively adjusted. There was no material impact on the financial statements upon adoption. Utilization of the domestic NOL and research and development credit carryforwards may be subject to a substantial annual limitation due to ownership change limitations that may have occurred or that could occur in the future, as required by Section 382 and 383 of the Internal Revenue Code of 1986, as amended (the Code), as well as similar state and foreign provisions. These ownership changes may limit the amount of NOL and research and development credit carryforwards that can be utilized annually to offset future taxable income and tax, respectively. In general, an “ownership change” as defined by Section 382 of the Code results from a transaction or series of transactions over a three-year period resulting in an ownership change of more than 50 percentage points of the outstanding stock of a company by certain stockholders. Since the Company’s formation, we raised capital through the issuance of capital stock on several occasions which on its own or combined with the purchasing stockholders’ subsequent disposition of those shares, has resulted in such an ownership change, and could result in an ownership change in the future. Upon the occurrence of an ownership change under Section 382 as outlined above, utilization of the NOL and research and development credit carryforwards become subject to an annual limitation under Section 382 of the Code, which is determined by first multiplying the value of our stock at the time of the ownership change by the applicable long-term, tax-exempt rate, which could be subject to additional adjustments. Any limitation may result in expiration of a portion of the NOL or research and development credit carryforwards before utilization. We completed an analysis through September 7, 2011, and had adjusted our NOL and research and development tax credit carryforwards accordingly. Ownership changes that may have occurred subsequent to September 7, 2011, and future ownership changes, including any ownership change resulting from this offering, may further limit our ability to utilize its remaining tax attributes. We recognize a tax benefit from an uncertain tax position when it is more likely than not that the position will be sustained upon examination, including resolutions of any related appeals or litigation processes, based on the technical merits. Income tax positions must meet a more-likely-than-not recognition threshold to be recognized. Our practice is to recognize interest and penalties related to income tax matters in income tax expense. We had no accrual for interest and penalties on our balance sheet and had not recognized interest or penalties in the consolidated statements of operations for the years ended December 31, 2015, 2014 and 2013. Due to the existence of the valuation allowance, future changes in unrecognized tax benefits will not impact our effective tax rate. Uncertain tax positions are evaluated based upon the facts and circumstances that exist at each reporting period. Subsequent changes in judgment based upon new information may lead to changes in recognition, derecognition, and measurement. Adjustments may result, for example, upon resolution of an issue with the taxing authorities, or expiration of a statute of limitations barring an assessment for an issue. The activity related to our unrecognized tax benefits is summarized as follows (in thousands): Balance as of December 31, 2012 $ 774 Increase related to prior year tax positions 79 Increase related to current year tax positions 94 Balance as of December 31, 2013 947 Increase related to prior year tax positions — Increase related to current year tax positions 177 Other decreases (18 ) Balance as of December 31, 2014 1,106 Increase related to prior year tax positions 2,404 Increase related to current year tax positions 1,523 Balance as of December 31, 2015 5,033 We do not anticipate that the amount of unrecognized tax benefits as of December 31, 2015 will change within the next twelve months. We are subject to taxation in the United States, Hong Kong and state jurisdictions. Our tax years from inception are subject to examination by the United States, Hong Kong and California authorities due to the carry forward of unutilized NOLs and research and development credits.

Quarterly Financial Data (Unaud

Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2015
Quarterly Financial Data [Abstract]
Quarterly Financial Data (Unaudited)	8. Quarterly Financial Data (Unaudited) The following financial information reflects all normal recurring adjustments, which are, in our opinion, necessary for a fair statement of the results of the interim periods. Summarized quarterly data for 2015 and 2014 are as follows (in thousands, except per share data): For the quarters ended March 31 June 30 September 30 December 31 2015: Operating expenses $ 8,922 $ 10,898 $ 11,313 $ 16,483 Net loss (9,071 ) (11,080 ) (11,329 ) (16,493 ) Basic and diluted net loss per share $ (9.39 ) $ (0.74 ) $ (0.48 ) (0.70 ) 2014: Operating expenses $ 5,930 $ 5,363 $ 6,231 $ 6,030 Net loss (6,093 ) (5,588 ) (6,631 ) (6,038 ) Basic and diluted net loss per share $ (7.87 ) $ (6.85 ) $ (8.02 ) $ (7.04 ) Net loss per share is computed independently for each of the quarters presented. Therefore, the sum of the quarterly per-share calculations will not necessarily equal the annual per share calculation.

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2015
Subsequent Events [Abstract]
Subsequent Events	9. Subsequent Events In January 2016, we granted to our executives, employees and certain consultants performance options with a market condition to purchase up to an aggregate 386,210 shares of common stock at an exercise price of $9.13. Upon achievement of specified goals by January 4, 2018, such performance options shall begin to vest over four years in equal monthly installments, otherwise the options will be subject to forfeiture. The grant date fair value of $1.93 was determined using a Monte Carlo simulation and will be expensed using the accelerated attribution method over the requisite service period of 5.0 years. In addition, our Compensation Committee of the Board of Directors approved annual option grants to our executives, members of board of directors, employees and certain consultants to purchase an aggregate of 574,482 shares of common stock at an exercise price of $6.14. The weighted average grant date fair value of the options granted to was $4.76.

Summary of Significant Accoun17

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2015
Accounting Policies [Abstract]
Organization and Business	Organization and Business We were incorporated in the state of Delaware on September 8, 2005. We are focused on the discovery and clinical development of innovative medicines for patients suffering from severe rare diseases.
Initial Public Offering	Initial Public Offering On May 12, 2015, we completed our initial public offering (IPO) of 6,164,000 shares of common stock at $14.00 per share, resulting in gross proceeds of approximately $86.3 million and net proceeds of $75.9 million, after underwriting and other expenses of approximately $10.4 million (consisting of approximately $6.0 million in underwriting discounts and commissions and approximately $4.4 million in other offering expenses). In connection with the IPO, all outstanding shares of redeemable convertible preferred stock were converted into 16,279,859 shares of our common stock and warrants to purchase 206,581 shares of redeemable convertible preferred stock were converted into warrants to purchase 25,970 shares of our common stock with a resultant reclassification of the warrant liabilities to additional paid-in capital. In addition, we filed an amended and restated certificate of incorporation on May 12, 2015, authorizing 150,000,000 shares of common stock and 7,285,456 shares of preferred stock, 5,000,000 of which is undesignated preferred stock. Upon the closing of the IPO, 1,574,566 shares of common stock were reserved for future issuance under the 2015 Stock Option and Incentive Plan (the 2015 Plan) and 227,623 shares of common stock were reserved for future issuance under the 2015 Employee Stock Purchase Plan (the 2015 ESPP).
Principles of Consolidation	Principles of Consolidation Our consolidated financial statements include our accounts, our 98% majority-owned subsidiary in Hong Kong, Pangu BioPharma Limited (Pangu BioPharma). All intercompany transactions and balances are eliminated in consolidation.
Reverse Stock Split	Reverse Stock Split On May 5, 2015, we filed an amendment to our amended and restated certificate of incorporation to effect a one-for-7.95413 reverse stock split of our common stock (the Reverse Stock Split). The par value and the authorized shares of the common and convertible preferred stock were not adjusted as a result of the Reverse Stock Split. All share information has been retroactively restated to reflect the Reverse Stock Split.
Use of Estimates	Use of Estimates Our consolidated financial statements are prepared in accordance with generally accepted accounting principles (GAAP). The preparation of our consolidated financial statements requires us to make estimates and assumptions that impact the reported amounts of assets, liabilities and expenses and the disclosure of contingent assets and liabilities in our consolidated financial statements and accompanying notes. The most significant estimates in our consolidated financial statements relate to the fair value of equity issuances and awards, and clinical trial and research and development expenses. Although these estimates are based on our knowledge of current events and actions we may undertake in the future, actual results may ultimately differ materially from these estimates and assumptions.
Segment Reporting	Financial Information about We operate in a single accounting segment. Refer to Note 1, “Organization, Business and Basis of Presentation” in the Notes to Consolidated Financial Statements included elsewhere in this report..
Cash and Cash Equivalents	Cash and Cash Equivalents Cash and cash equivalents consist primarily of readily available checking, money market accounts and money market funds. We consider all highly liquid investments that mature in three months or less when purchased to be cash equivalents.
Investment Securities	Investment Securities Investment securities primarily consist of investment grade corporate debt securities, asset-backed securities, commercial paper and United States Treasury securities. We classify all investment securities as available-for-sale. Investment securities are carried at fair value, with the unrealized gains and losses, if any, reported as a component of other comprehensive income (loss) in stockholders’ equity (deficit) until realized. Realized gains and losses from the sale of investment securities, if any, are determined on a specific identification basis. A decline in the market value of any investment security below cost that is determined to be other than temporary will result in an impairment charge to earnings and a new cost basis for the security is established. No such impairment charges were recorded for any period presented. Premiums and discounts are amortized or accreted over the life of the related security as an adjustment to yield using the straight-line method and are included in interest income. Interest income is recognized when earned. As of December 31, 2015, we held an aggregate total of $72.3 million of investment securities which consisted of corporate debt securities, asset-backed securities, commercial paper and United States Treasury securities, all of which will mature in less than two years and there was $0.2 million difference between the amortized cost and fair value of these investment securities. As of December 31, 2014, we held $2.0 million of corporate debt securities, all of which mature in less than three months, and there was no difference between the amortized cost and fair value of these investment securities.
Concentration of Credit Risk	Concentration of Credit Risk Financial instruments that potentially subject us to significant concentration of credit risk consist primarily of cash, cash equivalents and investment securities. We have established guidelines regarding diversification of investments and their maturities, which are designed to maintain principal and maximize liquidity. We maintain deposits in federally insured financial institutions in excess of federally insured limits. We have not experienced any losses in such accounts and we believe that we are not exposed to significant credit risk due to the financial position of the depository institutions in which those deposits are held.
Property and Equipment	Property and Equipment Property and equipment are stated at cost and depreciated on a straight-line basis over the estimated useful life of the related assets (generally three to seven years). Leasehold improvements are stated at cost and amortized on a straight-line basis over the lesser of the remaining term of the related lease or the estimated useful life of the leasehold improvements. Repairs and maintenance costs are charged to expense as incurred .
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets Long-lived assets consist primarily of property and equipment. An impairment loss is recorded if and when events and circumstances indicate that assets might be impaired and the undiscounted cash flows estimated to be generated by those assets are less than the carrying amount of those assets. While our current and historical operating losses are indicators of impairment, we believe that future cash flows to be received support the carrying value of our long-lived assets and, accordingly, have not recognized any impairment losses since inception.
Accrued Expenses	Accrued Expenses As part of the process of preparing our consolidated financial statements, we are required to estimate accrued expenses, including accrued research and development expenses for fees paid to investigative sites and CROs in connection with clinical trials; service providers in connection with preclinical development activities; service providers related to product manufacturing; and other professional services. The accrual process involves reviewing open contracts and purchase orders, communicating with our personnel to identify services that have been performed on our behalf and estimating the level of service performed and the associated cost incurred for the service when we have not yet been invoiced or otherwise notified of the actual cost. We make estimates of accrued expenses as of each balance sheet date in our consolidated financial statements based on facts and circumstances known to us at that time. Although we do not expect the estimates to be materially different from amounts actually incurred, if the estimates of the status and timing of services performed differs from the actual status and timing of services performed, we may report amounts that are too high or too low in any particular period.
Deferred Rent	Deferred Rent Rent expense, including the value of tenant improvement allowances received, is recorded on a straight-line basis over the term of the lease. The difference between rent expense and amounts paid under the lease agreements is recorded as deferred rent in the accompanying consolidated balance sheets.
Research and Development Costs	Research and Development Costs Research and development costs are expensed as incurred. Research and development costs include: salaries and employee-related expenses, including stock-based compensation and benefits for personnel in research and product development functions; costs associated with conducting our preclinical, development and regulatory activities, including fees paid to third-party professional consultants, service providers and our scientific, therapeutic and clinical advisory boards; costs to acquire, develop and manufacture preclinical study and clinical trial materials; costs incurred under clinical trial agreements with clinical research organizations and investigative sites; costs for laboratory supplies; payments related to licensed products and technologies; allocated facilities and information technology costs; and depreciation.
Patent Costs	Patent Costs Costs related to filing and pursuing patent applications are recorded as general and administrative expense and expensed as incurred since recoverability of such expenditures is uncertain.
Stock-Based Compensation	Stock-Based Compensation Stock-based compensation expense represents the cost of the grant date fair value of employee stock option grants recognized over the requisite service period of the awards (usually the vesting period) on a straight-line basis, net of estimated forfeitures. For stock option grants with performance-based milestones, the expense is recorded over the service period after the achievement of the milestone is probable or the performance condition is achieved. For stock option grants with market-based conditions, the expense is recorded using the accelerated attribution method over the requisite service period for each vesting tranche. We account for stock options granted to non-employees using the fair value approach. These option grants are subject to periodic revaluation over their vesting terms. We estimate the fair value of employee and non-employee stock option grants using the Black-Scholes option pricing model. We estimate the fair value of the market-based stock option grants using a Monte Carlo simulation.
Income Taxes	Income Taxes We account for income taxes under the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the financial statements. Under this method, deferred tax assets and liabilities are determined on the basis of the differences between the financial statements and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. The effect of a change in tax rates on deferred tax assets and liabilities is recognized as income in the period that includes the enactment date. We recognize net deferred tax assets to the extent that we believe these assets are more likely than not to be realized. In making such a determination, we consider all available positive and negative evidence, including future reversals of existing taxable temporary differences, projected future taxable income, tax-planning strategies and results of recent operations. If we determine that we would be able to realize the deferred tax assets in the future in excess of their net recorded amount, we would make an adjustment to the deferred tax asset valuation allowance, which would reduce the provision for income taxes. We record uncertain tax positions on the basis of a two-step process whereby (1) we determine whether it is more likely than not that the tax positions will be sustained on the basis of the technical merits of the position and (2) for those tax positions that meet the more-likely-than-not recognition threshold, we recognize the largest amount of tax benefit that is more than 50% likely to be realized upon ultimate settlement with the related tax authority. We recognize interest and penalties related to unrecognized tax benefits within income tax expense. Any accrued interest and penalties are included within the related tax liability. In November 2015, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) No. 2015-17, “Balance Sheet Classification of Deferred Taxes,” which requires all deferred tax assets and liabilities to be classified as noncurrent on the balance sheet. The new accounting guidance is effective for annual reporting periods beginning after December 15, 2016 and interim periods therein. Early adoption is permitted as of the beginning of interim or annual reporting periods. We elected to early adopt this guidance prospectively beginning in the year ended December 31, 2015 and prior periods were not retrospectively adjusted. There was no material impact on the financial statements upon adoption.
Net Loss Per Share	Net Loss Per Share Basic net loss per share is calculated by dividing the net loss attributable to common stockholders by the weighted average number of common shares outstanding for the period, without consideration for common stock equivalents and adjusted for the weighted average number of common shares outstanding that are subject to repurchase. We have excluded 61,814, 61,457 and 76,587 shares subject to repurchase from the weighted average number of common shares outstanding for the years ended December 31, 2015, 2014 and 2013, respectively. Diluted net loss per share attributable to common stockholders is calculated by dividing the net loss attributable to common stockholders by the weighted average number of common stock equivalents outstanding for the period determined using the treasury-stock method. Dilutive common stock equivalents are comprised of redeemable convertible preferred stock, redeemable convertible preferred stock issuable upon conversion of convertible promissory note, warrants for the purchase of redeemable convertible preferred stock, warrants for common stock and options outstanding under our stock option plan. For all periods presented, there is no difference in the number of shares used to calculate basic and diluted shares outstanding due to our net loss position. Potentially dilutive securities not included in the calculation of diluted net loss per share because to do so would be anti-dilutive are as follows (in common share equivalents): Years Ended December 31, 2015 2014 2013 Redeemable convertible preferred stock outstanding — 9,238,868 9,238,868 Redeemable convertible preferred stock issuable upon conversion of convertible promissory note — 94,455 94,455 Warrants for redeemable convertible preferred stock — 25,970 18,514 Warrants for common stock 25,970 — — Common stock options 2,625,280 1,514,471 821,057 Employee stock purchase plan 17,363 — — 2,668,613 10,873,764 10,172,894 The following table summarizes our net loss per share (in thousands, except per share data): Years Ended December 31, 2015 2014 2013 Numerator: Consolidated net loss (47,973 ) (24,350 ) (20,014 ) Accretion to redemption value (15 ) (416 ) (1,637 ) Net loss attributable to common stockholders (47,988 ) (24,766 ) (21,651 ) Denominator: Weighted average common shares outstanding 15,900,167 895,678 839,348 Weighted average common shares subject to repurchase (61,814 ) (61,457 ) (76,587 ) Weighted average common shares outstanding - basic and diluted 15,838,353 834,221 762,761 Net loss per share - basic and diluted $ (3.03 ) $ (29.69 ) $ (28.39 )
Recent Accounting Pronouncements	Recent Accounting Pronouncements In August 2014, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) No. 2014-15, Presentation of Financial Statements — Going Concern. ASU 2014-15 provides that in connection with preparing financial statements for each annual and interim reporting period, an entity’s management should evaluate whether there are conditions or events, considered in the aggregate, that raise substantial doubt about the entity’s ability to continue as a going concern within one year after the date that the financial statements are issued (or within one year after the date that the financial statements are available to be issued when applicable). ASU 2014-15 is effective for the annual reporting period ending after December 15, 2016, and for annual and interim periods thereafter. Early adoption is permitted. The adoption of ASU 2014-15 is not expected to have a material impact on our consolidated financial position or results of operations. In April 2015, the FASB issued ASU No. 2015-03, Interest - Imputation of Interest (Subtopic 835-30): Simplifying the Presentation of Debt Issuance Costs. ASU 2015-03 requires that debt issuance costs related to a recognized debt liability be presented in the balance sheet as a direct deduction from that debt liability, consistent with the presentation of a debt discount. The recognition and measurement guidance for debt issuance costs is not affected by ASU 2015-03. ASU 2015-03 is effective for fiscal years beginning after December 15, 2015, and interim periods within those fiscal years. Early application is permitted. The adoption of ASU 2015-03 is not expected to have a material impact on our consolidated financial position or results of operations. In November 2015, the FASB issued ASU No. 2015-17, “Balance Sheet Classification of Deferred Taxes,” which requires all deferred tax assets and liabilities to be classified as noncurrent on the balance sheet. The new accounting guidance is effective for annual reporting periods beginning after December 15, 2016 and interim periods therein. Early adoption is permitted as of the beginning of interim or annual reporting periods. We elected to early adopt this guidance prospectively beginning in the year ended December 31, 2015 and prior periods were not retrospectively adjusted. There was no material impact on our consolidated financial statements upon adoption. In January 2016, the FASB issued ASU No. 2016-01, “Financial Instruments-Overall: Recognition and Measurement of Financial Assets and Financial Liabilities,” which requires that (i) all equity investments, other than equity-method investments, in unconsolidated entities generally be measured at fair value through earnings and (ii) when the fair value option has been elected for financial liabilities, changes in fair value due to instrument-specific credit risk will be recognized separately in other comprehensive income. Additionally, the ASU changes the disclosure requirements for financial instruments. The new standard will be effective for fiscal years beginning after December 15, 2018, and interim periods within fiscal years beginning after December 15, 2019. Early adoption is permitted for certain provisions. The adoption of ASU 2015-03 is not expected to have a material impact on our consolidated financial position or results of operations. In February 2016, the FASB issued ASU 2016-02, Leases, to increase transparency and comparability among organizations by requiring recognition of lease assets and lease liabilities on the balance sheet and disclosure of key information about leasing arrangements. The standard will become effective for interim and annual periods beginning after December 15, 2018, with early adoption permitted. The guidance is required to be adopted at the earliest period presented using a modified retrospective approach. We are currently evaluating the impact the provisions will have on our consolidated financial statements and whether we will adopt the guidance early.

Summary of Significant Accoun18

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2015
Accounting Policies [Abstract]
Schedule of Potentially Dilutive Securities Not Included in Calculation of Diluted Net Loss Per Share	Potentially dilutive securities not included in the calculation of diluted net loss per share because to do so would be anti-dilutive are as follows (in common share equivalents): Years Ended December 31, 2015 2014 2013 Redeemable convertible preferred stock outstanding — 9,238,868 9,238,868 Redeemable convertible preferred stock issuable upon conversion of convertible promissory note — 94,455 94,455 Warrants for redeemable convertible preferred stock — 25,970 18,514 Warrants for common stock 25,970 — — Common stock options 2,625,280 1,514,471 821,057 Employee stock purchase plan 17,363 — — 2,668,613 10,873,764 10,172,894
Summary of Net Loss Per Share	The following table summarizes our net loss per share (in thousands, except per share data): Years Ended December 31, 2015 2014 2013 Numerator: Consolidated net loss (47,973 ) (24,350 ) (20,014 ) Accretion to redemption value (15 ) (416 ) (1,637 ) Net loss attributable to common stockholders (47,988 ) (24,766 ) (21,651 ) Denominator: Weighted average common shares outstanding 15,900,167 895,678 839,348 Weighted average common shares subject to repurchase (61,814 ) (61,457 ) (76,587 ) Weighted average common shares outstanding - basic and diluted 15,838,353 834,221 762,761 Net loss per share - basic and diluted $ (3.03 ) $ (29.69 ) $ (28.39 )

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2015
Fair Value Disclosures [Abstract]
Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis	Assets and liabilities measured at fair value on a recurring basis are as follows (in thousands): Fair Value Measurements Using Quoted Active for Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) As of December 31, 2015: Assets: Current: Cash and cash equivalents $ 53,025 $ 53,025 $ — $ — Short-term investments: Commercial paper 2,996 — 2,996 — Corporate debt securities 39,514 — 39,514 — Sub-total short-term investments 42,510 — 42,510 — Long-term investments: United States Treasury securities 1,999 1,999 — — Asset-backed securities 10,912 — 10,912 — Corporate debt securities 16,903 — 16,903 — Sub-total long-term investments 29,814 1,999 27,815 — Total assets measured at fair value $ 125,349 $ 55,024 $ 70,325 $ — As of December 31, 2014: Assets: Cash and cash equivalents $ 13,899 $ 13,899 $ — $ — Short-term investments - Corporate debt securities 1,954 — 1,954 — Total assets measured at fair value $ 15,853 $ 13,899 $ 1,954 $ — Liabilities: Preferred stock warrant liabilities $ 319 $ — $ — $ 319
Schedule of Available-for-sale Investments	As of December 31, 2015 and 2014, available-for-sale investments are detailed as follows (in thousands): December 31, 2015 Gross Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Market Value Short-term investments: Commercial paper $ 2,996 $ — $ — $ 2,996 Corporate debt securities 39,575 — (61 ) 39,514 $ 42,571 $ — $ (61 ) $ 42,510 Long-term investments: United States Treasury securities $ 2,006 $ — $ (7 ) $ 1,999 Asset-backed securities 10,928 — (16 ) 10,912 Corporate debt securities 16,990 — (87 ) 16,903 $ 29,924 $ — $ (110 ) $ 29,814 December 31, 2014 Gross Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Market Value Short-term: Corporate debt securities $ 1,954 $ — $ — $ 1,954
Schedule of Available-for-sale Investments in Unrealized Loss Position	Available-for-sale investments that are in an unrealized loss position as of December 31, 2015 are as follows (in thousands): Estimated Fair Value Gross Unrealized Losses United States Treasury securities $ 1,999 $ (7 ) Asset-backed securities 10,912 (16 ) Corporate debt securities 56,416 (148 ) $ 69,327 $ (171 )
Schedule of Reconciliation of All Liabilities Measured at Fair Value Using Level 3 Significant Unobservable Inputs	The following table provides a reconciliation of all liabilities measured at fair value using Level 3 significant unobservable inputs (in thousands): Warrant Liabilities Balance as of December 31, 2014 $ 319 Change in fair value (29 ) Balance as of May 5, 2015 $ 290 Reclassification to additional paid-in capital as of IPO on May 6, 2015 (290 ) Balance as of December 31, 2015 $ —

Balance Sheet Details (Tables)

Balance Sheet Details (Tables)	12 Months Ended
	Dec. 31, 2015
Balance Sheet Details [Abstract]
Summary of Property and Equipment	Property and equipment consist of the following (in thousands): December 31, 2015 2014 Computer and office equipment $ 336 $ 372 Scientific and laboratory equipment 3,518 2,848 Tenant improvements 1,687 1,668 5,541 4,888 Less accumulated depreciation and amortization (3,748 ) (2,963 ) $ 1,793 $ 1,925
Summary of Accrued Expenses	Accrued expenses consist of the following (in thousands): December 31, 2015 2014 Accrued salaries, wages and benefits 1,710 684 Other accrued expenses 2,885 2,248 $ 4,595 $ 2,932

Debt, Commitments and Conting21

Debt, Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2015
Debt Instruments [Abstract]
Schedule of Commercial Bank Debt and Unamortized Discount	Commercial bank debt and unamortized discount balances are as follows (in thousands): December 31, 2015 2014 Commercial bank debt $ 5,202 $ 8,439 Less debt discount, net of current portion (6 ) (60 ) Commercial bank debt, net of debt discount 5,196 8,379 Less current portion of commercial bank debt (3,420 ) (3,237 ) Commercial bank debt, net of current portion $ 1,776 $ 5,142 Current portion of commercial bank debt $ 3,420 $ 3,237 Current portion of debt discount (54 ) (103 ) Current portion of commercial bank debt $ 3,366 $ 3,134
Schedule of Future Minimum Principal and Interest Payments under Loan and Security Agreement SVB Including Final Payment	Future minimum principal and interest payments under our loan and security agreement with SVB, including the final payment, are as follows (in thousands): As of December 31, 2015 2016 $ 3,622 2017 2,311 $ 5,933 Less interest and final payment (731 ) Commercial bank debt $ 5,202
Schedule of Future Minimum Payments under Non-cancelable Operating Lease	Future minimum payments under the non-cancelable operating lease as of December 31, 2015 are as follows (in thousands): Operating Lease 2016 $ 610 2017 231 $ 841

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2015
Summary of Stock Option Activity	Stock option activity is summarized as follows: Number of Outstanding Options Weighted Average Price Weighted Remaining Contractual Term Aggregate Intrinsic Value (in 000s) Outstanding as of December 31, 2014 1,514,471 $ 4.60 Granted 1,715,028 $ 11.29 Exercised (197,253 ) $ 2.78 Canceled (406,966 ) $ 6.37 Outstanding as of December 31, 2015 2,625,280 $ 8.83 8.02 $ 6,684 Options vested and expected to vest as of December 31, 2015 2,625,280 $ 8.83 8.02 $ 6,684 Options exercisable as of December 31, 2015 899,931 $ 4.91 6.64 $ 5,063
Summary of Assumptions Used to Determine Fair Value of Employee Stock Purchase Plan	The assumptions used in the Black-Scholes option pricing model to determine the fair value of the ESPP offering were as follows: Year Ended December 31, 2015 Expected term (in years) 0.50 Risk-free interest rate 0.33 % Expected volatility 67.3 % Expected dividend yield 0.0 %
Schedule of Allocation of Stock-Based Compensation for All Options Including Performance Options with Market Condition	The allocation of stock-based compensation for all options, including performance options,with market condition is as follows (in thousands): Years Ended December 31, 2015 2014 2013 Research and development $ 2,524 $ 527 $ 96 General and administrative 2,332 1,264 59 $ 4,856 $ 1,791 $ 155
Summary of Warrants Outstanding	Warrants outstanding as of December 31, 2015: Number Exercise Price Expiration Outstanding Per Share Date 9,051 $ 6.63 September 2017 2,006 $ 7.48 March 2021 14,913 $ 20.12 July 2023 25,970
Summary of Common Stock Reserved for Future Issuance	Common stock reserved for future issuance is as follows: As of December 31, 2015 2014 Conversion of redeemable convertible preferred stock — 9,238,868 Conversion of redeemable convertible preferred stock issuable upon conversion of promissory note — 94,455 Redeemable convertible preferred stock warrants — 25,970 Common stock warrants 25,970 — Common stock options outstanding 2,625,280 1,514,471 Shares available under the 2014 Plan 984,357 180,190 Shares available under the 2015 Plan 903,350 — Shares available under the 2015 ESPP Plan 227,623 — 4,766,580 11,053,954
Employee Stock Option [Member]
Summary of Assumptions Used to Determine Fair Value of Employee Stock Option Grants and Performance Options with Market Condition	The assumptions used in the Black-Scholes option pricing model to determine the fair value of the employee stock option grants were as follows: Years Ended December 31, 2015 2014 2013 Expected term (in years) 5.50 – 6.08 5.77 – 6.56 6.52 – 6.56 Risk-free interest rate 1.5% – 1.9% 1.7% – 2.7% 2.0% – 2.2% Expected volatility 79.2% – 100.9% 111.0 % 109.0 % Expected dividend yield 0.0 % 0.0 % 0.0 %
Performance Options with Market Condition [Member]
Summary of Assumptions Used to Determine Fair Value of Employee Stock Option Grants and Performance Options with Market Condition	The assumptions used to determine the fair value of the performance options with market condition were as follows: Year Ended December 31, 2015 Expected term (in years) 4.8 Risk-free interest rate 2.05 % Expected volatility 80.6 % Expected dividend yield 0.0 %

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Schedule of Pretax Earnings (Loss) for Domestic and Foreign Operations	Pretax earnings (loss) were generated by both domestic and foreign operations as follows (in thousands): Years Ended December 31, 2015 2014 2013 United States $ (47,490 ) $ (34,885 ) $ (11,085 ) Foreign (483 ) 10,535 (8,929 ) $ (47,973 ) $ (24,350 ) $ (20,014 )
Schedule of Reconciliations of the Expected Statutory Federal Income Tax	A reconciliation of the expected statutory federal income tax provision to the actual income tax provision is summarized as follows (in thousands): Years Ended December 31, 2015 2014 2013 Expected income taxes benefit at federal statutory rate $ (16,311 ) $ (8,279 ) $ (6,804 ) State income taxes, net of federal benefit — (2,023 ) (634 ) Permanent items and other 19 (321 ) 2 Stock-based compensation 734 396 — Research credits (2,674 ) (372 ) (397 ) Unrecognized tax benefits 1,070 144 159 Foreign rate differential 84 (3,391 ) 2,978 Change in tax rate 3,551 — — Other, net 112 293 (26 ) Change in valuation allowance 13,415 13,553 4,722 Income tax (benefit) expense $ — $ — $ —
Schedule of Significant Components of Deferred Tax Assets	Significant components of our deferred tax assets are summarized as follows (in thousands): December 31, 2015 2014 2013 Net operating loss carryforwards $ 24,869 $ 20,066 $ 13,093 Capitalized research and development expenses 14,181 7,855 6,684 Research credits and other state credits 3,565 1,368 1,171 Intangible assets 4,176 4,926 28 Depreciation and amortization (120 ) (260 ) (360 ) Reserve and accruals 1,648 891 677 Valuation allowance (48,319 ) (34,846 ) (21,293 ) Net deferred tax assets $ — $ — $ —
Schedule of Activity Related to Unrecognized Tax Benefits	The activity related to our unrecognized tax benefits is summarized as follows (in thousands): Balance as of December 31, 2012 $ 774 Increase related to prior year tax positions 79 Increase related to current year tax positions 94 Balance as of December 31, 2013 947 Increase related to prior year tax positions — Increase related to current year tax positions 177 Other decreases (18 ) Balance as of December 31, 2014 1,106 Increase related to prior year tax positions 2,404 Increase related to current year tax positions 1,523 Balance as of December 31, 2015 5,033

Quarterly Financial Data (Una24

Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2015
Quarterly Financial Data [Abstract]
Summary of Quarterly Financial Data	The following financial information reflects all normal recurring adjustments, which are, in our opinion, necessary for a fair statement of the results of the interim periods. Summarized quarterly data for 2015 and 2014 are as follows (in thousands, except per share data): For the quarters ended March 31 June 30 September 30 December 31 2015: Operating expenses $ 8,922 $ 10,898 $ 11,313 $ 16,483 Net loss (9,071 ) (11,080 ) (11,329 ) (16,493 ) Basic and diluted net loss per share $ (9.39 ) $ (0.74 ) $ (0.48 ) (0.70 ) 2014: Operating expenses $ 5,930 $ 5,363 $ 6,231 $ 6,030 Net loss (6,093 ) (5,588 ) (6,631 ) (6,038 ) Basic and diluted net loss per share $ (7.87 ) $ (6.85 ) $ (8.02 ) $ (7.04 )

Organization, Business and Ba25

Organization, Business and Basis of Presentation - Additional Information (Detail) $ / shares in Units, $ in Thousands	May. 12, 2015USD ($)$ / sharesshares	May. 31, 2015shares	Dec. 31, 2015USD ($)$ / sharesshares	Apr. 30, 2015shares	Apr. 25, 2015shares	Dec. 31, 2014$ / sharesshares
Description Of Business [Line Items]
Initial public offering of common stock price per share | $ / shares			$ 0.001			$ 0.001
Net proceeds | $			$ 76,902
Common stock, shares authorized			150,000,000			95,500,000
Preferred stock, shares authorized						75,772,871
Common stock reserved for future issuance			4,766,580			11,053,954
Reverse stock split of company's common stock			On May 5, 2015, we filed an amendment to our amended and restated certificate of incorporation to effect a one-for-7.95413 reverse stock split of our common stock (the Reverse Stock Split). The par value and the authorized shares of the common and convertible preferred stock were not adjusted as a result of the Reverse Stock Split.
Ratio of reverse stock split of company's common stock			7.95413
Pangu BioPharma [Member] | Hong Kong [Member]
Description Of Business [Line Items]
Majority-owned subsidiary percentage			98.00%
2015 ESPP Plan [Member]
Description Of Business [Line Items]
Common stock reserved for future issuance			227,623
Redeemable Convertible Preferred Stock [Member]
Description Of Business [Line Items]
Shares issued upon conversion			(141,654,309)
Preferred stock, shares authorized			7,285,456			75,772,871
Common stock reserved for future issuance						9,238,868
Undesignated Preferred Stock [Member]
Description Of Business [Line Items]
Preferred stock, shares authorized			5,000,000			0
IPO [Member]
Description Of Business [Line Items]
Issuance of common stock through initial public offering, net, Shares	6,164,000
Initial public offering of common stock price per share | $ / shares	$ 14
Gross proceeds | $	$ 86,300
Net proceeds | $	75,900
Underwriting and other expenses | $	10,400
Underwriting discounts and commissions | $	6,000
Other offering expenses | $	$ 4,400
Common stock, shares authorized	150,000,000	150,000,000
Preferred stock, shares authorized	7,285,456	7,285,456
IPO [Member] | 2015 Stock Option and Incentive Plan [Member]
Description Of Business [Line Items]
Common stock reserved for future issuance				1,574,566	1,574,566
IPO [Member] | 2015 ESPP Plan [Member]
Description Of Business [Line Items]
Common stock reserved for future issuance				227,623	227,623
IPO [Member] | Redeemable Convertible Preferred Stock [Member]
Description Of Business [Line Items]
Warrants issued upon conversion	206,581	206,581
IPO [Member] | Undesignated Preferred Stock [Member]
Description Of Business [Line Items]
Preferred stock, shares authorized		5,000,000
Preferred stock, shares authorized	5,000,000
Common Stock [Member]
Description Of Business [Line Items]
Issuance of common stock through initial public offering, net, Shares			6,164,000
Shares issued upon conversion			16,279,859
Common Stock [Member] | IPO [Member]
Description Of Business [Line Items]
Shares issued upon conversion	16,279,859	16,279,859
Warrants issued upon conversion	25,970	25,970

Summary of Significant Accoun26

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Significant Accounting Policies [Line Items]
Impairment charges on investment securities	$ 0
Aggregate value of investment securities	72,300,000
Adjustment between amortized cost and fair value of investment securities	200,000
Impairment of long lived assets	$ 0
Weighted average shares subject to repurchase	61,814	61,457	76,587
Maximum [Member]
Significant Accounting Policies [Line Items]
Maturity period of investment securities	2 years
Estimated useful life of property and equipment	7 years
Minimum [Member]
Significant Accounting Policies [Line Items]
Estimated useful life of property and equipment	3 years
Percentage of tax benefit to be realized upon settlement	50.00%
Corporate Debt Securities [Member]
Significant Accounting Policies [Line Items]
Aggregate value of investment securities		$ 2,000,000
Adjustment between amortized cost and fair value of investment securities		$ 0
Corporate Debt Securities [Member] | Maximum [Member]
Significant Accounting Policies [Line Items]
Maturity period of investment securities		3 months

Summary of Significant Accoun27

Summary of Significant Accounting Policies - Schudule of Potentially Dilutive Securities Not Included in Calculation of Diluted Net Loss Per Share (Detail) - shares	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities not included in calculation of diluted net loss per share	2,668,613	10,873,764	10,172,894
Redeemable Convertible Preferred Stock Outstanding [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities not included in calculation of diluted net loss per share		9,238,868	9,238,868
Redeemable Convertible Preferred Stock Issuable upon Conversion of Convertible Promissory Note [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities not included in calculation of diluted net loss per share		94,455	94,455
Warrants for Redeemable Convertible Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities not included in calculation of diluted net loss per share		25,970	18,514
Warrants for Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities not included in calculation of diluted net loss per share	25,970
Common Stock Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities not included in calculation of diluted net loss per share	2,625,280	1,514,471	821,057
Employee Stock Purchase Plan [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities not included in calculation of diluted net loss per share	17,363

Summary of Significant Accoun28

Summary of Significant Accounting Policies - Summary of Net Loss Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Numerator:
Consolidated net loss	$ (16,493)	$ (11,329)	$ (11,080)	$ (9,071)	$ (6,038)	$ (6,631)	$ (5,588)	$ (6,093)	$ (47,973)	$ (24,350)	$ (20,014)
Accretion to redemption value of redeemable convertible preferred stock									(15)	(416)	(1,637)
Net loss attributable to common stockholders									$ (47,988)	$ (24,766)	$ (21,651)
Denominator:
Weighted average common shares outstanding									15,900,167	895,678	839,348
Weighted average common shares subject to repurchase									(61,814)	(61,457)	(76,587)
Weighted average common shares outstanding - basic and diluted									15,838,353	834,221	762,761
Net loss per share - basic and diluted	$ (0.70)	$ (0.48)	$ (0.74)	$ (9.39)	$ (7.04)	$ (8.02)	$ (6.85)	$ (7.87)	$ (3.03)	$ (29.69)	$ (28.39)

Fair Value Measurements - Sched

Fair Value Measurements - Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 53,025	$ 13,899
Total assets measured at fair value	125,349	15,853
Preferred Stock Warrant Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total liabilities measured at fair value		319
Short-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	42,510
Short-term Investments [Member] | Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	2,996
Short-term Investments [Member] | Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	39,514	1,954
Long-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	29,814
Long-term Investments [Member] | Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	16,903
Long-term Investments [Member] | United States Treasury securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,999
Long-term Investments [Member] | Asset-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	10,912
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	53,025	13,899
Total assets measured at fair value	55,024	13,899
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Long-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,999
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Long-term Investments [Member] | United States Treasury securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,999
Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	70,325	1,954
Significant Other Observable Inputs (Level 2) [Member] | Short-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	42,510
Significant Other Observable Inputs (Level 2) [Member] | Short-term Investments [Member] | Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	2,996
Significant Other Observable Inputs (Level 2) [Member] | Short-term Investments [Member] | Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	39,514	1,954
Significant Other Observable Inputs (Level 2) [Member] | Long-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	27,815
Significant Other Observable Inputs (Level 2) [Member] | Long-term Investments [Member] | Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	16,903
Significant Other Observable Inputs (Level 2) [Member] | Long-term Investments [Member] | Asset-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	$ 10,912
Significant Unobservable Inputs (Level 3) [Member] | Preferred Stock Warrant Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total liabilities measured at fair value		$ 319

Fair Value Measurements - Sch30

Fair Value Measurements - Schedule of Available-for-sale Investments (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	$ 42,571
Gross Unrealized Losses	(61)
Market Value	42,510
Short-term Investments [Member] | Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	39,575	$ 1,954
Gross Unrealized Losses	(61)
Market Value	39,514	$ 1,954
Short-term Investments [Member] | Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	2,996
Market Value	2,996
Long-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	29,924
Gross Unrealized Losses	(110)
Market Value	29,814
Long-term Investments [Member] | Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	16,990
Gross Unrealized Losses	(87)
Market Value	16,903
Long-term Investments [Member] | United States Treasury securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	2,006
Gross Unrealized Losses	(7)
Market Value	1,999
Long-term Investments [Member] | Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	10,928
Gross Unrealized Losses	(16)
Market Value	$ 10,912

Fair Value Measurements - Sch31

Fair Value Measurements - Schedule of Available-for-sale Investments in Unrealized Loss Position (Detail) $ in Thousands	Dec. 31, 2015USD ($)
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	$ 69,327
Gross Unrealized Losses	(171)
United States Treasury securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	1,999
Gross Unrealized Losses	(7)
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	10,912
Gross Unrealized Losses	(16)
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value	56,416
Gross Unrealized Losses	$ (148)

Fair value Measurements - Addit

Fair value Measurements - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2015Security
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Abstract]
Available-for-sale investments contractual maturity period	2 years
Available-for-sale investments in gross unrealized loss position	39
Available-for-sale investments in gross unrealized loss position	12 months

Fair Value Measurements - Sch33

Fair Value Measurements - Schedule of Reconciliation of All Liabilities Measured at Fair Value Using Level 3 Significant Unobservable Inputs (Detail) - Warrant Liabilities [Member] - USD ($) $ in Thousands	4 Months Ended	8 Months Ended
	May. 05, 2015	Dec. 31, 2015
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 319	$ 290
Change in fair value	(29)
Reclassification to additional paid-in capital as of IPO on May 6, 2015		$ (290)
Ending balance	$ 290

Balance Sheet Details - Summary

Balance Sheet Details - Summary of Property and Equipment (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Property Plant And Equipment [Line Items]
Property and equipment, Gross	$ 5,541	$ 4,888
Less accumulated depreciation and amortization	(3,748)	(2,963)
Property and equipment, Net	1,793	1,925
Computer and Office Equipment [Member]
Property Plant And Equipment [Line Items]
Property and equipment, Gross	336	372
Scientific and Laboratory Equipment [Member]
Property Plant And Equipment [Line Items]
Property and equipment, Gross	3,518	2,848
Tenant Improvements [Member]
Property Plant And Equipment [Line Items]
Property and equipment, Gross	$ 1,687	$ 1,668

Balance Sheet Details - Summa35

Balance Sheet Details - Summary of Accrued Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Payables And Accruals [Abstract]
Accrued salaries, wages and benefits	$ 1,710	$ 684
Other accrued expenses	2,885	2,248
Accrued expenses	$ 4,595	$ 2,932

Debt, Commitments and Conting36

Debt, Commitments and Contingencies - Schedule of Commercial Bank Debt and Unamortized Discount (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Commercial bank debt, net of current portion	$ 1,776	$ 5,142
Current portion of commercial bank debt	3,366	3,134
Commercial Bank Debt [Member]
Debt Instrument [Line Items]
Commercial bank debt	5,202	8,439
Less debt discount, net of current portion	(6)	(60)
Commercial bank debt, net of debt discount	5,196	8,379
Less current portion of commercial bank debt	(3,420)	(3,237)
Commercial bank debt, net of current portion	1,776	5,142
Current portion of commercial bank debt	3,420	3,237
Current portion of debt discount	(54)	(103)
Current portion of commercial bank debt	$ 3,366	$ 3,134

Debt, Commitments and Conting37

Debt, Commitments and Contingencies - Additional Information (Detail) - USD ($)	1 Months Ended								12 Months Ended
	May. 31, 2015	Jan. 31, 2015	Jun. 30, 2014	Jul. 31, 2013	Aug. 31, 2012	Apr. 30, 2012	Dec. 31, 2011	Oct. 31, 2007	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Noncancelable operating lease expiration period									2017-05
Extended term of lease							5 years
Rent expense									$ 400,000	$ 200,000	$ 200,000
Convertible promissory note	$ 2,000,000								2,000,000		500,000
Accrued interest on convertible promissory note	$ 500,000								925,000	415,000	254,000
Research and development expenses									34,504,000	16,777,000	13,832,000
The Scripps Research Institute [Member] | Maximum [Member]
Debt Instrument [Line Items]
Obligation to pay an aggregate amount under agreement								$ 2,750,000
Research Funding and Option Agreement [Member] | The Scripps Research Institute [Member]
Debt Instrument [Line Items]
Agreement renewal period								12 months
Written notice period to terminate the agreement								30 days
Issue of common stock, shares in consideration for adjustment of sublicense payments		119,840
Fair value of common stock issued		$ 1,400,000
Research and development expenses									700,000	700,000	600,000
Research Funding and Option Agreement [Member] | National Foundation for Cancer Research [Member]
Debt Instrument [Line Items]
Charitable donations									400,000	$ 400,000	$ 400,000
Master Services Agreement [Member]
Debt Instrument [Line Items]
Research and development expenses									$ 5,100,000
Milestones payment terms									up to the mid seven figures.
Silicon Valley Bank [Member]
Debt Instrument [Line Items]
Borrowings under loan and security agreement			$ 5,000,000	$ 5,000,000	$ 1,250,000	$ 1,250,000			$ 5,202,000
Debt instrument interest rate percentage			5.88%	5.00%	4.85%	4.89%
Increase in available credit under loan and security agreement				$ 10,000,000
Cash proceeds after repayment of debt				$ 2,900,000
Debt instrument fee									$ 500,000
Maturity date			Jun. 30, 2017		Dec. 31, 2015	Dec. 31, 2015
Silicon Valley Bank [Member] | 4.89% Commercial Bank Debt [Member]
Debt Instrument [Line Items]
Debt instrument payment terms									We were obligated to make interest-only payments through December 2012 and, beginning in December 2013, monthly payments of principal and interest through the maturity date in December 2015.
Silicon Valley Bank [Member] | 4.85% Commercial Bank Debt [Member]
Debt Instrument [Line Items]
Debt instrument payment terms									We were obligated to make interest-only payments through December 2012 and, beginning in December 2013, monthly payments of principal and interest through the maturity date in December 2015.
Silicon Valley Bank [Member] | 5.0% Commercial Bank Debt [Member]
Debt Instrument [Line Items]
Debt instrument payment terms									We were obligated to make interest-only payments on each $5.0 million borrowing through June 2014 and, beginning in July 2014, monthly payments of principal and interest through the maturity date in June 2017.
Silicon Valley Bank [Member] | 5.88% Commercial Bank Debt [Member]
Debt Instrument [Line Items]
Debt instrument payment terms									We were obligated to make interest-only payments on each $5.0 million borrowing through June 2014 and, beginning in July 2014, monthly payments of principal and interest through the maturity date in June 2017.
Silicon Valley Bank [Member] | Series D Redeemable Convertible Preferred Stock [Member]
Debt Instrument [Line Items]
Warrant issued to purchase shares				59,312
Exercise price of warrant issued				$ 2.529
Warrant exercisable			118,624
Warrants issued upon conversion	14,913
Subordinated Unsecured Convertible Promissory Note [Member]
Debt Instrument [Line Items]
Debt instrument interest rate percentage							8.00%
Convertible promissory note borrowed							$ 2,000,000

Debt, Commitments and Conting38

Debt, Commitments and Contingencies - Schedule of Future Minimum Principal and Interest Payments under Loan and Security Agreement SVB Including Final Payment (Detail) - Silicon Valley Bank [Member] - USD ($) $ in Thousands	Dec. 31, 2015	Jun. 30, 2014	Jul. 31, 2013	Aug. 31, 2012	Apr. 30, 2012
Debt Instrument [Line Items]
2,016	$ 3,622
2,017	2,311
Commercial bank debt, net of debt discount	5,933
Less interest and final payment	(731)
Commercial bank debt	$ 5,202	$ 5,000	$ 5,000	$ 1,250	$ 1,250

Debt, Commitments and Conting39

Debt, Commitments and Contingencies - Schedule of Future Minimum Payments under Non-cancelable Operating Lease (Detail) $ in Thousands	Dec. 31, 2015USD ($)
Leases Operating [Abstract]
2,016	$ 610
2,017	231
Operating Lease , total	$ 841

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Detail) - USD ($)

May. 12, 2015

Mar. 31, 2015

Oct. 31, 2015

May. 31, 2015

Apr. 30, 2015

Dec. 31, 2014

Dec. 31, 2015

Dec. 31, 2014

Dec. 31, 2013

Sep. 30, 2015

Mar. 01, 2016

Apr. 25, 2015

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Convertible preferred stock, Issuance costs

$ 999,000

Common stock, shares authorized

95,500,000

150,000,000

95,500,000

Preferred stock, shares authorized

75,772,871

75,772,871

Number of common stock shares reserved for issuance

11,053,954

4,766,580

11,053,954

Weighted average grant date fair value of stock options granted

$ 11.29

$ 10.18

$ 3.42

Aggregate intrinsic value of stock options exercised

$ 1,900,000

$ 400,000

$ 47,000

Unrecognized share based compensation expense related to unvested stock options

$ 12,700,000

Unrecognized cost expected to be recognized over a weighted average period

2 years 10 months 24 days

Number of Outstanding Options, Granted

1,715,028

Stock-based compensation

$ 4,856,000

1,791,000

$ 155,000

Performance Options Without Market Condition [Member] | Chief Executive Officer [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Incremental share based compensation costs

$ 700,000

Share based compensation expense recognized

$ 600,000

Performance Options with Market Condition [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Options vesting period

4 years

Weighted average grant date fair value of stock options granted

$ 4.23

Number of Outstanding Options, Granted

169,402

Performance options with market condition, exercise price

$ 10.24

Achievement of specified performance goals year and month

2017-10

Requisite service period

5 years

2014 Plan [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Maximum term for stock option plan grant

10 years

Options vesting period

4 years

4 years

6 years

Number of common stock shares reserved for issuance

180,190

984,357

180,190

Number of Outstanding Options, Granted

931,749

Stock-based compensation

$ 800,000

2015 ESPP Plan [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Number of common stock shares reserved for issuance

227,623

401 (k) Plan [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Policy approved month and year

2015-04

Date of Policy beginning

Jun. 1, 2015

Percentage of match employee contribution

50.00%

Percentage of participants's annual salary

6.00%

Discretionary contribution

$ 100,000

Amended and Restated [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Common stock, after increase in authorized shares

185,000,000

Preferred stock, after increase in authorized shares

143,939,765

Common Stock [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Conversion of redeemable convertible preferred stock in connection with IPO, Shares

16,279,859

Redeemable Convertible Preferred Stock [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Convertible preferred stock, shares issued

73,487,415

0

73,487,415

Conversion of redeemable convertible preferred stock in connection with IPO, Shares

(141,654,309)

Preferred stock, shares authorized

75,772,871

7,285,456

75,772,871

Number of common stock shares reserved for issuance

9,238,868

9,238,868

IPO [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Common stock, shares authorized

150,000,000

150,000,000

Preferred stock, shares authorized

7,285,456

7,285,456

IPO [Member] | 2015 Stock Option and Incentive Plan [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Effective date of plan

May 6, 2015

Number of common stock shares reserved for issuance

1,574,566

1,574,566

Stock option grants description

The number of shares reserved and available for issuance under the 2015 Plan will automatically increase each January 1, beginning on January 1, 2016 and thereafter until January 1, 2019, by the lesser of (i) 1,840,000 shares, (ii) 4% of the outstanding number of shares of the Company’s common stock on the immediately preceding December 31 or (iii) an amount determined by the Company’s board of directors.

Percentage threshold of outstanding shares as of December 31 of each year for calculation of annual increase in authorized shares under the plan

4.00%

IPO [Member] | 2015 Stock Option and Incentive Plan [Member] | Maximum [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Annual increase in shares authorized under plan, shares threshold

1,840,000

IPO [Member] | 2015 ESPP Plan [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Effective date of plan

May 6, 2015

Number of common stock shares reserved for issuance

227,623

227,623

Percentage threshold of outstanding shares as of December 31 of each year for calculation of annual increase in authorized shares under the plan

100.00%

IPO [Member] | Common Stock [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Conversion of redeemable convertible preferred stock in connection with IPO, Shares

16,279,859

16,279,859

Warrants issued upon conversion

25,970

25,970

IPO [Member] | Series E redeemable convertible preferred stock [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Convertible preferred stock, shares issued

68,166,894

Convertible preferred stock, purchase price

$ 1.119

Convertible preferred stock, total cash consideration

$ 76,300,000

Convertible preferred stock, Issuance costs

$ 600,000

Conversion of convertible preferred stock to common stock

0.12572

Conversion of redeemable convertible preferred stock in connection with IPO, Shares

7,040,991

IPO [Member] | Series E redeemable convertible preferred stock [Member] | Scenario Forecast [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Conversion of convertible preferred stock to common stock

0.10329

IPO [Member] | Undesignated Preferred Stock [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Preferred stock, shares authorized

5,000,000

IPO [Member] | Redeemable Convertible Preferred Stock [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Warrants issued upon conversion

206,581

206,581

Stockholders' Equity - Summary

Stockholders' Equity - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Outstanding Options, Beginning Balance	1,514,471
Number of Outstanding Options, Granted	1,715,028
Number of Outstanding Options, Exercised	(197,253)
Number of Outstanding Options, Canceled	(406,966)
Number of Outstanding Options, Ending Balance	2,625,280	1,514,471
Number of Outstanding Options, Vested and Expected to Vest, Ending Balance	2,625,280
Number of Outstanding Options, Exercisable, Ending Balance	899,931
Weighted Average Price	$ 8.83	$ 4.60
Weighted Average Price, Granted	11.29
Weighted Average Price, Exercised	2.78
Weighted Average Price, Canceled	6.37
Weighted Average Price, Options Vested and Expected to Vest, Ending Balance	8.83
Weighted Average Price, Options Exercisable, Ending Balance	$ 4.91
Weighted Average Contractual Term, Outstanding	8 years 7 days
Weighted Average Contractual Term, Options Vested and Expected to Vest	8 years 7 days
Weighted Average Contractual Term, Options Exercisable	6 years 7 months 21 days
Aggregate Intrinsic Value, Outstanding	$ 6,684
Aggregate Intrinsic Value, Options Vested and Expected to Vest	6,684
Aggregate Intrinsic Value, Options Exercisable	$ 5,063

Stockholders' Equity - Summar42

Stockholders' Equity - Summary of Assumptions Used to Determine Fair Value of Employee Stock Option Grants, Employee Stock Purchase Plan and Performance Options with Market Condition (Detail)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
ESPP [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 months
Expected volatility	67.30%
Expected dividend yield	0.00%
Risk-free interest rate	0.33%
Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate, minimum	1.50%	1.70%	2.00%
Risk-free interest rate, maximum	1.90%	2.70%	2.20%
Expected volatility		111.00%	109.00%
Expected volatility, minimum	79.20%
Expected volatility, maximum	100.90%
Expected dividend yield	0.00%	0.00%	0.00%
Employee Stock Option [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	5 years 6 months	5 years 9 months 7 days	6 years 6 months 7 days
Employee Stock Option [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 years 29 days	6 years 6 months 22 days	6 years 6 months 22 days
Performance Options with Market Condition [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	4 years 9 months 18 days
Expected volatility	80.60%
Expected dividend yield	0.00%
Risk-free interest rate	2.05%

Stockholders' Equity - Schedule

Stockholders' Equity - Schedule of Allocation of Stock-Based Compensation for All Options Including Performance Options with Market Condition (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 4,856	$ 1,791	$ 155
Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	2,524	527	96
General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 2,332	$ 1,264	$ 59

Stockholders' Equity - Summar44

Stockholders' Equity - Summary of Warrants Outstanding (Detail)	12 Months Ended
	Dec. 31, 2015$ / sharesshares
Class Of Warrant Or Right [Line Items]
Number Outstanding	25,970
Warrant One [Member]
Class Of Warrant Or Right [Line Items]
Number Outstanding	9,051
Exercise price of warrant issued | $ / shares	$ 6.63
Expiration Date	2017-09
Warrant Two [Member]
Class Of Warrant Or Right [Line Items]
Number Outstanding	2,006
Exercise price of warrant issued | $ / shares	$ 7.48
Expiration Date	2021-03
Warrant Three [Member]
Class Of Warrant Or Right [Line Items]
Number Outstanding	14,913
Exercise price of warrant issued | $ / shares	$ 20.12
Expiration Date	2023-07

Stockholders' Equity - Summar45

Stockholders' Equity - Summary of Common Stock Reserved for Future Issuance (Detail) - shares	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future issuance	4,766,580	11,053,954
Redeemable Convertible Preferred Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future issuance		9,238,868
Conversion of Redeemable Convertible Preferred Stock Issuable upon Conversion of Promissory Note [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future issuance		94,455
Redeemable Convertible Preferred Stock Warrants [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future issuance		25,970
Common Stock Warrants [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future issuance	25,970
2014 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future issuance	984,357	180,190
Shares Available Under the 2015 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future issuance	903,350
Shares Available Under the 2015 ESPP Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future issuance	227,623
Common Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future issuance	2,625,280	1,514,471

Income Taxes - Schedule of Pret

Income Taxes - Schedule of Pretax Earnings (Loss) for Domestic and Foreign Operations (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
United States	$ (47,490)	$ (34,885)	$ (11,085)
Foreign	(483)	10,535	(8,929)
Pretax earnings (loss)	$ (47,973)	$ (24,350)	$ (20,014)

Income Taxes - Schedule of Reco

Income Taxes - Schedule of Reconciliations of the Expected Statutory Federal Income Tax (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Expected income taxes benefit at federal statutory rate	$ (16,311)	$ (8,279)	$ (6,804)
State income taxes, net of federal benefit		(2,023)	(634)
Permanent items and other	19	(321)	2
Stock-based compensation	734	396
Research credits	(2,674)	(372)	(397)
Unrecognized tax benefits	1,070	144	159
Foreign rate differential	84	(3,391)	2,978
Change in tax rate	3,551
Other, net	112	293	(26)
Change in valuation allowance	$ 13,415	$ 13,553	$ 4,722

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Sep. 30, 2014Entity	Dec. 31, 2013USD ($)
Income Tax [Line Items]
Valuation allowance	$ 48,319,000	$ 34,846,000		$ 21,293,000
Number of variable interest entities | Entity			6
Recognized intangible assets		$ 0
Amortization of intellectual property		15 years
Period of change in ownership	3 years
Percentage of change in ownership	50.00%
Unrecognized tax benefits, income tax penalties and interest accrued	$ 0	$ 0		$ 0
Change in unrecognized tax benefits	0
California [Member] | Operating Loss Carryforwards Expire in 2016 [Member]
Income Tax [Line Items]
Net operating loss carryforwards	$ 200,000
Net operating loss carryforwards expiration year	2,016
California [Member] | Operating Loss Carryforwards Expire in 2017 [Member]
Income Tax [Line Items]
Net operating loss carryforwards	$ 1,400,000
Net operating loss carryforwards expiration year	2,017
California [Member] | Operating Loss Carryforwards Expiring from 2028 through 2034 [Member]
Income Tax [Line Items]
Net operating loss carryforwards	$ 47,300,000
Federal [Member]
Income Tax [Line Items]
Net operating loss carryforwards	$ 65,400,000
Net operating loss carryforwards expiration year	2,025
Research and development credit carryforward	$ 1,800,000
Research and development credit carryforward expiration year	2,026
Federal [Member] | Orphan Drug Credits [Member]
Income Tax [Line Items]
Research and development credit carryforward	$ 2,900,000
Research and development credit carryforward expiration year	2,035
State [Member]
Income Tax [Line Items]
Net operating loss carryforwards	$ 48,900,000
Net operating loss carryforwards expiration year	2,016
Research and development credit carryforward	$ 1,900,000
Foreign [Member]
Income Tax [Line Items]
Net operating loss carryforwards	$ 5,800,000
Minimum [Member] | California [Member] | Operating Loss Carryforwards Expiring from 2028 through 2034 [Member]
Income Tax [Line Items]
Net operating loss carryforwards expiration year	2,028
Maximum [Member] | California [Member] | Operating Loss Carryforwards Expiring from 2028 through 2034 [Member]
Income Tax [Line Items]
Net operating loss carryforwards expiration year	2,034

Income Taxes - Schedule of Sign

Income Taxes - Schedule of Significant Components of Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Net operating loss carryforwards	$ 24,869	$ 20,066	$ 13,093
Capitalized research and development expenses	14,181	7,855	6,684
Research credits and other state credits	3,565	1,368	1,171
Intangible assets	4,176	4,926	28
Depreciation and amortization	(120)	(260)	(360)
Reserve and accruals	1,648	891	677
Valuation allowance	$ (48,319)	$ (34,846)	$ (21,293)

Income Taxes - Schedule of Acti

Income Taxes - Schedule of Activity Related to Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Beginning balance	$ 1,106	$ 947	$ 774
Increase related to prior year tax positions	2,404		79
Increase related to current year tax positions	1,523	177	94
Other decreases		(18)
Ending balance	$ 5,033	$ 1,106	$ 947

Quarterly Financial Data (Una51

Quarterly Financial Data (Unaudited) - Schedule of Quarterly Data for Interim Periods (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]
Operating expenses	$ 16,483	$ 11,313	$ 10,898	$ 8,922	$ 6,030	$ 6,231	$ 5,363	$ 5,930	$ 47,616	$ 23,554	$ 19,542
Net loss	$ (16,493)	$ (11,329)	$ (11,080)	$ (9,071)	$ (6,038)	$ (6,631)	$ (5,588)	$ (6,093)	$ (47,973)	$ (24,350)	$ (20,014)
Basic and diluted net loss per share	$ (0.70)	$ (0.48)	$ (0.74)	$ (9.39)	$ (7.04)	$ (8.02)	$ (6.85)	$ (7.87)	$ (3.03)	$ (29.69)	$ (28.39)

Subsequent Events - Additional

Subsequent Events - Additional Information (Detail) - $ / shares	1 Months Ended		12 Months Ended
	Jan. 31, 2016	Oct. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Subsequent Event [Line Items]
Number of Outstanding Options, Granted			1,715,028
Weighted Average Price, Granted			$ 11.29
Weighted average grant date fair value of options granted			$ 11.29	$ 10.18	$ 3.42
Performance Options with Market Condition [Member]
Subsequent Event [Line Items]
Number of Outstanding Options, Granted		169,402
Options vesting period		4 years
Weighted average grant date fair value of options granted		$ 4.23
Requisite service period		5 years
Subsequent Event [Member] | Common Stock [Member] | Executives, Employees And Consultants [Member] | Performance Options with Market Condition [Member]
Subsequent Event [Line Items]
Number of Outstanding Options, Granted	386,210
Weighted Average Price, Granted	$ 9.13
Options vesting period	4 years
Weighted average grant date fair value of options granted	$ 1.93
Requisite service period	5 years
Subsequent Event [Member] | Common Stock [Member] | Executives, Board of Directors, Employees and Consultants [Member | Performance Options Without Market Condition [Member]
Subsequent Event [Line Items]
Number of Outstanding Options, Granted	574,482
Weighted Average Price, Granted	$ 6.14
Weighted average grant date fair value of options granted	$ 4.76