Document and Entity Information - USD ($) $ in Millions	12 Months Ended
Document and Entity Information - USD ($) $ in Millions	Dec. 31, 2017	Mar. 01, 2018	Jun. 30, 2017
Document and Entity Information
Entity Registrant Name	Corvus Pharmaceuticals, Inc.
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Central Index Key	0001626971
Document Type	10-K
Document Period End Date	Dec. 31, 2017
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Filer Category	Accelerated Filer
Entity Public Float			$ 102.8
Entity Common Stock, Shares Outstanding		21,065,834
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	FY

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 45,106	$ 5,050
Marketable securities	44,949	129,846
Prepaid and other current assets	1,179	1,137
Total current assets	91,234	136,033
Property and equipment, net	2,672	3,248
Other assets	869	869
Total assets	94,775	140,150
Current liabilities:
Accounts payable	3,454	1,900
Accrued and other liabilities	5,515	4,044
Total current liabilities	8,969	5,944
Other liabilities	971	1,405
Total liabilities	9,940	7,349
Commitments and contingencies (Note 13)
Stockholders' equity:
Preferred stock: $0.0001 par value; 10,000,000 shares authorized at December 31, 2017 and December 31, 2016 ; 0 shares issued and outstanding at December 31, 2017 and December 31, 2016
Common stock: $0.0001 par value; 290,000,000 shares authorized at December 31, 2017 and December 31, 2016; 21,041,250 and 20,922,428 shares issued and outstanding at December 31, 2017 and December 31, 2016, respectively	2	2
Additional paid-in capital	208,408	200,709
Accumulated other comprehensive loss	(41)	(39)
Accumulated deficit	(123,534)	(67,871)
Total stockholders' equity	84,835	132,801
Total liabilities and stockholders' equity	$ 94,775	$ 140,150

CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
CONSOLIDATED BALANCE SHEETS
Preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock, shares authorized	290,000,000	290,000,000
Common stock, shares issued	21,041,250	20,922,428
Common stock, shares outstanding	21,041,250	20,922,428

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating expenses:
Research and development	$ 46,305	$ 29,356	$ 11,352
General and administrative	10,219	7,620	2,418
Total operating expenses	56,524	36,976	13,770
Loss from operations	(56,524)	(36,976)	(13,770)
Change in fair value of convertible preferred stock liability			(17,600)
Interest income and other expense, net	861	601	35
Net loss	$ (55,663)	$ (36,375)	$ (31,335)
Net loss per share, basic and diluted (in dollars per share)	$ (2.72)	$ (2.36)	$ (83.86)
Shares used to compute net loss per share, basic and diluted (in shares)	20,488,506	15,422,041	373,643
Other comprehensive income (loss):
Unrealized gain (loss) on marketable securities	$ (2)	$ 6	$ (45)
Comprehensive loss	$ (55,665)	$ (36,369)	$ (31,380)

Statements of Changes in Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	IPO Common Stock.	IPO Additional Paid-In Capital	IPO	Convertible Preferred Stock	Series A convertible preferred stock	Series B convertible preferred stock	Series A and Series B convertible preferred stock	Common Stock.	Additional Paid-In Capital	Accumulated Other Comprehensive Income	Accumulated Deficit	Total
Balance (in shares) at Dec. 31, 2014				3,395,468				1,046,749
Increase (Decrease) in Shares
Issuance of common stock to founders, net of repurchase (in shares)								384,866
Issuance of stock (in shares)					5,525,961	5,353,312
Balance (in shares) at Dec. 31, 2015				14,274,741				1,431,615
Balance at Dec. 31, 2014				$ 10,011
Increase (Decrease) in Convertible Preferred Stock
Issuance of convertible preferred stock					$ 20,730	$ 74,839
Reclassification of convertible preferred stock liability				20,200
Balance at Dec. 31, 2015				$ 125,780
Balance at Dec. 31, 2014									$ 2		$ (161)	$ (159)
Increase (Decrease) in Stockholders' Deficit
Issuance of common stock for cash upon early exercise of stock options and lapse of restrictions									10			10
Stock-based compensation expense									428			428
Unrealized loss on marketable securities										$ (45)		(45)
Net loss											(31,335)	(31,335)
Balance at Dec. 31, 2015									440	(45)	(31,496)	(31,101)
Increase (Decrease) in Shares
Number of common stock issued with the conversion of the preferred stock (in shares)							(14,274,741)	14,274,741
Issuance of stock (in shares)	5,202,618
Issuance of common stock on exercise of stock options (in shares)								13,454
Balance (in shares) at Dec. 31, 2016								20,922,428
Increase (Decrease) in Stockholders' Deficit
Conversion of convertible preferred stock into common stock							$ (125,780)	$ 1	125,779			125,780
Issuance of common stock	$ 1	$ 70,624	$ 70,625
Common stock issued on exercise of stock options									4			4
Vesting of restricted stocks issued upon early exercise of stock options									34			34
Stock-based compensation expense									3,828			3,828
Unrealized loss on marketable securities										6		6
Net loss											(36,375)	(36,375)
Balance at Dec. 31, 2016								$ 2	200,709	(39)	(67,871)	132,801
Increase (Decrease) in Shares
Issuance of stock (in shares)								52,569
Issuance of common stock on exercise of stock options (in shares)								66,253
Balance (in shares) at Dec. 31, 2017								21,041,250
Increase (Decrease) in Stockholders' Deficit
Issuance of common stock in conjunction with the sales Agreement, net									711			711
Common stock issued on exercise of stock options									731			731
Vesting of restricted stocks issued upon early exercise of stock options									28			28
Stock-based compensation expense									6,229			6,229
Unrealized loss on marketable securities										(2)		(2)
Net loss											(55,663)	(55,663)
Balance at Dec. 31, 2017								$ 2	$ 208,408	$ (41)	$ (123,534)	$ 84,835

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities
Net loss	$ (55,663)	$ (36,375)	$ (31,335)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	842	594	148
Amortization (accretion) related to marketable securities	(195)	648	(41)
Stock-based compensation	6,229	3,828	428
Change in fair value of convertible preferred stock liability			17,600
Other			40
Changes in operating assets and liabilities:
Prepaid and other current assets	(44)	290	(1,265)
Other assets		(869)
Accounts payable	1,554	518	1,240
Accrued and other liabilities	1,499	2,814	1,218
Other long-term liabilities	(434)	695	639
Net cash used in operating activities	(46,212)	(27,857)	(11,328)
Cash flows from investing activities
Purchases of marketable securities	(88,309)	(258,281)	(104,385)
Sales of marketable securities		4,199
Maturities of marketable securities	173,401	213,725	14,100
Purchase of property and equipment	(266)	(2,199)	(1,747)
Net cash used in investing activities	84,826	(42,556)	(92,032)
Cash flows from financing activities
Proceeds from issuance of common stock, net	711	71,354
Proceeds from issuance of convertible preferred stock, net			95,569
Payment of offering costs			(729)
Proceeds from exercise of common stock options	731	4	108
Net cash provided by financing activities	1,442	71,358	94,948
Net increase in cash and cash equivalents	40,056	945	(8,412)
Cash and cash equivalents at beginning of the period	5,050	4,105	12,517
Cash and cash equivalents at end of the period	$ 45,106	5,050	4,105
Supplemental disclosures of cash flow information
Purchases of property and equipment incurred but not paid		$ 84	286
Deferred offering costs incurred but not paid			$ 222

Organization	12 Months Ended
Organization	Dec. 31, 2017
Organization
Organization	1. Organization Corvus Pharmaceuticals, Inc. ("Corvus" or the "Company") was incorporated in Delaware on January 27, 2014 and commenced operations in November 2014. Corvus is a clinical stage biopharmaceutical company focused on the development and commercialization of novel immuno-oncology therapies that are designed to harness the immune system to attack cancer cells. The Company's operations are located in Burlingame, California. The Company has four insignificant subsidiaries.

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2017

Summary of Significant Accounting Policies

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Basis of Presentation

The accompanying consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America ("U.S. GAAP"). The Company's functional and reporting currency is the U.S. dollar. The accompanying consolidated financial statements have been prepared on a going-concern basis, which contemplates the realization of assets and discharge of liabilities in the normal course of business. Since its inception, the Company has incurred significant losses and negative cash flows from operations. As of December 31, 2017, the Company had an accumulated deficit of $123.5 million and cash, cash equivalents and marketable securities of $90.1 million. The Company has financed its operations primarily with the proceeds from the sale of stock. The Company will need to raise additional capital to meet its business objectives. The Company believes that its current cash, cash equivalents and marketable securities will be sufficient to fund its planned expenditures and meet its obligations through at least the next twelve months from the issuance of these financial statements.

Use of Estimates

The preparation of the Company's consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the amounts reported in the consolidated financial statements and accompanying notes. Actual results could differ from such estimates.

Initial Public Offering

On March 22, 2016, the Company's registration statement on Form S-1 (File No. 333-208850) relating to its initial public offering ("IPO") of its common stock was declared effective by the Securities and Exchange Commission ("SEC") and the shares of its common stock began trading on the Nasdaq Global Market on March 23, 2016. The public offering price of the shares sold in the IPO was $15.00 per share. The IPO closed on March 29, 2016, pursuant to which the Company sold 4,700,000 shares of its common stock. On April 26, 2016, the Company sold an additional 502,618 shares of its common stock to the underwriters upon partial exercise of their over-allotment option, at the initial offering price of $15.00 per share. The Company received aggregate net proceeds of approximately $70.6 million, after underwriting discounts, commissions and offering expenses. Immediately prior to the consummation of the IPO, all outstanding shares of convertible preferred stock were converted into common stocks.

Concentrations of Credit Risk and Other Risks and Uncertainties

Substantially all of the Company's cash and cash equivalents are deposited in accounts with two financial institutions that management believes are of high credit quality. Such deposits may, at times, exceed federally insured limits. The Company maintains its cash with an accredited financial institution and accordingly, such funds are subject to minimal credit risk. The Company's marketable securities consist of investments in U.S. Treasury securities, U.S. government agency securities and corporate debt obligations, which can be subject to certain credit risks. However, the Company mitigates the risks by investing in high-grade instruments, limiting its exposure to any one issuer, and monitoring the ongoing creditworthiness of the financial institutions and issuers. The Company has not experienced any losses on its deposits of cash, cash equivalents or marketable securities.

The Company is subject to a number of risks similar to other early stage biopharmaceutical companies, including, but not limited to, the need to obtain adequate additional funding, possible failure of preclinical testing or clinical trials, its reliance on third parties to conduct its clinical trials, the need to obtain marketing approval for its product candidates, competitors developing new technological innovations, the need to successfully commercialize and gain market acceptance of the Company's product candidates, its right to develop and commercialize its product candidates pursuant to the terms and conditions of the licenses granted to the Company, and protection of proprietary technology. If the Company does not successfully commercialize or partner any of its product candidates, it will be unable to generate product revenue or achieve profitability.

Segments

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. The Company views its operations and manages its business in one operating segment, that of the development of and commercialization of novel immuno-oncology therapies that are designed to harness the immune system to attack cancer cells.

Cash, Cash Equivalents and Marketable Securities

The Company considers all highly liquid investment securities with remaining maturities at the date of purchase of three months or less to be cash equivalents.

Investments with remaining maturities, at the date of purchase, greater than three months, but less than one year are considered short-term. The Company determines the appropriate classification of marketable securities at the time of purchase and evaluates such designation as of each balance sheet date. To date, all marketable securities have been classified as available-for-sale and are carried at fair value with unrealized gains and losses, if any, included as a component of accumulated other comprehensive income (loss) in stockholders' equity (deficit). Interest and realized gains and losses are included in interest income. Realized gains and losses are recognized based on the specific identification method.

Fair Value Measurements

Fair value accounting is applied for all financial assets and liabilities and non-financial assets and liabilities that are recognized or disclosed at fair value in the consolidated financial statements on a recurring basis (at least annually). The carrying amount of the Company's financial instruments, including cash equivalents, accounts payable and accrued liabilities, approximate fair value due to their short-term maturities.

Deferred Offering Costs

Deferred offering costs consist primarily of direct incremental costs related to the Company's initial public offering of its common stock. Upon completion of the initial public offering in March 2016, these amounts were offset against the proceeds of the offering.

Property and Equipment, Net

Property and equipment are stated at cost and depreciated using the straight-line method over the estimated useful lives of the respective assets:

Laboratory equipment		5 years
Computer equiipment and purchased software		3 years
Leasehold improvements		Shorter of asset's useful life or remaining term of lease

Maintenance and repairs that do not extend the life or improve the asset are expensed when incurred. When assets are retired or otherwise disposed of, the cost and accumulated depreciation are removed from the balance sheet and any resulting gain or loss is reflected in operations.

Impairment of Long-Lived Assets

The Company regularly reviews the carrying value and estimated lives of all of its long-lived assets, including property and equipment, to determine whether indicators of impairment may exist which warrant adjustments to carrying values or estimated useful lives. The determinants used for this evaluation include management's estimate of the asset's ability to generate positive income from operations and positive cash flow in future periods as well as the strategic significance of the assets to the Company's business objectives. Should impairment exist, the impairment loss to be recognized is measured by the amount by which the carrying amount of the asset exceeds the projected discounted future net cash flows arising from the asset. All long-lived assets are maintained in the United States of America.

Consolidation

The consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries. Intercompany balances and transactions have been eliminated in consolidation

Research and Development Expenses

The Company records research and development expenses as incurred. The Company accounts for nonrefundable advance payments for goods and services that will be used in future research and development activities as expenses when the goods have been received or when the service has been performed rather than when the payment is made. Research and development expenses consist of costs incurred by the Company for the discovery and development of the Company's product candidates and include:


	•	employee-related expenses, including salaries, benefits, travel and non-cash stock-based compensation expense;
	•	external research and development expenses incurred under arrangements with third parties, such as contract research organizations, contract manufacturing organizations, academic and non-profit institutions and consultants;
	•	costs to acquire technologies to be used in research and development that have not reached technological feasibility and have no alternative future use;
	•	license fees; and
	•	other expenses, which include direct and allocated expenses for laboratory, facilities and other costs.

Clinical Trial Accruals

Costs for preclinical studies and clinical trial activities are recognized based on an evaluation of the vendors' progress towards completion of specific tasks, using data such as clinical site activations, patient enrollment or information provided to the Company by its vendors regarding their actual costs incurred. Payments for these activities are based on the terms of individual contracts and payment timing may differ significantly from the period in which the services are performed. The Company determines accrual estimates through reports from and discussions with applicable personnel and outside service providers as to the progress or state of completion, or the services completed. The Company's estimates of accrued expenses as of each balance sheet date are based on the facts and circumstances known at the time.

Stock-Based Compensation

The Company maintains incentive plans under which incentive stock options and nonqualified stock options may be granted to employees and non-employee service providers.

The Company accounts for stock-based employee compensation arrangements in accordance with the provisions of ASC 718, "Compensation—Stock Compensation." For stock options granted to employees, the Company recognizes compensation expense for all stock-based awards based on the grant-date estimated fair values. The value of the award is recognized as an expense ratably over the requisite service period. The fair value of stock options is determined using the Black-Scholes option pricing model. Forfeitures are accounted for when they occur.

Stock-based compensation expense related to stock options granted to non-employees is recognized based on the fair value of the stock options, determined using the Black-Scholes option pricing model. The expense for options granted to non-employees is periodically re-measured as the underlying options vest. The awards generally vest over the time period the Company expects to receive service from the non-employee.

Income Taxes

The Company accounts for income taxes under the asset and liability method. The Company estimates actual current tax exposure together with assessing temporary differences resulting from differences in accounting for reporting purposes and tax purposes for certain items, such as accruals and allowances not currently deductible for tax purposes. These temporary differences result in deferred tax assets and liabilities, which are included in the Company's balance sheets. In general, deferred tax assets represent future tax benefits to be received when certain expenses previously recognized in the Company's statements of operations and comprehensive loss become deductible expenses, under applicable income tax laws or when net operating loss or credit carryforwards are utilized. Accordingly, realization of the Company's deferred tax assets is dependent on future taxable income against which these deductions, losses and credits can be utilized.

The Company must assess the likelihood that the Company's deferred tax assets will be recovered from future taxable income and a valuation allowance is recorded when it is more likely than not that the deferred tax asset will not be recovered. The Company applies judgment in the determination of the financial statement recognition and measurement of a tax position taken or expected to be taken in a tax return. Based on the available evidence, the Company is unable, at this time, to support the determination that it is more likely than not that its deferred tax assets will be utilized in the future. Accordingly, the Company recorded a full valuation allowance for all periods presented. The Company intends to maintain a valuation allowance until sufficient evidence exists to support its reversal.

The Company recognizes benefits of uncertain tax positions if it is more likely than not such positions will be sustained upon examination based solely on their technical merits as the largest amount of benefit that is more likely than not to be realized upon the ultimate settlement. The Company recognizes any material interest and penalties related to unrecognized tax benefits in income tax expense. The Company is required to file income tax returns in the U.S. federal jurisdiction. The Company currently is not under examination by the Internal Revenue Service or other jurisdictions for any tax years.

On December 22, 2017, the 2017 Tax Cuts and Jobs Act (the "Tax Act") was enacted into law and the new legislation contains several key tax provisions that affected us, including a one-time mandatory transition tax on accumulated foreign earnings and a reduction of the corporate income tax rate to 21% effective January 1, 2018, among others. We are required to recognize the effect of the tax law changes in the period of enactment, such as determining the transition tax, remeasuring our U.S. deferred tax assets and liabilities as well as reassessing the net realizability of our deferred tax assets and liabilities. In December 2017, the SEC staff issued Staff Accounting Bulletin No. 118, Income Tax Accounting Implications of the Tax Cuts and Jobs Act (SAB 118), which allows us to record provisional amounts during a measurement period not to extend beyond one year of the enactment date. We consider the accounting of the transition tax and deferred tax re-measurements to be complete—however, since the Tax Act was passed late in the fourth quarter of 2017, and ongoing guidance and accounting interpretation are expected over the next 12 months, we are not yet able to reasonably estimate the effect of Global Intangible Low Taxed Income ("GILTI") and have not yet made a policy decision regarding whether to record deferred taxes on GILTI. We expect to complete our analysis within the measurement period in accordance with SAB 118. See Note 12 in these notes to the consolidated financial statements for additional information.

Comprehensive Loss

Comprehensive loss includes net loss as well as other changes in stockholders' equity (deficit) that result from transactions and economic events other than those with stockholders. The Company's only element of other comprehensive loss in any period presented was unrealized gains and losses on available for sale marketable securities.

Net Loss per Share

Basic net loss per share is calculated by dividing the net loss by the weighted average number of common shares outstanding during the period, without consideration of potentially dilutive securities. Diluted net loss per share is computed by dividing the net loss by the weighted average number of common shares and potentially dilutive securities outstanding for the period. For purposes of the diluted net loss per share calculation, the convertible preferred stock, common stock subject to repurchase, and stock options are considered to be potentially dilutive securities. Because the Company has reported a net loss for all periods presented, diluted net loss per common share is the same as basic net loss per common share for those periods.

Recent Accounting Pronouncements

In May 2014, the Financial Accounting Standards Board ("FASB") issued Accounting Standards Update ("ASU") No. 2014-09, Revenue from Contracts with Customers, which required an entity to recognize the amount of revenue to which it expects to be entitled for the transfer of promised goods or services to customers. ASU No. 2014-09 will replace most existing revenue recognition guidance in U.S. GAAP when it becomes effective. The new standard is effective January 1, 2018 for public companies. Early application is permitted as of January 1, 2017. The standard permits the use of either the retrospective or cumulative effect transition method. In March 2016, the FASB issued ASU No. 2016-08, Revenue from Contracts with Customers (Topic 606): Principal versus Agent Considerations (Reporting Revenue Gross versus Net), which clarifies the implementation guidance on principal versus agent considerations in ASU No. 2014-09. In April 2016, the FASB issued ASU No. 2016-10, Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing, which clarifies certain aspects of identifying performance obligations and licensing implementation guidance. In May 2016, the FASB issued ASU No. 2016-12, Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Practical Expedients, which relates to disclosures of remaining performance obligations, as well as other amendments to guidance on collectability, non-cash consideration and the presentation of sales and other similar taxes collected from customers. These standards have the same effective date and transition date of January 1, 2018. The Company does not believe adopting this guidance will have a material impact on its consolidated financial statements as the Company is not yet generating revenues.

In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) that replaces existing lease guidance. The new standard requires lessees to record right of use assets and corresponding lease liabilities on the balance sheet. The new guidance will continue to classify leases as either finance or operating, with classification affecting the pattern of expense recognition in the statement of income. The standard is effective for the Company beginning January 1, 2019, with early application permitted. The new standard is required to be applied with a modified retrospective approach to each prior reporting period presented with various optional practical expedients. The Company is currently assessing the impact of this guidance on its consolidated financial statements.

In May 2017, the FASB issued ASU No 2017-09, Compensation—Stock Compensation (Topic 718)—Scope of Modification Accounting, to clarify when to account for a change to the terms or conditions of a share-based payment award as a modification. Under the new standard, modification is required only if the fair value, the vesting conditions, or the classification of an award as equity or liability changes as a result of the change in terms or conditions. ASU 2017-09 will be effective for the Company beginning January 1, 2018 and will be applied prospectively. Early adoption is permitted. The Company is currently evaluating the impact of this standard on its consolidated financial statements.

Net Loss per Share

12 Months Ended

Dec. 31, 2017

Net Loss per Share

Net Loss per Share

3. Net Loss per Share

The following table shows the calculation of net loss per share (in thousands, except share and per share data):

	Year Ended December 31,
	2017			2016			2015
Numerator:
Net loss—basic and diluted	$	(55,663	)	$	(36,375	)	$	(31,335	)


Denominator:
Weighted average common shares outstanding		20,958,557			16,188,980			1,269,315
Less: weighted average common shares subject to repurchase		(470,051	)		(766,939	)		(895,672	)

Weighted average common shares outstanding used to compute basic and diluted net loss per share		20,488,506			15,422,041			373,643


Net loss per share, basic and diluted	$	(2.72	)	$	(2.36	)	$	(83.86	)

The amounts in the table below were excluded from the calculation of diluted net loss per share, due to their anti-dilutive effect:

	Year Ended December 31,
	2017		2016		2015
Convertible preferrd stock		—		—		14,274,741
Common stock subject to repurchase		319,203		611,698		924,535
Outstanding options		3,013,394		2,350,582		784,136

Total shares of common stock equivalents		3,332,597		2,962,280		15,983,412

Fair Value Measurements

12 Months Ended

Dec. 31, 2017

Fair Value Measurements

Fair Value Measurements

4. Fair Value Measurements

Financial assets and liabilities are measured and recorded at fair value. The Company is required to disclose information on all assets and liabilities reported at fair value that enables an assessment of the inputs used in determining the reported fair values. The fair value hierarchy prioritizes valuation inputs based on the observable nature of those inputs. The fair value hierarchy applies only to the valuation inputs used in determining the reported fair value of the investments and is not a measure of the investment credit quality. The hierarchy defines three levels of valuation inputs:


	•	Level 1—Quoted prices in active markets for identical assets or liabilities
	•	Level 2—Inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly
	•	Level 3—Unobservable inputs that reflect the Company's own assumptions about the assumptions market participants would use in pricing the asset or liability

There have been no transfers of assets and liabilities between levels of hierarchy.

The Company's Level 2 investments are valued using third-party pricing sources. The pricing services utilize industry standard valuation models, including both income and market-based approaches, for which all significant inputs are observable, either directly or indirectly, to estimate fair value. These inputs include reported trades of and broker/dealer quotes on the same or similar investments, issuer credit spreads, benchmark investments, prepayment/default projections based on historical data and other observable inputs.

The following tables present information as of December 31, 2017 and 2016 about the Company's assets that are measured at fair value on a recurring basis and indicate the level of the fair value hierarchy the Company utilized to determine such fair values (in thousands):

	December 31, 2017
	Fair Value Measured Using
	Fair Value Measured Using						Total Balance
	(Level 1)		(Level 2)		(Level 3)		Total Balance
Assets
Cash equivalents	$	—	$	44,555	$	—	$	44,555
Marketable securities		—		44,949		—		44,949

	$	—	$	89,504	$	—	$	89,504

	December 31, 2016
	Fair Value Measured Using
	Fair Value Measured Using						Total Balance
	(Level 1)		(Level 2)		(Level 3)		Total Balance
Assets
Cash equivalents	$	2,999	$	—	$	—	$	2,999
Marketable securities		129,846		—		—		129,846

	$	132,845	$	—	$	—	$	132,845

As of December 31, 2017, marketable securities had a maximum remaining maturity of nine months.

As of December 31, 2017 and 2016, the fair value of available for sale marketable securities by type of security were as follows (in thousands):

	December 31, 2017
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Government agency securities	$	32,311	$	—	$	(39	)	$	32,272
Corporate debt obligations		12,679		—		(2	)		12,677

	$	44,990	$	—	$	(41	)	$	44,949

	December 31, 2016
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
U.S. Treasury securities	$	129,885	$	—	$	(39	)	$	129,846

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

License and Collaboration Agreements	12 Months Ended
License and Collaboration Agreements	Dec. 31, 2017
License and Collaboration Agreements
License and Collaboration Agreements	5. License and Collaboration Agreements Scripps Licensing Agreement In December 2014, the Company entered into a license agreement with The Scripps Research Institute ("Scripps"), pursuant to which it was granted a non-exclusive, world-wide license for all fields of use under Scripps' rights in certain know-how and technology related to a mouse hybridoma clone expressing an anti-human CD73 antibody, and to progeny, mutants or unmodified derivatives of such hybridoma and any antibodies expressed by such hybridoma. Scripps also granted the Company the right to grant sublicenses in conjunction with other proprietary rights the Company holds, or to others collaborating with or performing services for the Company. Under this license agreement, Scripps has agreed not to grant any additional commercial licenses with respect to such materials, other than march-in rights granted to the U.S. government. Upon execution of the agreement, the Company made a one-time cash payment to Scripps of $10,000 in 2015 and is also obligated to pay a minimum annual fee to Scripps of $25,000. The one-time cash payment was recorded as research and development expense as technological feasibility of the asset had not been established and there was no alternative future use. The first minimum annual fee payment is due on the first anniversary of effective date of the agreement and will be due on each subsequent anniversary of the effective date for the term of the agreement. The Company is also required to make performance-based cash payments upon successful completion of clinical and sales milestones. The aggregate potential milestone payments are $2.6 million. The Company is also required to pay royalties on net sales of licensed products sold by it, its affiliates and its sublicensees at a rate in the low-single digits. In addition, should the Company sublicense the rights licensed under the agreement, it has agreed to pay a percentage of sublicense revenue received at specified rates that start at double digit percentages and decrease to single digit percentages based on the elapsed time from the effective date of the agreement and the time of entry into such sublicense. To date, no milestone payments have been made. The Company's license agreement with Scripps will terminate upon expiration of its obligation to pay royalties to Scripps under the license agreement. The Company's license agreement with Scripps is terminable by the consent of the parties, at will by the Company upon providing 90 days written notice to Scripps, or by Scripps for certain material breaches, or if the Company undergoes a bankruptcy event. In addition, Scripps may terminate the license on a product-by-product basis, or the entire agreement, if the Company fails to meet specified diligence obligations related to the development and commercialization of licensed products. Scripps may also terminate the agreement after the third anniversary of the effective date of the agreement if it reasonably believes, based on reports the Company provides to Scripps, that the Company has not used commercially reasonable efforts as required under the agreement, subject to a specified notice and cure period. Vernalis Licensing Agreement In February 2015, the Company entered into a license agreement with Vernalis (R&D) Limited ("Vernalis"), which was subsequently amended as of November 5, 2015, and, pursuant to which the Company was granted an exclusive, worldwide license under certain patent rights and know-how, including a limited right to grant sublicenses, for all fields of use to develop, manufacture and commercialize products containing certain adenosine receptor antagonists, including CPI-444. Pursuant to this agreement, the Company made a one-time cash payment to Vernalis in the amount of $1.0 million, which was recorded as research and development expense as technological feasibility of the asset had not been established and there was no alternative future use. The Company is also required to make cash milestone payments to Vernalis upon the successful completion of clinical and regulatory milestones for licensed products depending on the indications for which such licensed products are developed and upon achievement of certain sales milestones. In February 2017, the Company made a milestone payment of $3.0 million to Vernalis following the expansion of a cohort of patients with renal cell cancer treated with single agent CPI-444 in the Company's Phase 1/1b clinical trial. The aggregate potential milestone payments exceed $200 million for all indications. The Company has also agreed to pay Vernalis tiered incremental royalties based on the annual net sales of licensed products containing CPI-444 on a product-by-product and country-by-country basis, subject to certain offsets and reductions. The tiered royalty rates for products containing CPI-444 range from the mid-single digits up to the low-double digits on a country-by-country net sales basis. The royalties on other licensed products that do not include CPI-444 also increase with the amount of net sales on a product-by-product and country-by-country basis and range from the low-single digits up to the mid-single digits on a country-by-country net sales basis. The Company is also obligated to pay to Vernalis certain sales milestones as indicated above when worldwide net sales reach specified levels over an agreed upon time period. The agreement will expire on a product-by-product and country-by-country basis upon the expiration of the Company's payment obligations to Vernalis in respect of a particular product and country. Both parties have the right to terminate the agreement for an uncured material breach by the other party. The Company may also terminate the agreement at its convenience by providing 90 days written notice, provided that the Company has not received notice of its own default under the agreement at the time the Company exercises such termination right. Vernalis may also terminate the agreement if the Company challenges a licensed patent or undergoes a bankruptcy event. Genentech Collaboration Agreement In October 2015, the Company entered into a clinical trial collaboration agreement with Genentech to evaluate the safety, tolerability and preliminary efficacy of CPI-444 combined with Genentech's investigational cancer immunotherapy, Tecentriq, a fully humanized monoclonal antibody targeting protein programmed cell death ligand 1("PD-L1"), in a variety of solid tumors in a Phase 1/1b clinical trial. Pursuant to this agreement, the Company will be responsible for the conduct and cost of the relevant studies, under the supervision of a joint development committee made up of representatives of the Company and representatives of Genentech. Genentech will supply Tecentriq. As part of the agreement, the Company granted Genentech certain rights of first negotiation to participate in future clinical trials that the Company may conduct evaluating the administration of CPI-444 in combination with an anti-PD-1 or anti-PD-L1 antibody. If the Company and Genentech do not reach agreement on the terms of any such participation by Genentech within a specified time period, the Company retains the right to collaborate with third parties in such activities. The Company also granted Genentech certain rights of first negotiation should it decide to license development and commercialization rights to CPI-444. Should the Company and Genentech not reach agreement on the terms of such a license within a specified time period, it retains the right to enter into a license with another third party. The Company and Genentech each have the right to terminate the agreement for material breach by the other party. In addition, the agreement may be terminated by either party due to safety considerations, if directed by a regulatory authority or if development of CPI-444 or Tecentriq is discontinued. Further, the agreement will expire after a set period of time following the provision by the Company of the final clinical study report to Genentech. In May 2017, the Company signed a second clinical trial collaboration agreement with Genentech. Under the new agreement, CPI-444 administered in combination with Tecentriq will be evaluated in a Phase 1b/2 randomized, controlled clinical study as second-line therapy in patients with non-small cell lung cancer who are resistant and/or refractory to prior therapy with an anti-PD-(L)1 antibody. It is anticipated that the study will enroll up to 65 patients in the treatment arm. Genentech will be responsible for the conduct of the study and the parties will share the cost of the Phase 1b/2 trial, which began enrolling patients in the fourth quarter of 2017. The Company is responsible for supplying CPI-444 and retains global development and commercialization rights to CPI-444. The Company and Genentech each have the right to terminate the agreement for material breach by the other party. In addition, the agreement may be terminated by either party due to safety considerations, if directed by a regulatory authority or if development of CPI-444 or Tecentriq is discontinued.

Balance Sheet Components

12 Months Ended

Dec. 31, 2017

Balance Sheet Components

Balance Sheet Components

6. Balance Sheet Components (in thousands):

	December 31,
	2017			2016
Prepaid and Other Current Assets
Interest receivable	$	132		$	365
Prepaid research and development manufacturing expenses		327			—
Other		720			772

	$	1,179		$	1,137


Property and Equipment, net
Laboratory equipment	$	2,034		$	1,868
Computer equipment and purchased software		130			58
Leasehold improvements		2,078			2,051

		4,242			3,977
Less: accumulated depreciation and amortization		(1,570	)		(729	)

	$	2,672		$	3,248


Accrued and Other Liabilities
Accrued clinical trial related	$	2,870		$	1,617
Accrued manufacturing expense		1,056			955
Personnel related		572			526
Deferred rent		410			378
Accrued legal and accounting		224			255
Other accrued expenses		383			313

	$	5,515		$	4,044


Other Liabilities
Deferred rent	$	960		$	1,370
Shares subject to vesting		11			35

	$	971		$	1,405

Convertible Preferred Stock Liability	12 Months Ended
Convertible Preferred Stock Liability	Dec. 31, 2017
Convertible Preferred Stock Liability
Convertible Preferred Stock Liability	8. Convertible Preferred Stock Liability On November 26, 2014, the Company executed the Series A Convertible Preferred Stock Purchase Agreement for the issuance of up to 8,921,438 shares of Series A convertible preferred stock and issued 3,395,468 shares for net proceeds of $12.6 million in connection with the first closing of the first tranche. In January 2015, in connection with the second closing of the first tranche, the Company issued 1,065,246 shares of Series A convertible preferred stock for net proceeds of $4.0 million and in June 2015, in connection with the closing of the second tranche, an additional 4,460,715 shares of Series A convertible preferred stock were issued for net proceeds of $16.7 million. The Series A Convertible Preferred Stock Purchase Agreement provided that, upon the earliest to occur of any of three defined triggers, each investor of the first tranche agreed to purchase its pro-rata portion of the shares to be issued in the second tranche and the Company agreed to sell and issue said shares of Series A convertible preferred stock on the same terms as the first tranche. A convertible preferred stock liability was recorded for the Company's obligation to sell the second tranche of the Series A convertible preferred stock to the first tranche stockholders at a fixed price of $3.755 per share upon the satisfaction of certain conditions. A liability was initially recorded in connection with the first tranche of the Series A convertible preferred stock financing at its initial estimated fair value of $2.6 million, with gains and losses arising from changes in fair value recognized in the statements of operations at each period while such instrument was classified as a liability. A $17.6 million charge was recorded for the change in estimated fair value of the Series A convertible preferred stock liability for the period from January 1, 2015 to the closing of the second tranche in June 2015. Upon the closing of the second tranche in June 2015, the liability terminated and the balance of the liability of $20.2 million was reclassified to convertible preferred stock. The preferred stock liability related to Series A convertible preferred stock was valued at issuance and at December 31, 2014 and March 31, 2015 using a black scholes option-pricing method based on the consideration paid for the Series A convertible preferred stock and the convertible preferred stock liability using an assumed term of 1.0 years and 0.75 years, an interest rate of 0.13% and 0.20% and a volatility of 85% and 85%, respectively. Immediately prior to its exercise on June 10, 2015, the convertible preferred stock liability's fair value was estimated based on its intrinsic value, with the fair value of the Series A convertible preferred stock estimated as of June 10, 2015 and compared to the exercise price of the Series A convertible preferred stock liability. To estimate the fair value of the Series A convertible preferred stock as of June 10, 2015, the enterprise value of the Company was estimated based on potential IPO and sale estimates. The enterprise value was then allocated to the various classes of securities using an option pricing model that assumed a term of two years to a liquidity event, an interest rate of 0.75% and a volatility of 75% based on market conditions and expectations as of the June valuation date.

Common Stock

12 Months Ended

Dec. 31, 2017

Common Stock

Common Stock

9. Common Stock

As of December 31, 2017, the amended and restated certificate of incorporation authorizes the Company to issue 290 million shares of common stock and 10 million shares of preferred stock.

Each share of common stock is entitled to one vote. Common stockholders are entitled to dividends if and when declared by the board of directors. As of December 31, 2017, no dividends on common stock had been declared.

On September 20, 2017, the Company entered into a sales agreement (the "Sales Agreement") with Cowen and Company, LLC ("Cowen") to sell shares of the Company's common stock, from time to time, with aggregate gross sales proceeds of up to $125,000,000, through an at-the-market equity offering program under which Cowen will act as its sales agent. The issuance and sale of shares of common stock by the Company pursuant to the Sales Agreement are deemed an "at-the-market" offering under the Securities Act of 1933, as amended. Cowen is entitled to compensation for its services equal to up to 3.0% of the gross proceeds of any shares of common stock sold through Cowen under the Sales Agreement. During the year ended December 31, 2017, the Company received net proceeds of approximately $0.7 million from the sale of 52,569 shares of common stock pursuant to the Sales Agreement.

The Company has reserved shares of common stock, for issuance as follows:

	December 31,
	2017		2016		2015
Convertible preferred stock		—		—		14,274,741
Shares available for future option grants		2,576,535		2,475,600		2,559,499
Outstanding options		3,013,394		2,350,582		784,136
Unvested restricted common stock (founders and early exercise of stock options)		319,203		611,698		924,535
Shares reserved for employee stock purchase plan		200,000		200,000		—

Total		6,109,132		5,637,880		18,542,911

- Definition

The entire disclosure for shareholders' equity comprised of portions attributable to the parent entity and noncontrolling interest, including other comprehensive income. Includes, but is not limited to, balances of common stock, preferred stock, additional paid-in capital, other capital and retained earnings, accumulated balance for each classification of other comprehensive income and amount of comprehensive income.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section S99
-Paragraph 4
-Subparagraph (SAB Topic 4.E)
-URI http://asc.fasb.org/extlink&oid=27010918&loc=d3e74567-122707

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21484-112644

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(29))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21488-112644

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(e))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(31))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 4
-Subparagraph (SAB Topic 4.C)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187143-122770

Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(30))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 9: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21506-112644

Reference 10: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(d))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 11: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

Reference 12: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 30
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6405834&loc=d3e23285-112656

Reference 13: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21475-112644

Reference 14: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Preferred Stock
-URI http://asc.fasb.org/extlink&oid=6521494

Reference 15: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644

Reference 16: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21564-112644

+ Details

Stock Option Plans

12 Months Ended

Dec. 31, 2017

Stock Option Plans

Stock Option Plans

10. Stock Option Plans

In February 2014, the Company adopted the 2014 Equity Incentive Plan (the "2014 Plan"), which was subsequently amended in November 2014, July 2015 and September 2015, under which it granted incentive stock options ("ISOs") or non-qualified stock options ("NSOs"). Terms of stock agreements, including vesting requirements, are determined by the board of directors or a committee authorized by the board of directors, subject to the provisions of the 2014 Plan. In general, awards granted by the Company vest over four years and have maximum exercise term of 10 years. The 2014 Plan provides that grants must be at an exercise price of 100% of fair market value of the Company's common stock as determined by the board of directors on the date of the grant.

In connection with the consummation of the IPO in March 2016, the 2016 Equity Incentive Award Plan (the "2016 Plan"), became effective. Under the 2016 Plan, incentive stock options, non-statutory stock options, stock purchase rights and other stock-based awards may be granted. Terms of stock agreements, including vesting requirements, are determined by the board of directors or a committee authorized by the board of directors, subject to the provisions of the 2016 Plan. In general, awards granted by the Company vest over four years and have maximum exercise term of 10 years. The 2016 Plan provides that grants must be at an exercise price of 100% of fair market value of the Company's common stock as determined by the board of directors on the date of the grant. In conjunction with adopting the 2016 Plan, the 2014 Plan was terminated and no further awards will be granted under the 2014 Plan. Options outstanding under the 2014 Plan as of the effective date of the 2016 Plan that are forfeited or lapse unexercised may be re-issued under the 2016 Plan, up to a maximum of 1,136,229 shares.

Activity under the Company's stock option plans is set forth below:

				Options Outstanding
	Shares Available for Grant			Number of Options			Weighted- Average Exercise Price
Balance at December 31, 2016		2,475,600			2,350,582		$	11.88
Additional shares authorized		830,000
Options granted		(969,500	)		969,500			11.80
Options exercised		—			(66,253	)		11.02
Options forfeited		240,435			(240,435	)		13.09

Balance at December 31, 2017		2,576,535			3,013,394		$	11.78

The following table summarizes information about stock options outstanding at December 31, 2017 and 2016:

	Options Outstanding at December 31, 2017						Options Vested at December 31, 2017
Exercise Price	Number		Weighted Average Remaining Contractual Life (in Years)		Weighted Average Exercise Price		Number		Weighted Average Remaining Contractual Life (in Years)		Weighted Average Exercise Price
$0.28 - $4.65		535,136		7.55	$	1.46		325,397		7.53	$	1.34
$6.75 - $10.60		701,600		9.86	$	10.44		2,688		7.87	$	6.75
$11.21 - $16.70		1,776,658		8.57	$	15.41		596,727		8.28	$	15.23

		3,013,394		8.69	$	11.78		924,812		8.02	$	11.40

	Options Outstanding at December 31, 2016						Options Vested at December 31, 2016
Exercise Price	Number		Weighted Average Remaining Contractual Life (in Years)		Weighted Average Exercise Price		Number		Weighted Average Remaining Contractual Life (in Years)		Weighted Average Exercise Price
$0.28		419,682		8.46	$	0.28		159,732		8.44	$	0.28
$4.65		144,500		8.81	$	4.65		42,449		8.81	$	4.65
$6.75		5,000		8.87	$	6.75		1,438		8.87	$	6.75
$13.34 - 16.37		1,781,400		9.42	$	15.21		193,867		9.18	$	15.14

		2,350,582		9.21	$	11.88		397,486		8.84	$	8.02

The weighted average grant date fair value of options granted for the years ended December 31, 2017, 2016 and 2015, was $8.93, $10.78 and $4.37, respectively.

Options outstanding and exercisable that had vested or were expected to vest at December 31, 2017 were as follows:

	Number of shares		Weighted Average Exercise Price		Weighted Average Remaining Contractual Life (years)		Aggregate Intrinsic Value (in thousands)
Vested		924,812	$	10.38		8.02	$	2,946
Expected to vest		2,088,582	$	12.40		8.98	$	1,903

In the table above, aggregate intrinsic value represents the difference between the exercise price of the options to purchase common stock and the estimated fair value of the Company's common stock of $10.36

The aggregate intrinsic value of stock options exercised in the years ended December 31, 2017, 2016 and 2015, was $332,000, $216,000 and $714,000, respectively.

The total fair value of options that vested in the year ended December 31, 2017, 2016 and 2015, was $5.8 million, $3.2 million, and $0.1 million, respectively.

Stock-Based Compensation

12 Months Ended

Dec. 31, 2017

Stock-Based Compensation

Stock-Based Compensation

11. Stock-Based Compensation

The Company's results of operations include expenses relating to employee and non-employee stock-based awards as follows (in thousands):

	Year Ended December 31,
	2017		2016		2015
Research and development	$	2,692	$	1,685	$	292
General and administrative		3,537		2,143		136

Total	$	6,229	$	3,828	$	428

Valuation Assumptions

The Company estimated the fair value of employee stock options using the Black-Scholes valuation model. The fair value of employee stock options is being amortized on a straight-line basis over the requisite service period of the awards. The fair value of employee stock options were estimated using the following assumptions for the years ended December 31, 2017, 2016 and 2015:

	Year Ended December 31,
	2017			2016			2015
Risk-free interest rate		2.1	%		1.6	%		1.7	%
Expected volatility		91.4	%		84.6	%		83.3	%
Expected term (in years)		6.0			6.0			6.1
Expected dividend yield		0	%		0	%		0	%

Risk-free Interest Rate: The Company based the risk-free interest rate over the expected term of the options based on the constant maturity rate of U.S. Treasury securities with similar maturities as of the date of the grant.

Volatility: The Company used an average historical stock price volatility of comparable public companies within the biotechnology and pharmaceutical industry using an average of historical volatilities of the Company's industry peers.

Expected Term: The Company uses the simplified method prescribed in the ASC 718, Compensation—Stock Compensation, to calculate the expected term of options granted to employees and directors.

Expected Dividends: The Company has not paid and does not anticipate paying any dividends in the near future.

At December 31, 2017, 2016 and 2015, the unrecognized compensation expense associated with respect to options granted to employees was $18.6 million, $18.5 million and $4.7 million, respectively, and is expected to be recognized on a straight-line basis over 2.85, 3.28, and 3.64 years, respectively.

Stock-based compensation expense related to awards to non-employees is recognized based on the then-current fair value at each measurement date over the associated service period of the award, which is generally the vesting term, on a straight line basis. The Company used the Black-Scholes valuation model to assist it in determining the fair value of stock-based awards. Stock-based compensation expense for non-employees was $74,000, $142,500 and $42,848 for the years ended December 31, 2017, 2016 and 2015, respectively.

The following assumptions were used in valuation of non-employee stock options:

	Year Ended December 31,
	2017	2016	2015
Risk-free interest rate	2.17% - 2.33%	1.90% - 1.94%	1.8% - 2.2%
Expected volatility	80.3% - 83.0%	92.1% - 92.7%	82% - 84.7%
Expected term (in years)	7.1 - 7.6	8.1 - 8.6	9.1 - 10.0
Expected dividend yield	0%	0%	0%

- Definition

The entire disclosure for compensation-related costs for equity-based compensation, which may include disclosure of policies, compensation plan details, allocation of equity compensation, incentive distributions, equity-based arrangements to obtain goods and services, deferred compensation arrangements, employee stock ownership plan details and employee stock purchase plan details.

+ References + Details

Income Taxes

12 Months Ended

Dec. 31, 2017

Income Taxes

Income Taxes

12. Income Taxes

The components of loss before income tax is as follows (in thousands):

	December 31,
	2017			2016			2015
Domestic	$	(28,253	)	$	(11,375	)	$	(31,335	)
Foreign		(27,410	)		(25,000	)		—

	$	(55,663	)	$	(36,375	)	$	(31,335	)

During the years ended December 31, 2017, 2016 and 2015, the Company recorded no income tax benefits for the net operating losses (NOLs) incurred due to the uncertainty of realizing a benefit from those items.

A reconciliation of the Company's effective tax rate to the U.S. Federal statutory rate is as follows:

	December 31,
	2017			2016			2015
Federal tax benefit at statutory rate		34	%		34	%		34	%
State tax, net of Federal benefit		7	%		8	%		3	%
Loss due to change in fair value of convertible preferred stock liability		—			—			(19	)%
Foreign rate differential		(17	)%		(23	)%		—
Federal rate change impact		(9	)%		—			—
Change in valuation allowance		(16	)%		(19	)%		(18	)%
Research and development tax credits		2	%		2	%		1	%
Other		(1	)%		(2	)%		(1	)

Effective income tax rate		0	%		0	%		0	%

The effective tax rate is different from the federal statutory tax rate primarily due to a foreign rate differential and a valuation allowance against deferred tax assets as a result of the Company's history of losses.

The principal components of the Company's net deferred tax assets are as follows (in thousands)

	December 31,
	2017			2016			2015
Net operating loss carryforwards	$	15,438		$	9,339		$	4,671
Tax credit carryforwards		4,351			1,960			445
Capitalized tax assets		131			241			528
Accruals		183			230			108
Stock compensation		1,730			954			43
Other		52			67			71

Total deferred tax assets		21,885			12,791			5,866
Valuation allowance		(21,885	)		(12,791	)		(5,866	)

Net deferred tax assets	$	—		$	—		$	—

The Company recorded a valuation allowance against its deferred tax assets at December 31, 2017 and 2016 because Company management believed that it was more likely than not that these assets would not be fully realized in the future. The valuation allowance increased by approximately $9.1 million and $7.0 million for the years ended December 31, 2017 and 2016, respectively. Changes in the valuation allowance for deferred tax assets relate primarily to the increase in the Company's net operating loss carryforward.

As of December 31, 2017, the Company had federal NOL carryforwards of approximately $42.1 million and state NOL carryforwards of approximately $94.5 million which are available to reduce future taxable income. The NOLs will begin to expire in 2034, if not utilized.

As of December 31, 2017, the Company also had $3.0 million of federal and $2.1 million of state research and development tax credit carryforwards available to reduce future income taxes. The federal research and development tax credits will begin to expire 2035, if not utilized. The state research and development tax credits have no expiration date.

Utilization of NOL carryforwards and credits may be subject to an annual limitation due to the ownership change provisions provided by the Internal Revenue Code of 1986, as amended ("Code"), and similar state provisions. An annual limitation may result in the expiration of NOLs and credits before utilization. In March and April 2016, the Company issued a total of 5.2 million shares of common stock associated with its IPO. In addition, during the third quarter of 2015, the Company issued a new series of convertible preferred stock. Such common and preferred stock issuances may have created an ownership change under these provisions of the Code and similar state provisions. As of December 31, 2017, NOLs and credits are not expected to expire unused in the carryforward period as a result of these issuances of convertible preferred shares.

On December 22, 2017, the U.S. government enacted comprehensive tax legislation commonly referred to as the Tax Cuts and Jobs Act (the "Tax Act"). The Tax Act makes broad and complex changes to the U.S. tax code, including, but not limited to, (1) reducing the U.S. federal corporate tax rate from 35% to 21%; (2) requiring companies to pay a one-time transition tax on certain unrepatriated earnings of foreign subsidiaries; (3) generally eliminating U.S. federal income taxes on dividends from foreign subsidiaries; (4) requiring a current inclusion in U.S. federal taxable income of certain earnings of controlled foreign corporations; (5) eliminating the corporate alternative minimum tax (AMT) and changing how existing AMT credits can be realized; (6) creating the base erosion anti-abuse tax (BEAT), a new minimum tax; (7) creating a new limitation on deductible interest expense; and (8) changing rules related to uses and limitations of net operating loss carryforwards created in tax years beginning after December 31, 2017.

The SEC staff issued SAB 118, which provides guidance on accounting for the tax effects of the Tax Act. SAB 118 provides a measurement period that should not extend beyond one year from the Tax Act enactment date for companies to complete the accounting under ASC 740. In accordance with SAB 118, a company must reflect the income tax effects of those aspects of the Tax Act for which the accounting under ASC 740 is complete. To the extent that a company's accounting for certain income tax effects of the Tax Act is incomplete but it is able to determine a reasonable estimate, it must record a provisional estimate in the financial statements. If a company cannot determine a provisional estimate to be included in the financial statements, it should continue to apply ASC 740 on the basis of the provisions of the tax laws that were in effect immediately before the enactment of the Tax Act.

Our accounting for the following elements of the Tax Act is complete:

Reduction of US federal Corporate Tax Rate: The Tax Act reduces the corporate tax rate to 21%, effective January 1, 2018. Accordingly, we have re-measured all deferred taxes at 21% as of December 31, 2017. Consequently, we have recorded a decrease related to the net deferred tax asset balance of $4.9 million, with a corresponding net adjustment to the Company's valuation allowance.

Deemed Repatriation Transition Tax: The Deemed Repatriation Transition Tax ("Transition Tax") is a tax on previously untaxed accumulated and current earnings and profits (E&P) of certain of our foreign subsidiaries. To determine the amount of the Transition Tax, we must determine, in addition to other factors, the amount of post-1986 E&P of the relevant subsidiaries, as well as the amount of non-U.S. income taxes paid on such earnings. the Company has a cumulative foreign E&P deficit as of December 31, 2017. As such, the Company does not have a transition tax liability in the 2017 tax year.

Our accounting for the following elements of the Tax Act is incomplete:

Global Intangible Low Taxed Income ("GILTI"): The Tax Act creates a new requirement that certain income (i.e., GILTI) earned by a controlled foreign corporations ("CFCs") must be included currently in the gross income of the CFCs' U.S. shareholder. GILTI is the excess of the shareholder's "net CFC tested income" over the net deemed tangible income return, which is currently defined as the excess of (1) 10% of the aggregate of the U.S. shareholder's pro rata share of the qualified business asset investment of each CFC with respect to which it is a U.S. shareholder over (2) the amount of certain interest expense taken into account in the determination of net CFC-tested income. Because of the complexity of the new GILTI tax rules, we are continuing to evaluate this provision of the Tax Act and the application of ASC 740. Under U.S. GAAP, we are allowed to make an accounting policy choice of either (1) treating taxes due on future U.S. inclusions in taxable income related to GILTI as a current-period expense when incurred (the "period cost method") or (2) factoring such amounts into a company's measurement of its deferred taxes (the "deferred method"). Our selection of an accounting policy with respect to the new GILTI tax rules will depend, in part, on analyzing our global income to determine whether we expect to have future U.S. inclusions in taxable income related to GILTI and, if so, what the impact is expected to be. Because whether we expect to have future U.S. inclusions in taxable income related to GILTI depends on not only our current structure and estimated future results of global operations but also our intent and ability to modify our structure and/or our business, we are not yet able to reasonably estimate the effect of this provision of the Tax Act. Therefore, we have not made any adjustments related to potential GILTI tax in our financial statements and have not made a policy decision regarding whether to record deferred taxes on GILTI.

As of December 31, 2017, the Company had unrecognized tax benefits ("UTBs") of approximately $1.2 million. All of the deferred tax assets associated with these UTBs are fully offset by a valuation allowance. The following table summarizes the activity related to UTBs:

	December 31,
	2017			2016		2015
Unrecognized tax benefits beginning of the period	$	604		$	135	$	—
Increase related to the prior year		(51	)		6		—
Increased related to the current year		666			463		135

Unrecognized tax benefits, end of the period	$	1,219		$	604	$	135

The Company follows the provisions of ASC 740, Accounting for Income Taxes, and the accounting guidance related to accounting for uncertainty in income taxes. The Company determines its uncertain tax positions based on a determination of whether and how much of a tax benefit taken by the Company in its tax filings or positions is more likely than not to be sustained upon examination by the relevant income tax authorities. The Company will recognize both accrued interest and penalties related to unrecognized benefits in income tax expense. Since the Company is in a loss carryforward position, the Company is generally subject to examination by the U.S. federal, state and local income tax authorities for all tax years in which a loss carryforward is available.

Commitments and Contingencies

12 Months Ended

Dec. 31, 2017

Commitments and Contingencies

Commitments and Contingencies

13. Commitments and Contingencies

Facility Lease

In January 2015, the Company signed an initial operating lease, effective February 1, 2015 for 8,138 square feet of office and laboratory space with a one-year term. Between January 2015 and August 2016, the Company entered into a series of lease amendments to increase the amount of leased space to 28,633 square feet and extend the expiration of the lease to February 2021. The lease agreement includes an annual rent escalation clause and a right to extend the term at the then current market rate for three years. Under the lease and subsequent amendments, the landlord provided approximately $1.9 million in free rent and lease incentives. The Company records rent expense on a straight-line basis over the effective term of the lease, including any free rent periods and incentives. The lease requires the Company to pay additional amounts for operating and maintenance expenses. Rent expense related to the facilities lease for the years ended years ended December 31, 2017, 2016 and 2015 was approximately $734,000, $584,000 and $347,000, respectively. As of December 31, 2017, future minimum lease payments under the facility lease were as follows (in thousands):

	Payments Due by Period
	Total		Less than 1 Year		2 - 3 years		4 -5 Years		More than 5 Years
Contractual obligations:
Operating lease obligations	$	3,632	$	1,144	$	2,387	$	101	$	—

Total contractual obligations	$	3,632	$	1,144	$	2,387	$	101	$	—

In August 2015 the Company entered into an agreement for a line of credit of $0.1 million for the purpose of issuing its landlord a letter of credit of $0.1 million as a security deposit under its facility lease. The Company pledged money market funds and marketable securities as collateral for the line of credit. Pursuant to the Company's license agreements with each of Vernalis and Scripps, it has obligations to make future milestone and royalty payments to these parties, respectively. However, because these amounts are contingent, they have not been included on the Company's balance sheet.

Indemnifications

In the ordinary course of business, the Company enters into agreements that may include indemnification provisions. Pursuant to such agreements, the Company may indemnify, hold harmless and defend an indemnified party for losses suffered or incurred by the indemnified party. Some of the provisions will limit losses to those arising from third-party actions. In some cases, the indemnification will continue after the termination of the agreement. The maximum potential amount of future payments the Company could be required to make under these provisions is not determinable. The Company has never incurred material costs to defend lawsuits or settle claims related to these indemnification provisions. The Company has also entered into indemnification agreements with its directors and officers that may require the Company to indemnify its directors and officers against liabilities that may arise by reason of their status or service as directors or officers to the fullest extent permitted by Delaware corporate law. There have been no claims to date and the Company has a directors and officers insurance policy that may enable it to recover a portion of any amounts paid for future claims.

Legal Proceedings

The Company is not a party to any material legal proceedings.

Related Party Transaction	12 Months Ended
Related Party Transaction	Dec. 31, 2017
Related Party Transaction
Related Party Transaction	14. Related Party Transaction In 2017, the Company purchased $461,000 of research services from a vendor during the normal course of business, where a Corvus director is also a member of the vendor's board of directors.

Quarterly Selected Financial Data (unaudited)

12 Months Ended

Dec. 31, 2017

Quarterly Selected Financial Data (unaudited)

Quarterly Selected Financial Data (unaudited)

15. Quarterly Selected Financial Data (unaudited)

	Quarter Ended
	December 31, 2017			September 30, 2017			June 30, 2017			March 31, 2017
Operating Expenses	$	12,189		$	12,944		$	15,174		$	16,217
Net loss		(11,929	)		(12,717	)		(14,981	)		(16,036	)
Net loss per share, basic and diluted	$	(0.58	)	$	(0.62	)	$	(0.73	)	$	(0.79	)

	Quarter Ended
	December 31, 2016			September 30, 2016			June 30, 2016			March 31, 2016
Operating Expenses	$	11,249		$	10,476		$	8,825		$	6,426
Net loss		(11,086	)		(10,297	)		(8,645	)		(6,347	)
Net loss per share, basic and diluted	$	(0.55	)	$	(0.51	)	$	(0.43	)	$	(5.39	)

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2017

Summary of Significant Accounting Policies

Basis of Presentation

Use of Estimates

Initial Public Offering

Concentrations of Credit Risk and Other Risks and Uncertainties

Segments

Cash and Cash Equivalents and Marketable Securities

Cash, Cash Equivalents and Marketable Securities

The Company considers all highly liquid investment securities with remaining maturities at the date of purchase of three months or less to be cash equivalents.

Fair Value Measurements

Deferred Offering Costs

Property and Equipment, Net

Property and equipment are stated at cost and depreciated using the straight-line method over the estimated useful lives of the respective assets:

Laboratory equipment		5 years
Computer equiipment and purchased software		3 years
Leasehold improvements		Shorter of asset's useful life or remaining term of lease

Impairment of Long-Lived Assets

Consolidation

The consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries. Intercompany balances and transactions have been eliminated in consolidation

Research and Development Expenses


	•	employee-related expenses, including salaries, benefits, travel and non-cash stock-based compensation expense;
	•	external research and development expenses incurred under arrangements with third parties, such as contract research organizations, contract manufacturing organizations, academic and non-profit institutions and consultants;
	•	costs to acquire technologies to be used in research and development that have not reached technological feasibility and have no alternative future use;
	•	license fees; and
	•	other expenses, which include direct and allocated expenses for laboratory, facilities and other costs.

Clinical Trial Accruals

Stock-Based Compensation

The Company maintains incentive plans under which incentive stock options and nonqualified stock options may be granted to employees and non-employee service providers.

Income Taxes

Comprehensive Loss

Net Loss per Share

Recent Accounting Pronouncements

Net Loss per Share (Tables)

12 Months Ended

Dec. 31, 2017

Net Loss per Share

Schedule of net loss per share, basic and diluted

The following table shows the calculation of net loss per share (in thousands, except share and per share data):

	Year Ended December 31,
	2017			2016			2015
Numerator:
Net loss—basic and diluted	$	(55,663	)	$	(36,375	)	$	(31,335	)


Denominator:
Weighted average common shares outstanding		20,958,557			16,188,980			1,269,315
Less: weighted average common shares subject to repurchase		(470,051	)		(766,939	)		(895,672	)

Weighted average common shares outstanding used to compute basic and diluted net loss per share		20,488,506			15,422,041			373,643


Net loss per share, basic and diluted	$	(2.72	)	$	(2.36	)	$	(83.86	)

Schedule of antidilutive securities excluded from calculation of diluted net loss per share

	Year Ended December 31,
	2017		2016		2015
Convertible preferrd stock		—		—		14,274,741
Common stock subject to repurchase		319,203		611,698		924,535
Outstanding options		3,013,394		2,350,582		784,136

Total shares of common stock equivalents		3,332,597		2,962,280		15,983,412

Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2017

Fair Value Measurements

Schedule of fair value of assets and liabilities recorded at fair value on a recurring basis, by level within fair value hierarchy

	December 31, 2017
	Fair Value Measured Using
	Fair Value Measured Using						Total Balance
	(Level 1)		(Level 2)		(Level 3)		Total Balance
Assets
Cash equivalents	$	—	$	44,555	$	—	$	44,555
Marketable securities		—		44,949		—		44,949

	$	—	$	89,504	$	—	$	89,504

	December 31, 2016
	Fair Value Measured Using
	Fair Value Measured Using						Total Balance
	(Level 1)		(Level 2)		(Level 3)		Total Balance
Assets
Cash equivalents	$	2,999	$	—	$	—	$	2,999
Marketable securities		129,846		—		—		129,846

	$	132,845	$	—	$	—	$	132,845

Schedule of fair value of available for sale marketable securities

As of December 31, 2017 and 2016, the fair value of available for sale marketable securities by type of security were as follows (in thousands):

	December 31, 2017
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Government agency securities	$	32,311	$	—	$	(39	)	$	32,272
Corporate debt obligations		12,679		—		(2	)		12,677

	$	44,990	$	—	$	(41	)	$	44,949

	December 31, 2016
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
U.S. Treasury securities	$	129,885	$	—	$	(39	)	$	129,846

Balance Sheet Components (Tables)

12 Months Ended

Dec. 31, 2017

Balance Sheet Components

Schedule of prepaid and other current assets

	December 31,
	2017		2016
Prepaid and Other Current Assets
Interest receivable	$	132	$	365
Prepaid research and development manufacturing expenses		327		—
Other		720		772

	$	1,179	$	1,137

Schedule of property and equipment, net

Property and Equipment, net
Laboratory equipment	$	2,034		$	1,868
Computer equipment and purchased software		130			58
Leasehold improvements		2,078			2,051

		4,242			3,977
Less: accumulated depreciation and amortization		(1,570	)		(729	)

	$	2,672		$	3,248

Schedule of accrued and other liabilities

Accrued and Other Liabilities
Accrued clinical trial related	$	2,870	$	1,617
Accrued manufacturing expense		1,056		955
Personnel related		572		526
Deferred rent		410		378
Accrued legal and accounting		224		255
Other accrued expenses		383		313

	$	5,515	$	4,044

Schedule of other liabilities

Other Liabilities
Deferred rent	$	960	$	1,370
Shares subject to vesting		11		35

	$	971	$	1,405

Common Stock (Tables)

12 Months Ended

Dec. 31, 2017

Common Stock

Schedule of shares of common stock reserved for issuance

	December 31,
	2017		2016		2015
Convertible preferred stock		—		—		14,274,741
Shares available for future option grants		2,576,535		2,475,600		2,559,499
Outstanding options		3,013,394		2,350,582		784,136
Unvested restricted common stock (founders and early exercise of stock options)		319,203		611,698		924,535
Shares reserved for employee stock purchase plan		200,000		200,000		—

Total		6,109,132		5,637,880		18,542,911

Stock Option Plans (Tables)

12 Months Ended

Dec. 31, 2017

Stock Option Plans

Summary of stock option activity

				Options Outstanding
	Shares Available for Grant			Number of Options			Weighted- Average Exercise Price
Balance at December 31, 2016		2,475,600			2,350,582		$	11.88
Additional shares authorized		830,000
Options granted		(969,500	)		969,500			11.80
Options exercised		—			(66,253	)		11.02
Options forfeited		240,435			(240,435	)		13.09

Balance at December 31, 2017		2,576,535			3,013,394		$	11.78

Summary of information about stock options outstanding

	Options Outstanding at December 31, 2017						Options Vested at December 31, 2017
Exercise Price	Number		Weighted Average Remaining Contractual Life (in Years)		Weighted Average Exercise Price		Number		Weighted Average Remaining Contractual Life (in Years)		Weighted Average Exercise Price
$0.28 - $4.65		535,136		7.55	$	1.46		325,397		7.53	$	1.34
$6.75 - $10.60		701,600		9.86	$	10.44		2,688		7.87	$	6.75
$11.21 - $16.70		1,776,658		8.57	$	15.41		596,727		8.28	$	15.23

		3,013,394		8.69	$	11.78		924,812		8.02	$	11.40

	Options Outstanding at December 31, 2016						Options Vested at December 31, 2016
Exercise Price	Number		Weighted Average Remaining Contractual Life (in Years)		Weighted Average Exercise Price		Number		Weighted Average Remaining Contractual Life (in Years)		Weighted Average Exercise Price
$0.28		419,682		8.46	$	0.28		159,732		8.44	$	0.28
$4.65		144,500		8.81	$	4.65		42,449		8.81	$	4.65
$6.75		5,000		8.87	$	6.75		1,438		8.87	$	6.75
$13.34 - 16.37		1,781,400		9.42	$	15.21		193,867		9.18	$	15.14

		2,350,582		9.21	$	11.88		397,486		8.84	$	8.02

Schedule of options outstanding and exercisable that had vested or were expected to vest

Options outstanding and exercisable that had vested or were expected to vest at December 31, 2017 were as follows:

	Number of shares		Weighted Average Exercise Price		Weighted Average Remaining Contractual Life (years)		Aggregate Intrinsic Value (in thousands)
Vested		924,812	$	10.38		8.02	$	2,946
Expected to vest		2,088,582	$	12.40		8.98	$	1,903

Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2017

Schedule of expenses relating to employee and non-employee stock-based awards

The Company's results of operations include expenses relating to employee and non-employee stock-based awards as follows (in thousands):

	Year Ended December 31,
	2017		2016		2015
Research and development	$	2,692	$	1,685	$	292
General and administrative		3,537		2,143		136

Total	$	6,229	$	3,828	$	428

Schedule of fair value of share-based awards to employees was estimated using Black Scholes model

	Year Ended December 31,
	2017			2016			2015
Risk-free interest rate		2.1	%		1.6	%		1.7	%
Expected volatility		91.4	%		84.6	%		83.3	%
Expected term (in years)		6.0			6.0			6.1
Expected dividend yield		0	%		0	%		0	%

Non-employees

Schedule of fair value of share-based awards to employees was estimated using Black Scholes model

	Year Ended December 31,
	2017	2016	2015
Risk-free interest rate	2.17% - 2.33%	1.90% - 1.94%	1.8% - 2.2%
Expected volatility	80.3% - 83.0%	92.1% - 92.7%	82% - 84.7%
Expected term (in years)	7.1 - 7.6	8.1 - 8.6	9.1 - 10.0
Expected dividend yield	0%	0%	0%

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2017

Income Taxes

Schedule of components of loss before income tax

The components of loss before income tax is as follows (in thousands):

	December 31,
	2017			2016			2015
Domestic	$	(28,253	)	$	(11,375	)	$	(31,335	)
Foreign		(27,410	)		(25,000	)		—

	$	(55,663	)	$	(36,375	)	$	(31,335	)

Schedule of reconciliation of the effective tax rate to the U.S. Federal statutory rate

	December 31,
	2017			2016			2015
Federal tax benefit at statutory rate		34	%		34	%		34	%
State tax, net of Federal benefit		7	%		8	%		3	%
Loss due to change in fair value of convertible preferred stock liability		—			—			(19	)%
Foreign rate differential		(17	)%		(23	)%		—
Federal rate change impact		(9	)%		—			—
Change in valuation allowance		(16	)%		(19	)%		(18	)%
Research and development tax credits		2	%		2	%		1	%
Other		(1	)%		(2	)%		(1	)

Effective income tax rate		0	%		0	%		0	%

Schedule of principal components of the net deferred tax assets

The principal components of the Company's net deferred tax assets are as follows (in thousands)

	December 31,
	2017			2016			2015
Net operating loss carryforwards	$	15,438		$	9,339		$	4,671
Tax credit carryforwards		4,351			1,960			445
Capitalized tax assets		131			241			528
Accruals		183			230			108
Stock compensation		1,730			954			43
Other		52			67			71

Total deferred tax assets		21,885			12,791			5,866
Valuation allowance		(21,885	)		(12,791	)		(5,866	)

Net deferred tax assets	$	—		$	—		$	—

Schedule of unrecognized tax benefits activity

	December 31,
	2017			2016		2015
Unrecognized tax benefits beginning of the period	$	604		$	135	$	—
Increase related to the prior year		(51	)		6		—
Increased related to the current year		666			463		135

Unrecognized tax benefits, end of the period	$	1,219		$	604	$	135

Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2017

Commitments and Contingencies

Schedule of future minimum lease payments under the facility lease

As of December 31, 2017, future minimum lease payments under the facility lease were as follows (in thousands):

	Payments Due by Period
	Total		Less than 1 Year		2 - 3 years		4 -5 Years		More than 5 Years
Contractual obligations:
Operating lease obligations	$	3,632	$	1,144	$	2,387	$	101	$	—

Total contractual obligations	$	3,632	$	1,144	$	2,387	$	101	$	—

Quarterly Selected Financial Data (unaudited) (Tables)

12 Months Ended

Dec. 31, 2017

Quarterly Selected Financial Data (unaudited)

Quarterly Selected Financial Data (unaudited)

	Quarter Ended
	December 31, 2017			September 30, 2017			June 30, 2017			March 31, 2017
Operating Expenses	$	12,189		$	12,944		$	15,174		$	16,217
Net loss		(11,929	)		(12,717	)		(14,981	)		(16,036	)
Net loss per share, basic and diluted	$	(0.58	)	$	(0.62	)	$	(0.73	)	$	(0.79	)

	Quarter Ended
	December 31, 2016			September 30, 2016			June 30, 2016			March 31, 2016
Operating Expenses	$	11,249		$	10,476		$	8,825		$	6,426
Net loss		(11,086	)		(10,297	)		(8,645	)		(6,347	)
Net loss per share, basic and diluted	$	(0.55	)	$	(0.51	)	$	(0.43	)	$	(5.39	)

Summary of Significant Accounting Policies - Basis of Presentation, Credit Risk and Income Taxes (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017 USD ($) segment item	Dec. 31, 2016 USD ($)	Dec. 31, 2015
Accumulated deficit		$ (123,534)	$ (67,871)
Cash, cash equivalents and marketable securities		$ 90,100
Concentrations of Credit Risk and Other Risks and Uncertainties
Number of highly credit qualified financial institutions \| item		2
Segments
Number of operating segments \| segment		1
Income Taxes
Corporate tax rate		34.00%	34.00%	34.00%
Forecast
Income Taxes
Corporate tax rate	21.00%

- Definition

Cash includes currency on hand as well as demand deposits with banks or financial institutions. It also includes other kinds of accounts that have the general characteristics of demand deposits in that the customer may deposit additional funds at any time and effectively may withdraw funds at any time without prior notice or penalty. Cash equivalents, excluding items classified as marketable securities, include short-term, highly liquid Investments that are both readily convertible to known amounts of cash, and so near their maturity that they present minimal risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three years ago does not become a cash equivalent when its remaining maturity is three months. Short-term investments, exclusive of cash equivalents, generally consist of marketable securities intended to be sold within one year (or the normal operating cycle if longer) and may include trading securities, available-for-sale securities, or held-to-maturity securities (if maturing within one year), as applicable.

+ References + Details

Summary of Significant Accounting Policies - IPO (Details) - USD ($) $ / shares in Units, $ in Millions	Apr. 26, 2016	Mar. 29, 2016
IPO
Initial Public Offering
Share Price		$ 15.00
Issuance of stock (in shares)		4,700,000
Net proceeds from IPO		$ 70.6
Over-Allotment Option
Initial Public Offering
Share Price	$ 15.00
Issuance of stock (in shares)	502,618

Summary of Significant Accounting Policies - Property and Equipment, Net (Details)	12 Months Ended
	Dec. 31, 2017
Laboratory equipment
Property and Equipment, Net
Estimated Useful lives	5 years
Computer equipment and purchased software
Property and Equipment, Net
Estimated Useful lives	3 years

Net Loss per Share - Net Loss per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net loss - basic and diluted	$ (11,929)	$ (12,717)	$ (14,981)	$ (16,036)	$ (11,086)	$ (10,297)	$ (8,645)	$ (6,347)	$ (55,663)	$ (36,375)	$ (31,335)
Denominator:
Weighted average common shares outstanding									20,958,557	16,188,980	1,269,315
Less: weighted average common shares subject to repurchase									(470,051)	(766,939)	(895,672)
Weighted average common shares outstanding used to compute basic and diluted net loss per share									20,488,506	15,422,041	373,643
Net loss per share, basic and diluted (in dollars per share)	$ (0.58)	$ (0.62)	$ (0.73)	$ (0.79)	$ (0.55)	$ (0.51)	$ (0.43)	$ (5.39)	$ (2.72)	$ (2.36)	$ (83.86)

Net Loss per Share - Anti-Dilutive Securities (Details) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Excluded from the calculation of diluted net loss per share, due to anti-dilutive effect
Shares of common stock equivalents	3,332,597	2,962,280	15,983,412
Convertible Preferred Stock
Excluded from the calculation of diluted net loss per share, due to anti-dilutive effect
Shares of common stock equivalents			14,274,741
Common stock subject to repurchase
Excluded from the calculation of diluted net loss per share, due to anti-dilutive effect
Shares of common stock equivalents	319,203	611,698	924,535
Outstanding options
Excluded from the calculation of diluted net loss per share, due to anti-dilutive effect
Shares of common stock equivalents	3,013,394	2,350,582	784,136

Fair Value Measurements - Recurring (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets
Marketable securities	$ 44,949	$ 129,846
Recurring
Assets
Cash equivalents	44,555	2,999
Marketable securities	44,949	129,846
Total Assets	89,504	132,845
Recurring \| Level 1
Assets
Cash equivalents		2,999
Marketable securities		129,846
Total Assets		$ 132,845
Recurring \| Level 2
Assets
Cash equivalents	44,555
Marketable securities	44,949
Total Assets	$ 89,504

Fair Value Measurements - Available For Sale (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair value of available for sale marketable securities
Amortized Cost	$ 44,990
Gross Unrealized Losses	(41)
Fair Value	44,949	$ 129,846
U.S. Treasury securities
Fair value of available for sale marketable securities
Amortized Cost		129,885
Gross Unrealized Losses		(39)
Fair Value		$ 129,846
Government agency securities
Fair value of available for sale marketable securities
Amortized Cost	32,311
Gross Unrealized Losses	(39)
Fair Value	32,272
Corporate debt obligations
Fair value of available for sale marketable securities
Amortized Cost	12,679
Gross Unrealized Losses	(2)
Fair Value	$ 12,677

License and Collaboration Agreements (Details)	1 Months Ended	12 Months Ended
License and Collaboration Agreements (Details)	May 31, 2017 item	Dec. 31, 2017 USD ($)	Dec. 31, 2015 USD ($)	Feb. 28, 2017 USD ($)
License and Collaboration Agreements
Collaboration Agreement, Maximum Number of Patients in Treatment Arm \| item	65
Licensing Agreement \| Scripps Research Institute
License and Collaboration Agreements
Minimum annual fee			$ 25,000
Aggregate potential milestone payments		$ 2,600,000
Milestone payments made		$ 0
Period of written notice to terminate		90 days
Licensing Agreement \| Scripps Research Institute \| Research and development
License and Collaboration Agreements
One-time cash payment			$ 10,000
Licensing Agreement \| Vernalis (R&D) Limited
License and Collaboration Agreements
Milestone payments made				$ 3,000,000
Period of written notice to terminate		90 days
Licensing Agreement \| Vernalis (R&D) Limited \| Minimum
License and Collaboration Agreements
Aggregate potential milestone payments		$ 200,000,000
Licensing Agreement \| Vernalis (R&D) Limited \| Research and development
License and Collaboration Agreements
One-time cash payment		$ 1,000,000

Balance Sheet Components - Prepaid and Other Current Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Prepaid and Other Current Assets
Interest receivable	$ 132	$ 365
Prepaid research and development manufacturing expenses	327
Other	720	772
Total of prepaid and other current assets	$ 1,179	$ 1,137

Balance Sheet Components - Property and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property and Equipment, net
Property and equipment, gross	$ 4,242	$ 3,977
Less: accumulated depreciation and amortization	(1,570)	(729)
Property and equipment, net	2,672	3,248
Laboratory equipment
Property and Equipment, net
Property and equipment, gross	2,034	1,868
Computer equipment and purchased software
Property and Equipment, net
Property and equipment, gross	130	58
Leasehold improvements
Property and Equipment, net
Property and equipment, gross	$ 2,078	$ 2,051

Balance Sheet Components - Accrued and Other Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accrued and Other Liabilities
Accrued clinical trial related	$ 2,870	$ 1,617
Accrued manufacturing expense	1,056	955
Personnel related	572	526
Deferred rent	410	378
Accrued legal and accounting	224	255
Other accrued expenses	383	313
Total of accrued and other liabilities	$ 5,515	$ 4,044

Balance Sheet Components - Other Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Other Liabilities
Deferred rent	$ 960	$ 1,370
Shares subject to vesting	11	35
Total of other liabilities	$ 971	$ 1,405

Convertible Preferred Stock (Details) - shares	Mar. 29, 2016	Dec. 31, 2016
Convertible preferred stock
Shares Outstanding		0
Conversion
Number of common stock issued with the conversion of the preferred stock (in shares)	14,274,741
Preferred stock conversion ratio	1

Convertible Preferred Stock Liability (Details) $ / shares in Units, $ in Thousands			1 Months Ended		3 Months Ended	6 Months Ended	12 Months Ended
	Jun. 10, 2015	Nov. 26, 2014 USD ($) item shares	Jun. 30, 2015 USD ($) $ / shares shares	Jan. 31, 2015 USD ($) shares	Mar. 31, 2015	Jun. 30, 2015 USD ($)	Dec. 31, 2015 USD ($) shares	Dec. 31, 2014
Convertible preferred stock
Shares Authorized \| shares							14,274,741
Net proceeds							$ 95,569
Change in fair value of convertible preferred stock liability							$ 17,600
Series A convertible preferred stock
Convertible preferred stock
Shares Authorized \| shares		8,921,438
Issuance of stock (in shares) \| shares		3,395,468	4,460,715	1,065,246			5,525,961
Net proceeds		$ 12,600	$ 16,700	$ 4,000
Number of defined triggers for issuance of additional shares to first tranche investors on the same terms \| item		3
Convertible preferred stock liability		$ 2,600
Change in fair value of convertible preferred stock liability						$ 17,600
Reclassification of liability to convertible preferred stock			$ 20,200
Series A convertible preferred stock \| Obligation to issue second tranche
Convertible preferred stock
Fixed price for second tranche obligation (in dollars per share) \| $ / shares			$ 3.755
Assumed term	2 years				9 months			1 year
Interest rate (as a percent)	0.75%				0.20%			0.13%
Volatility (as a percent)	75.00%				85.00%			85.00%

Common Stock (Details)		12 Months Ended
Common Stock (Details)	Sep. 20, 2017 USD ($)	Dec. 31, 2017 USD ($) Vote shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 shares
Common Stock
Common stock, shares authorized for issuance		290,000,000	290,000,000
Preferred stock, shares authorized for issuance		10,000,000	10,000,000
Number of votes per share of common stock \| Vote		1
Dividends on common stock \| $		$ 0
Proceeds from Issuance of Common Stock \| $		$ 711,000	$ 71,354,000
Shares of common stock reserved for issuance, on an as-converted basis
Convertible preferred stock, shares authorized				14,274,741
Shares available for future option grants		2,576,535	2,475,600	2,559,499
Outstanding options		3,013,394	2,350,582	784,136
Unvested restricted common stock (founders and early exercise of stock options)		319,203	611,698	924,535
Shares reserved for employee stock purchase plan		200,000	200,000
Total		6,109,132	5,637,880	18,542,911
Sales Agreement \| Cowen
Common Stock
Proceeds from Issuance of Common Stock \| $	$ 125,000,000	$ 700,000
Issuance of stock (in shares)		52,569
Compensation on sale of stock, percentage	3.00%
Common Stock.
Common Stock
Issuance of stock (in shares)		52,569
Shares of common stock sold to founders				384,866

Stock Option Plans - Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended		12 Months Ended
	Mar. 31, 2016	Feb. 28, 2014	Dec. 31, 2017	Dec. 31, 2016
Shares Available for Grant
Beginning balance			2,475,600	2,559,499
Exercised			$ 731	$ 4
Ending balance			2,576,535	2,475,600
Number of Options
Beginning balance			2,350,582	784,136
Ending balance			3,013,394	2,350,582
2014 Plan
Stock Option Plans
Vesting period		4 years
Maximum exercise term		10 years
Exercise price of common stock of its fair value (as a percent)		100.00%
Maximum number of options that may be re-issued under the 2016 plan	1,136,229
2016 Plan
Stock Option Plans
Vesting period	4 years
Maximum exercise term	10 years
Exercise price of common stock of its fair value (as a percent)	100.00%
Shares Available for Grant
Beginning balance			2,475,600
Shares authorized for plan			830,000
Granted			(969,500)
Forfeited			240,435
Ending balance			2,576,535	2,475,600
Number of Options
Beginning balance			2,350,582
Granted			969,500
Exercised				(66,253)
Forfeited			(240,435)
Ending balance			3,013,394	2,350,582
Weighted-Average Exercise Price
Beginning balance (in dollars per share)			$ 11.88
Granted (in dollars per share)			11.80
Exercised (in dollars per share)				$ 11.02
Forfeited (in dollars per share)			13.09
Ending balance (in dollars per share)			$ 11.78	$ 11.88

Stock Option Plan - Exercise Price Range (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Information about stock options outstanding
Options Outstanding, Number	3,013,394	2,350,582
Options Outstanding, Weighted Average Remaining Contractual Life	8 years 8 months 9 days	9 years 2 months 16 days
Weighted average exercise price of options outstanding (in dollars per share)	$ 11.78	$ 11.88
Options Vested, Number	924,812	397,486
Options Vested, Weighted Average Remaining Contractual Life	8 years 7 days	8 years 10 months 2 days
Weighted average exercise price of options vested (in dollars per share)	$ 11.40	$ 8.02
Weighted average grant date fair value of options granted (in dollars per share)	8.93	10.78	$ 4.37
$0.28 - $4.65
Information about stock options outstanding
Exercise Price, low end of range (in dollars per share)	0.28
Exercise Price, high end of range (in dollars per share)	$ 4.65
Options Outstanding, Number	535,136
Options Outstanding, Weighted Average Remaining Contractual Life	7 years 6 months 18 days
Weighted average exercise price of options outstanding (in dollars per share)	$ 1.46
Options Vested, Number	325,397
Options Vested, Weighted Average Remaining Contractual Life	7 years 6 months 11 days
Weighted average exercise price of options vested (in dollars per share)	$ 1.34
$6.75 - $10.60
Information about stock options outstanding
Exercise Price, low end of range (in dollars per share)	6.75
Exercise Price, high end of range (in dollars per share)	$ 10.60
Options Outstanding, Number	701,600
Options Outstanding, Weighted Average Remaining Contractual Life	9 years 10 months 10 days
Weighted average exercise price of options outstanding (in dollars per share)	$ 10.44
Options Vested, Number	2,688
Options Vested, Weighted Average Remaining Contractual Life	7 years 10 months 13 days
Weighted average exercise price of options vested (in dollars per share)	$ 6.75
$11.21 - $16.70
Information about stock options outstanding
Exercise Price, low end of range (in dollars per share)	11.21
Exercise Price, high end of range (in dollars per share)	$ 16.70
Options Outstanding, Number	1,776,658
Options Outstanding, Weighted Average Remaining Contractual Life	8 years 6 months 26 days
Weighted average exercise price of options outstanding (in dollars per share)	$ 15.41
Options Vested, Number	596,727
Options Vested, Weighted Average Remaining Contractual Life	8 years 3 months 11 days
Weighted average exercise price of options vested (in dollars per share)	$ 15.23
$0.28
Information about stock options outstanding
Exercise Price, low end of range (in dollars per share)		$ 0.28
Options Outstanding, Number		419,682
Options Outstanding, Weighted Average Remaining Contractual Life		8 years 5 months 16 days
Weighted average exercise price of options outstanding (in dollars per share)		$ 0.28
Options Vested, Number		159,732
Options Vested, Weighted Average Remaining Contractual Life		8 years 5 months 9 days
Weighted average exercise price of options vested (in dollars per share)		$ 0.28
$4.65
Information about stock options outstanding
Exercise Price, low end of range (in dollars per share)		$ 4.65
Options Outstanding, Number		144,500
Options Outstanding, Weighted Average Remaining Contractual Life		8 years 9 months 22 days
Weighted average exercise price of options outstanding (in dollars per share)		$ 4.65
Options Vested, Number		42,449
Options Vested, Weighted Average Remaining Contractual Life		8 years 9 months 22 days
Weighted average exercise price of options vested (in dollars per share)		$ 4.65
$6.75
Information about stock options outstanding
Exercise Price, low end of range (in dollars per share)		$ 6.75
Options Outstanding, Number		5,000
Options Outstanding, Weighted Average Remaining Contractual Life		8 years 10 months 13 days
Weighted average exercise price of options outstanding (in dollars per share)		$ 6.75
Options Vested, Number		1,438
Options Vested, Weighted Average Remaining Contractual Life		8 years 10 months 13 days
Weighted average exercise price of options vested (in dollars per share)		$ 6.75
$13.34 - $16.37
Information about stock options outstanding
Exercise Price, low end of range (in dollars per share)		13.34
Exercise Price, high end of range (in dollars per share)		$ 16.37
Options Outstanding, Number		1,781,400
Options Outstanding, Weighted Average Remaining Contractual Life		9 years 5 months 1 day
Weighted average exercise price of options outstanding (in dollars per share)		$ 15.21
Options Vested, Number		193,867
Options Vested, Weighted Average Remaining Contractual Life		9 years 2 months 5 days
Weighted average exercise price of options vested (in dollars per share)		$ 15.14

Stock Option Plan - Vested or Expected to Vest (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Options outstanding and exercisable
Vested, Number of Shares (in shares)	924,812
Vested, Weighted Average Exercise Price (in dollars per share)	$ 10.38
Vested, Weighted Average Remaining Contractual Life	8 years 7 days
Vested, Aggregate Intrinsic Value	$ 2,946,000
Expected to vest, Number of Shares (in shares)	2,088,582
Expected to vest, Weighted Average Exercise Price (in dollars per share)	$ 12.40
Expected to vest, Weighted Average Remaining Contractual Life	8 years 11 months 23 days
Expected to vest, Aggregate Intrinsic Value	$ 1,903,000
Fair value of common stock (in dollars per share)	$ 10.36
Aggregate intrinsic value of stock options exercised	$ 332,000	$ 216,000	$ 714,000
Fair value of options vested	$ 5,800,000	$ 3,200,000	$ 100,000

Stock-Based Compensation - Expense Allocation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock-based compensation
Stock-based compensation expense	$ 6,229	$ 3,828	$ 428
Research and development
Stock-based compensation
Stock-based compensation expense	2,692	1,685	292
General and administrative
Stock-based compensation
Stock-based compensation expense	$ 3,537	$ 2,143	$ 136

Stock Based Compensation -Valuation Assumptions Employees (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Weighted-average assumptions used to estimate fair value of share-based awards
Risk-free interest rate	2.10%	1.60%	1.70%
Expected volatility	91.40%	84.60%	83.30%
Expected term (in years)	6 years	6 years	6 years 1 month 6 days
Expected dividend yield	0.00%	0.00%	0.00%
Unrecognized compensation expense	$ 18.6	$ 18.5	$ 4.7
Expected period for recognizing compensation expense	2 years 10 months 6 days	3 years 3 months 11 days	3 years 7 months 21 days

Stock Based Compensation -Valuation Assumptions Non-Employees (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock Option Plans
Stock-based compensation expense	$ 6,229,000	$ 3,828,000	$ 428,000
Weighted-average assumptions used to estimate fair value of share-based awards
Risk-free interest rate, minimum			1.80%
Risk-free interest rate, maximum			2.20%
Expected Volatility, minimum			82.00%
Expected Volatility, maximum			84.70%
Expected term (in years)	6 years	6 years	6 years 1 month 6 days
Expected dividend yield	0.00%	0.00%	0.00%
Minimum
Weighted-average assumptions used to estimate fair value of share-based awards
Expected term (in years)			9 years 1 month 6 days
Maximum
Weighted-average assumptions used to estimate fair value of share-based awards
Expected term (in years)			10 years
Non-employee stock options
Stock Option Plans
Stock-based compensation expense	$ 74,000	$ 142,500	$ 42,848
Weighted-average assumptions used to estimate fair value of share-based awards
Risk-free interest rate, minimum	2.17%	1.90%
Risk-free interest rate, maximum	2.33%	1.94%
Expected Volatility, minimum	80.30%	92.10%
Expected Volatility, maximum	83.00%	92.70%
Expected dividend yield	0.00%	0.00%
Non-employee stock options \| Minimum
Weighted-average assumptions used to estimate fair value of share-based awards
Expected term (in years)	7 years 1 month 6 days	8 years 1 month 6 days
Non-employee stock options \| Maximum
Weighted-average assumptions used to estimate fair value of share-based awards
Expected term (in years)	7 years 7 months 6 days	8 years 7 months 6 days

Income Taxes - Components of loss before income tax and Income tax benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes
Domestic	$ (28,253)	$ (11,375)	$ (31,335)
Foreign	(27,410)	(25,000)
Total	(55,663)	(36,375)	(31,335)
Income tax benefits	$ 0	$ 0	$ 0

Income Taxes - Reconciliation of effective tax rate (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of the effective tax rate to the U.S. Federal statutory rate
Federal tax benefit at statutory rate	34.00%	34.00%	34.00%
State tax, net of Federal benefit	7.00%	8.00%	3.00%
Loss due to change in fair value of convertible preferred stock liability			(19.00%)
Foreign rate differential	(17.00%)	(23.00%)
Federal rate change impact	(9.00%)
Change in valuation allowance	(16.00%)	(19.00%)	(18.00%)
Research and development tax credits	2.00%	2.00%	1.00%
Other	(1.00%)	(2.00%)	(1.00%)
Effective income tax rate	0.00%	0.00%	0.00%

Income Taxes - Deferred tax assets net (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred tax assets:
Net operating loss carryforwards	$ 15,438	$ 9,339	$ 4,671
Tax credit carryforwards	4,351	1,960	445
Capitalized tax assets	131	241	528
Accruals	183	230	108
Stock compensation	1,730	954	43
Other	52	67	71
Total deferred tax assets	21,885	12,791	5,866
Valuation allowance	(21,885)	(12,791)	(5,866)
Net deferred tax assets	0	0	$ 0
Increased in valuation allowance	$ 9,100	$ 7,000

Income Taxes - Tax credit carry forwards (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Apr. 30, 2016
Tax credit carryforwards
Common stock, shares issued	21,041,250	20,922,428
Research and development credits \| Federal
Tax credit carryforwards
Research and development tax credit carryforwards	$ 3.0
Research and development credits \| State
Tax credit carryforwards
Research and development tax credit carryforwards	$ 2.1
IPO
Tax credit carryforwards
Common stock, shares issued			5,200,000

Income Taxes - Tax Cuts and Jobs Act (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Corporate tax rate		34.00%	34.00%	34.00%
Net decrease to the DTA		$ 4.9
Forecast
Corporate tax rate	21.00%

Income Taxes - Unrecognized tax benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Summary of unrecognized tax benefits
Unrecognized tax benefits beginning of the period	$ 604	$ 135
Increase related to the prior year		6
Decrease related to the prior year	(51)
Increased related to the current year	666	463	$ 135
Unrecognized tax benefits, end of the period	$ 1,219	$ 604	$ 135

Commitments and Contingencies (Details)	1 Months Ended	12 Months Ended			20 Months Ended
Commitments and Contingencies (Details)	Jan. 31, 2015 ft²	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Aug. 31, 2016 ft²
Facility Lease
Landlord Provided Free Rent And Lease Incentives		$ 1,900,000
Rent expense		734,000	$ 584,000	$ 347,000
Operating lease obligations
Total		3,632,000
Less than 1 Year		1,144,000
2-3 years		2,387,000
4-5 years		101,000
Contractual obligations
Total		3,632,000
Less than 1 Year		1,144,000
2-3 years		2,387,000
4-5 years		$ 101,000
Facilities lease \| Office and laboratory space in Burlingame, California
Facility Lease
Area of leased facility \| ft²	8,138
Lease term	1 year
Additional area of lease \| ft²					28,633
Lease agreement, right to extend the term		3 years

Commitments and Contingencies - Line of Credit (Details) $ in Millions	Aug. 31, 2015 USD ($)
Line of credit
Security deposit	$ 0.1
Secured debt
Line of credit
Line of credit	$ 0.1

Quarterly Selected Financial Data (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Selected Financial Data (unaudited)
Operating Expenses	$ 12,189	$ 12,944	$ 15,174	$ 16,217	$ 11,249	$ 10,476	$ 8,825	$ 6,426	$ 56,524	$ 36,976	$ 13,770
Net loss	$ (11,929)	$ (12,717)	$ (14,981)	$ (16,036)	$ (11,086)	$ (10,297)	$ (8,645)	$ (6,347)	$ (55,663)	$ (36,375)	$ (31,335)
Net loss per share, basic and diluted (in dollars per share)	$ (0.58)	$ (0.62)	$ (0.73)	$ (0.79)	$ (0.55)	$ (0.51)	$ (0.43)	$ (5.39)	$ (2.72)	$ (2.36)	$ (83.86)

Convertible Preferred Stock	12 Months Ended
Convertible Preferred Stock	Dec. 31, 2017
Convertible Preferred Stock
Convertible Preferred Stock	7. Convertible Preferred Stock Immediately prior to the consummation of the IPO on March 29, 2016, all outstanding shares of Series A and B convertible preferred stock were converted into 14,274,741 shares of common stock on a one-for-one basis. As such, no convertible preferred stock shares were outstanding as of December 31, 2016.

Organization (Details)	12 Months Ended
Organization (Details)	Dec. 31, 2017 subsidiary
Organization
Number of insignificant subsidiaries	4

Related Party Transaction (Details) $ in Thousands	12 Months Ended
Related Party Transaction (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Director
Related Party Transaction
Purchase of research services	$ 461,000

Income Taxes - Operating loss carryforwards (Details) $ in Millions	Dec. 31, 2017 USD ($)
Federal
Net operating loss carryforwards
NOL carryforwards	$ 42.1
State
Net operating loss carryforwards
NOL carryforwards	$ 94.5