Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2017	Mar. 02, 2018	Jun. 30, 2017
Document And Entity Information [Abstract]
Entity Registrant Name	FATE THERAPEUTICS INC
Entity Central Index Key	0001434316
Document Type	10-K
Document Period End Date	Dec. 31, 2017
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Accelerated Filer
Trading Symbol	FATE
Entity Public Float			$ 122,330,000
Entity Common Stock, Shares Outstanding		52,769,156
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	$ 88,952	$ 88,609
Short-term investments and related maturity receivables	11,997	3,503
Prepaid expenses and other current assets	1,647	1,211
Total current assets	102,596	93,323
Property and equipment, net	2,550	1,579
Restricted cash	122	122
Other assets	24	24
Total assets	105,292	95,048
Current liabilities:
Accounts payable	1,678	934
Accrued expenses	7,254	3,957
Current portion of deferred rent	12	4
Current portion of deferred revenue	2,105	2,105
Long–term debt, current portion		8,187
Total current liabilities	11,049	15,187
Deferred rent	1,347	101
Deferred revenue	724	2,829
Accrued expenses	175	1,276
Long–term debt, net of current portion	14,808	2,501
Commitments and contingencies (Note 5)
Stockholders’ Equity:
Preferred stock, $0.001 par value; authorized shares—5,000,000 at December 31, 2017 and December 31, 2016; 2,819,549 Class A convertible preferred shares issued and outstanding at December 31, 2017 and December 31, 2016	3	3
Common stock, $0.001 par value; authorized shares—150,000,000 at December 31, 2017 and December 31, 2016; issued and outstanding—52,648,601 at December 31, 2017 and 41,386,506 at December 31, 2016	53	41
Additional paid–in capital	295,934	248,957
Accumulated other comprehensive loss	(3)	(1)
Accumulated deficit	(218,798)	(175,846)
Total stockholders’ equity	77,189	73,154
Total liabilities and stockholders’ equity	$ 105,292	$ 95,048

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
Preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001
Preferred stock, shares authorized	5,000,000	5,000,000
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, authorized shares	150,000,000	150,000,000
Common stock issued	52,648,601	41,386,506
Common stock, outstanding shares	52,648,601	41,386,506
Class A Convertible Preferred Shares
Preferred stock, issued shares	2,819,549	2,819,549
Preferred stock, outstanding shares	2,819,549	2,819,549

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Collaboration revenue	$ 4,106	$ 4,402	$ 2,431
Operating expenses:
Research and development	34,358	26,452	19,861
General and administrative	11,873	9,913	10,352
Total operating expenses	46,231	36,365	30,213
Loss from operations	(42,125)	(31,963)	(27,782)
Other income (expense):
Interest income	559	138	10
Interest expense	(1,268)	(1,637)	(2,220)
Loss on extinguishment of debt	(118)
Total other expense, net	(827)	(1,499)	(2,210)
Net loss	(42,952)	(33,462)	(29,992)
Other comprehensive loss:
Unrealized loss on available-for-sale securities, net	(2)	(1)
Comprehensive loss	$ (42,954)	$ (33,463)	$ (29,992)
Net loss per common share, basic and diluted	$ (1.02)	$ (1.05)	$ (1.18)
Weighted-average common shares used to compute basic and diluted net loss per share	41,982,167	31,754,140	25,484,262

Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid In Capital	Accumulated Other Comprehensive Loss	Accumulated Deficit	Convertible Preferred Stock
Balance at Dec. 31, 2014	$ 28,340	$ 21	$ 140,711		$ (112,392)
Balance (in shares) at Dec. 31, 2014		20,569,399
Exercise of stock options, net of issuance costs	420		420
Exercise of stock options, net of issuance costs (in shares)		227,215
Repurchase liability for unvested equity awards	44		44
Stock–based compensation	2,400		2,400
Senior executive incentive bonuses paid in common stock	97		97
Senior executive incentive bonuses paid in common stock (in Shares)		19,956
Public offering of common stock, net of offering costs	32,149	$ 7	32,142
Public offering of common stock, net of offering costs (in shares)		6,900,000
Issuance of common stock to collaboration partner	4,580	$ 1	4,579
Issuance of common stock to collaboration partner (in Shares)		1,000,000
Net loss	(29,992)				(29,992)
Balance at Dec. 31, 2015	38,038	$ 29	180,393		(142,384)
Balance (in shares) at Dec. 31, 2015		28,716,570
Exercise of stock options, net of issuance costs	187		187
Exercise of stock options, net of issuance costs (in shares)		136,368
Repurchase liability for unvested equity awards	1		1
Stock–based compensation	3,184		3,184
Private placement issuances of common stock, net of offering costs	28,797	$ 12	28,785
Private placement issuances of common stock, net of offering costs (in shares)		12,486,837
Private placement issuance of Series A convertible preferred stock, net of offering costs	36,289		36,286			$ 3
Private placement issuance of Series A convertible preferred stock, net of offering costs (in shares)						2,819,549
Senior executive incentive bonuses paid in common stock	121		121
Senior executive incentive bonuses paid in common stock (in Shares)		46,731
Unrealized loss on short-term investments	(1)			$ (1)
Net loss	(33,462)				(33,462)
Balance at Dec. 31, 2016	73,154	$ 41	248,957	(1)	(175,846)	$ 3
Balance (in shares) at Dec. 31, 2016		41,386,506				2,819,549
Exercise of stock options, net of issuance costs	226		226
Exercise of stock options, net of issuance costs (in shares)		83,220
Issuance of common stock upon vesting of restricted stock units		$ 1	(1)
Issuance of common stock upon vesting of restricted stock units (in shares)		225,125
Stock–based compensation	3,606		3,606
Issuance of warrants for common stock	217		217
Private placement issuances of common stock, net of offering costs	(13)		(13)
Private placement issuance of Series A convertible preferred stock, net of offering costs	(26)		(26)
Public offering of common stock, net of offering costs	42,979	$ 11	42,968
Public offering of common stock, net of offering costs (in shares)		10,953,750
Unrealized loss on short-term investments	(2)			(2)
Net loss	(42,952)				(42,952)
Balance at Dec. 31, 2017	$ 77,189	$ 53	$ 295,934	$ (3)	$ (218,798)	$ 3
Balance (in shares) at Dec. 31, 2017		52,648,601				2,819,549

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	$ (42,952)	$ (33,462)	$ (29,992)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation and amortization	971	881	687
Stock–based compensation	3,606	3,184	2,400
Amortization of debt discounts and debt issuance costs	81	139	176
Amortization of premiums and discounts on investments, net	(25)	171
Noncash interest expense	321	492	651
Deferred rent	1,085	(7)	(24)
Deferred revenue	(2,105)	(2,401)	7,335
Loss on extinguishment of debt	30
Changes in assets and liabilities:
Prepaid expenses and other assets	(428)	(381)	(107)
Accounts payable and accrued expenses	2,511	1,561	477
Net cash used in operating activities	(36,905)	(29,823)	(18,397)
Cash flows from investing activities
Proceeds from sale of property and equipment		18
Purchase of property and equipment	(1,725)	(457)	(1,498)
Purchases of short-term investments	(39,971)	(19,675)
Maturities of short-term investments	31,500	16,000
Net cash used in investing activities	(10,196)	(4,114)	(1,498)
Cash flows from financing activities
Issuance of common stock from equity incentive plans, net of repurchases and issuance costs	205	186	420
Proceeds from public offering of common stock, net of issuance costs	43,206		32,149
Proceeds from sale of common stock to collaboration partner			4,580
Proceeds from long–term debt	15,000
Payments on long–term debt	(10,764)	(7,689)	(1,546)
Payments for the issuance of debt	(10)
Net cash provided by financing activities	47,444	57,737	35,603
Net change in cash, cash equivalents and restricted cash	343	23,800	15,708
Cash, cash equivalents and restricted cash at beginning of the period	88,731	64,931	49,223
Cash, cash equivalents and restricted cash at end of the period	89,074	88,731	64,931
Supplemental disclosure of cash flow information
Interest paid	2,314	1,067	$ 1,353
Supplemental schedule of noncash investing and financing activities
Issuance of warrants for common stock in connection with long–term debt	217
Common Stock
Cash flows from financing activities
Proceeds from private placement issuances of stock, net of issuance costs	(65)	28,849
Preferred Stock
Cash flows from financing activities
Proceeds from private placement issuances of stock, net of issuance costs	$ (128)	$ 36,391

Organization and Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2017

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Organization and Summary of Significant Accounting Policies

1. Organization and Summary of Significant Accounting Policies

Organization

Fate Therapeutics, Inc. (the “Company”) was incorporated in the state of Delaware on April 27, 2007 and has its principal operations in San Diego, California. The Company is a clinical-stage biopharmaceutical company dedicated to the development of programmed cellular immunotherapies for cancer and immune disorders. The Company’s cell therapy pipeline is comprised of NK- and T-cell immuno-oncology programs, including off-the-shelf engineered product candidates derived from clonal master iPSC lines, and immuno-regulatory programs, including product candidates to prevent life-threatening complications in patients undergoing hematopoietic cell transplantation and to promote immune tolerance in patients with autoimmune disease. Its adoptive cell therapy programs are based on the Company’s novel ex vivo cell programming approach, which it applies to modulate the therapeutic function and direct the fate of immune cells.

As of December 31, 2017, the Company has devoted substantially all of its efforts to product development, raising capital and building infrastructure and has not generated any revenues from any sales of its therapeutic products. To date, the Company’s revenues have been derived from collaboration agreements and government grants.

Public Equity Offerings

In May 2015, the Company completed a public offering of common stock in which investors purchased 6,900,000 shares of its common stock at an offering price of $5.00 per share. Gross proceeds from the offering were $34.5 million. After giving effect to costs related to the offering, total net proceeds from the offering were $32.1 million.

In December 2017, the Company completed a public offering of common stock in which investors purchased 10,953,750 shares of its common stock at a price of $4.20 per share under the Company’s shelf registration statement. Gross proceeds from the offering were $46.0 million, and after giving effect to an estimated $3.0 million of costs related to the offering (of which $0.2 million was not paid as of December 31, 2017), net proceeds are estimated to be $43.0 million.

Private Placements of Common Stock and Convertible Preferred Stock

In August 2016, the Company completed a private placement of common stock in which investors purchased 5,250,000 shares of the Company’s common stock at a price of $1.96 per share. Gross proceeds from the private placement were $10.3 million. After giving effect to costs related to the private placement, net proceeds were $10.2 million. The Company also registered all of the shares issued in the private placement transaction for resale on a Form S-3 filed with the Securities and Exchange Commission (the SEC), as required under a registration rights agreement entered into by the Company with the purchasers of the common stock, and the registration statement was declared effective in September 2016.

In November 2016, the Company completed a private placement of common and preferred stock in which investors, including investors affiliated with the Company’s directors and officers, purchased convertible preferred stock and common stock of the Company. The Company issued 2,819,549 shares of non-voting Class A Preferred Stock at $13.30 per share, each of which is convertible into five shares of common stock upon certain conditions. The Company also issued 7,236,837 shares of common stock at $2.66 per share. Gross proceeds from the private placement were $56.7 million. After giving effect to costs related to the private placement, net proceeds were $54.9 million. The Company also entered into a registration rights agreement (the Registration Rights Agreement) with certain of the purchasers in the November 2016 placement, excluding those purchasers affiliated with the Company’s directors and officers, requiring the Company to register for the resale of the relevant shares. The Company registered all of the relevant shares issued in the placement for resale on a Form S-3 filed with the SEC, as required under the Registration Rights Agreement, and the registration statement was declared effective in January 2017. See Note 6 to the Consolidated Financial Statements for additional information related to this offering.

Use of Estimates

The Company’s consolidated financial statements are prepared in accordance with United States generally accepted accounting principles (GAAP). The preparation of the Company’s consolidated financial statements requires it to make estimates and assumptions that impact the reported amounts of assets, liabilities, revenues and expenses and the disclosure of contingent assets and liabilities in the Company’s consolidated financial statements and accompanying notes. The most significant estimates in the Company’s consolidated financial statements relate to accrued expenses. Although these estimates are based on the Company’s knowledge of current events and actions it may undertake in the future, actual results may ultimately materially differ from these estimates and assumptions.

Principles of Consolidation

The consolidated financial statements include the accounts of the Company and its subsidiaries, Fate Therapeutics (Canada), Inc. or Fate Canada, incorporated in Canada and which was dissolved in November 2016, Fate Therapeutics Ltd., incorporated in the United Kingdom, and Tfinity Therapeutics, Inc., incorporated in the United States. To date, the aggregate operations of these subsidiaries have not been significant and all intercompany transactions and balances have been eliminated in consolidation.

Segment Reporting

Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision-maker in making decisions regarding resource allocation and assessing performance. The Company views its operations and manages its business in one operating segment.

Fair Value of Financial Instruments

The carrying amounts of accounts payable and accrued liabilities are considered to be representative of their respective fair values because of the short-term nature of those instruments. Based on the borrowing rates available to the Company for loans with similar terms, which is considered a Level 2 input as described below, and because of the relatively recent financing date, the Company believes that the fair value of long-term debt approximates its carrying value.

The accounting guidance defines fair value, establishes a consistent framework for measuring fair value and expands disclosure for each major asset and liability category measured at fair value on either a recurring or nonrecurring basis. Fair value is defined as an exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As such, fair value is a market-based measurement that should be determined based on assumptions that market participants would use in pricing an asset or liability. As a basis for considering such assumptions, the accounting guidance establishes a three- tier fair value hierarchy, which prioritizes the inputs used in measuring fair value as follows:

Level 1: Observable inputs such as quoted prices in active markets;

Level 2: Inputs, other than the quoted prices in active markets, that are observable either directly or indirectly; and

Level 3: Unobservable inputs in which there is little or no market data, which require the reporting entity to develop its own assumptions.

Financial assets measured at fair value on a recurring basis consist of the Company’s cash equivalents and short-term investments. Cash equivalents consisted of money market funds and short-term investments consisted of U.S. treasuries. The following table presents the Company’s assets which were measured at fair value on a recurring basis as of December 31, 2017 and 2016 (in thousands):

			Fair Value Measurements at Reporting Date Using
	Total		Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
As of December 31, 2017:
Cash equivalents	$	88,952	$	88,952	$	—	$	—
U.S. Treasury debt securities		11,997		11,997		—		—
Total assets	$	100,949	$	100,949	$	—	$	—

As of December 31, 2016:
Cash equivalents	$	88,609	$	88,609	$	—	$	—
U.S. Treasury debt securities		3,503		3,503		—		—
Total assets	$	92,112	$	92,112	$	—	$	—

The Company obtains pricing information from its investment manager and generally determines the fair value of investment securities using standard observable inputs, including reported trades, broker/dealer quotes, and bid and/or offers.

None of the Company’s non-financial assets or liabilities is recorded at fair value on a non-recurring basis. No transfers between levels have occurred during the periods presented.

As of December 31, 2017 and 2016, the Company had no material liabilities measured at fair value on a recurring basis.

Cash, Cash Equivalents and Restricted Cash

Cash and cash equivalents include cash in readily available checking and savings accounts, and money market funds. The Company considers all highly liquid investments with an original maturity of three months or less from the date of purchase to be cash equivalents.

The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the Consolidated Balance Sheets that sum to the total of the same such amounts shown in the Consolidated Statements of Cash Flows as of December 31, 2017 (in thousands):

	December 31, 2017		December 31, 2016		December 31, 2015
Cash and cash equivalents	$	88,952	$	88,609	$	64,809
Restricted cash		122		122		122
Total cash, cash equivalents, and restricted cash shown in the statement of cash flows	$	89,074	$	88,731	$	64,931

Amounts included in restricted cash represent security deposits required to secure the Company’s credit card limit and its facilities lease.

Short-Term Investments

Available-for-sale securities are carried at fair value, with the unrealized gains and losses reported in comprehensive income. The amortized cost of available-for-sale debt securities is adjusted for amortization of premiums and accretion of discounts to maturity. Such amortization and accretion is included in interest income. Realized gains and losses and declines in value judged to be other-than-temporary, if any, on available-for-sale securities are included in other income or expense. The cost of securities sold is based on the specific identification method. Interest and dividends on securities classified as available-for-sale are included in interest income.

Concentration of Credit Risk

Financial instruments, which potentially subject the Company to a significant concentration of credit risk, consist primarily of cash and cash equivalents, and short-term investments. The Company maintains deposits in federally insured financial institutions in excess of federally insured limits. The Company has not experienced any losses in such accounts and management believes that the Company is not exposed to significant credit risk due to the financial position of the depository institutions in which those deposits and investments are held.

Property and Equipment

Property and equipment are recorded at cost and depreciated using the straight-line method over the estimated useful lives of the assets (generally two to five years) and generally consist of furniture and fixtures, computers, and scientific and office equipment. Repairs and maintenance costs are charged to expense as incurred.

Impairment of Long-Lived Assets

Long-lived assets consist primarily of property and equipment. An impairment loss is recorded if and when events and circumstances indicate that assets might be impaired and the undiscounted cash flows estimated to be generated by those assets are less than the carrying amount of those assets. While the Company’s current and historical operating losses and negative cash flows are indicators of impairment, management believes that future cash flows to be received support the carrying value of its long-lived assets and, accordingly, has not recognized any impairment losses since inception.

Deferred Rent

Deferred rent consists of the difference between cash payments and the recognition of rent expense on a straight-line basis for the facilities the Company occupies. The Company’s lease for its facilities provides for fixed increases in minimum annual rental payments. The total amount of rental payments due over the lease term are charged to rent expense ratably over the life of the lease.

Revenue Recognition

The Company recognizes revenues when all four of the following criteria are met: (i) persuasive evidence that an agreement exists; (ii) delivery of the products and/or services has occurred; (iii) the selling price is fixed or determinable; and (iv) collectability is reasonably assured.

Revenue arrangements with multiple elements are analyzed to determine whether the elements can be divided into separate units of accounting or whether the elements must be accounted for as a single unit of accounting. The Company divides the elements into separate units of accounting and applies the applicable revenue recognition criteria to each of the elements, if the delivered elements have value to the customer on a stand-alone basis, if the arrangement includes a general right of return relative to the delivered elements, and if the delivery or performance of the undelivered elements is considered probable and substantially within the Company’s control.

Revenue has been allocated to each element at the inception of the arrangement using the relative selling price method that is based on a three-tier hierarchy. The relative selling price method requires that the estimated selling price for each element be based on vendor-specific objective evidence (VSOE) of fair value, which represents the price charged for each element when it is sold separately or, for an element not yet being sold separately, the price established by management. When VSOE of fair value is not available, third-party evidence (TPE) of fair value is acceptable, or a best estimate of selling price is used if neither VSOE nor TPE is available. A best estimate of selling price should be consistent with the objective of determining the price at which the Company would transact if the element were sold regularly on a stand-alone basis and should also take into account market conditions and company-specific factors.

Revenue arrangements with multiple elements may include license fees, research and development payments, milestone payments, other contingent payments, and royalties on any product sales derived from collaborations. The Company recognizes nonrefundable license fees with stand-alone value as revenue at the time that the Company has satisfied all performance obligations, and recognizes license fees without stand-alone value as revenue in combination with any undelivered performance obligations. The Company recognizes a research and development payment as revenue over the term of the collaboration agreement as contracted amounts are earned, or reimbursable costs are incurred, under the agreement, where contracted amounts are considered to be earned in relative proportion to the performance required under the applicable agreement. The Company recognizes a milestone payment, which is contingent upon the achievement of a milestone in its entirety, as revenue in the period in which the milestone is achieved only if the milestone meets all criteria to be considered substantive. These criteria include the following: (i) the consideration being earned should be commensurate with either the Company’s performance to achieve the milestone or the enhancement of the value of the item delivered as a result of a specific outcome resulting from the Company’s performance to achieve the milestone; (ii) the consideration being earned should relate solely to past performance; (iii) the consideration being earned should be reasonable relative to all deliverables and payment terms in the arrangement; and (iv) the milestone should be considered in its entirety and cannot be bifurcated into substantive and nonsubstantive components. Any amounts received pursuant to revenue arrangements with multiple elements prior to satisfying the Company’s revenue recognition criteria are recorded as deferred revenue on the Company’s consolidated balance sheets.

Revenue from government grants is recorded when reimbursable expenses are incurred under the grant in accordance with the terms of the grant award.

Research and Development Costs

All research and development costs are expensed as incurred.

Patent Costs

Costs related to filing and pursuing patent applications are recorded as general and administrative expense and expensed as incurred since recoverability of such expenditures is uncertain.

Stock-Based Compensation

Stock-based compensation expense represents the cost of the grant date fair value of employee stock option and restricted stock unit grants recognized over the requisite service period of the awards (usually the vesting period) on a straight-line basis. For stock option grants for which vesting is subject to performance-based milestones, the expense is recorded over the remaining service period after the point when the achievement of the milestone is probable or the performance condition has been achieved. For stock option grants for which vesting is subject to both performance-based milestones and market conditions, expense is recorded over the derived service period after the point when the achievement of the performance-based milestone is probable or the performance condition has been achieved. The Company estimates the fair value of stock option grants using the Black-Scholes option pricing model, with the exception of option grants for which vesting is subject to both performance-based milestones and market conditions, which are valued using a lattice-based model. The fair value of restricted stock units is based on the closing price of the Company’s common stock as reported on The NASDAQ Global Market on the date of grant.

The Company accounts for stock options and restricted stock awards to non-employees using the fair value approach. Stock options and restricted stock awards to non-employees are subject to periodic revaluation over their vesting terms. For stock option grants for which vesting is subject to performance-based milestones, the expense is recorded over the remaining service period after the point when the performance condition is determined to be probable of achievement or when it has been achieved.

Convertible Preferred Stock

The Company applies the relevant accounting standards to distinguish liabilities from equity when assessing the classification and measurement of preferred stock. Preferred shares subject to mandatory redemptions are considered liabilities and measured at fair value. Conditionally redeemable preferred shares are considered temporary equity. All other preferred shares are considered as stockholders’ equity.

The Company applies the relevant accounting standards for derivatives and hedging (in addition to distinguishing liabilities from equity) when accounting for hybrid contracts that contain conversion options. Conversion options must be bifurcated from the host instruments and accounted for as free standing financial instruments according to certain criteria. These criteria include circumstances when (i) the economic characteristics and risks of the embedded derivative instruments are not clearly and closely related to the economic characteristics and risks of the host contract, (ii) the hybrid instrument that embodies both the embedded derivative instrument and the host contract is not re-measured at fair value under otherwise applicable accounting principles with changes in fair value reported in earnings as they occurred, and (iii) a separate instrument with the same terms as the embedded derivative instrument would be considered a derivative instrument. The derivative is subsequently measured at fair value at each reporting date, with the changes in fair value reported in earnings.

Income Taxes

The Company accounts for income taxes under the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the financial statements. Under this method, deferred tax assets and liabilities are determined on the basis of the differences between the financial statements and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. The effect of a change in tax rates on deferred tax assets and liabilities is recognized in income in the period that includes the enactment date.

The Company recognizes net deferred tax assets to the extent that the Company believes these assets are more likely than not to be realized. In making such a determination, management considers all available positive and negative evidence, including future reversals of existing taxable temporary differences, projected future taxable income, tax-planning strategies, and results of recent operations. If management determines that the Company would be able to realize its deferred tax assets in the future in excess of their net recorded amount, management would make an adjustment to the deferred tax asset valuation allowance, which would reduce the provision for income taxes.

The Company records uncertain tax positions on the basis of a two-step process whereby (1) management determines whether it is more likely than not that the tax positions will be sustained on the basis of the technical merits of the position and (2) for those tax positions that meet the more- likely-than-not recognition threshold, management recognizes the largest amount of tax benefit that is more than 50 percent likely to be realized upon ultimate settlement with the related tax authority. The Company recognizes interest and penalties related to unrecognized tax benefits within income tax expense. Any accrued interest and penalties are included within the related tax liability.

Comprehensive Loss

Comprehensive loss is defined as a change in equity during a period from transactions and other events and circumstances from non-owner sources. Other comprehensive loss included unrealized losses on available-for-sale securities, which was the only difference between net loss and comprehensive loss for the applicable periods.

Net Loss Per Common Share

Basic net loss per common share is calculated by dividing the net loss by the weighted-average number of common shares outstanding for the period, without consideration for common stock equivalents. Excluded from the weighted-average number of shares outstanding are shares which have been issued upon the early exercise of stock options and are subject to future vesting and unvested restricted stock totaling zero shares, 3,284 shares, and 44,381 shares for the years ended December 31, 2017, 2016, and 2015, respectively. Dilutive common stock equivalents are comprised of convertible preferred stock, warrants for the purchase of common stock, and common stock options and restricted stock units outstanding under the Company’s stock option plans. For all periods presented, there is no difference in the number of common shares used to calculate basic and diluted common shares outstanding due to the Company’s net loss position.

Potentially dilutive securities not included in the calculation of diluted net loss per common share because to do so would be anti-dilutive are as follows (in common stock equivalent shares):

	As of December 31,
	2017		2016		2015
Warrants for common stock		225,756		134,113		134,113
Common stock options		5,458,043		3,910,350		2,587,474
Restricted stock units		212,625		525,250		525,250
Series A convertible preferred stock (if converted)		14,097,745		14,097,745		—
		19,994,169		18,667,458		3,246,837

Recent Accounting Pronouncements

In November 2016, the Financial Accounting Standards Board (the FASB) issued Accounting Standards Update (ASU) No. 2016-18 (ASU 2016-18). ASU 2016-18 requires that a statement of cash flows explain the change during the period in the total of cash, cash equivalents, and amounts generally described as restricted cash or restricted cash equivalents. ASU 2016-18 is effective for fiscal years beginning after December 15, 2017. As early adoption of this amendment is permitted, the Company has adopted the update retrospectively to each period presented. The adoption of this guidance did not have a material impact on the Company’s Consolidated Financial Statements.

In March 2016, the FASB issued ASU 2016-09, which simplifies several aspects of the accounting for share-based payment transactions, including the income tax consequences, classification of awards as either equity or liabilities, and classification on the statement of cash flows. ASU 2016-09 became effective for the Company on January 1, 2017. The adoption of this guidance did not have a material impact on the Company’s Consolidated Financial Statements.

In February 2016, the FASB issued ASU 2016-02, which requires an entity to recognize assets and liabilities arising from a lease for both financing and operating leases, along with additional qualitative and quantitative disclosures. ASU 2016-02 is effective for fiscal years beginning after December 15, 2018, with early adoption permitted. The Company is currently evaluating the effect this standard will have on its Consolidated Financial Statements.

In May 2014, the FASB issued ASU 2014-09, which created a single, principle-based revenue recognition model that will supersede and replace nearly all existing U.S. GAAP revenue recognition guidance. Entities will recognize revenue in a manner that depicts the transfer of goods or services to customers at an amount that reflects the consideration to which the entity expects to be entitled to receive in exchange for those goods or services. The model provides that entities follow five steps: (i) identify the contract with a customer, (ii) identify the performance obligations in the contract, (iii) determine the transaction price, (iv) allocate the transaction price to the performance obligations, and (v) recognize revenue. For public business entities, ASU 2014-09 is effective beginning in the first quarter of 2018 using one of two prescribed transition methods: retrospectively to each prior reporting period presented (full retrospective method), or retrospectively with the cumulative effect of initially applying the guidance recognized at the date of initial application (the cumulative catch-up transition method). The Company will adopt ASU 2014-09 in the first quarter of 2018 using the full retrospective method. The Company has evaluated the effect that the updated standard will have on its internal processes, financial statements and related disclosures, and has determined that the adoption will not have a material impact on the Company’s historical Consolidated Financial Statements.

Going Concern Assessment

Pursuant to ASU 2014-15, the Company has assessed its ability to continue as a going concern for a period of one year from the date of the issuance of these financial statements. Substantial doubt about an entity’s ability to continue as a going concern exists when relevant conditions and events, considered in the aggregate, indicate that it is probable that the entity will be unable to meet its obligations as they become due within one year from the financial statement issuance date. The Company determined that there are no conditions or events that raise substantial doubt about its ability to continue as a going concern as of the date of the issuance of these financial statements.

Juno Collaboration and License Agreement	12 Months Ended
Juno Collaboration and License Agreement	Dec. 31, 2017
Collaboration And License Agreements Disclosure [Abstract]
Juno Collaboration and License Agreement	2. Juno Collaboration and License Agreement On May 4, 2015, the Company entered into a strategic research collaboration and license agreement (the Agreement) with Juno Therapeutics, Inc. (Juno) to screen for and identify small molecules that enhance the therapeutic properties of Juno’s genetically-engineered T-cell immunotherapies. Pursuant to the terms of the Agreement, Juno paid the Company a non-refundable upfront payment of $5.0 million and purchased 1,000,000 shares of the Company’s common stock at a price of $8.00 per share. Additionally, Juno agreed to fund all of the Company’s collaboration research activities for an initial four-year research term beginning on the effective date of the Agreement, with minimum annual research payments of $2.0 million to the Company. Juno has the option to extend the exclusive research term for an additional two years beyond the initial four-year term, subject to the payment of an extension fee of $3.0 million and the continued funding of the Company’s activities under the collaboration during the extended term, with minimum annual research payments of $4.0 million to the Company during the two-year extension period. Upon exercise of the research term extension, the Company has the option to require Juno to purchase up to $10.0 million of the Company’s common stock at a premium equal to 120% of the then thirty-day trailing volume weighted average trading price of the Company’s common stock. The Company applied Accounting Standards Codification (ASC) 605-25, Revenue Recognition — Multiple Element Arrangements, to evaluate the appropriate accounting for the Agreement. In accordance with this guidance, the Company assessed the potential deliverables, including an exclusive license granted by the Company to Juno for certain intellectual property and research services to be performed by the Company, and determined that the deliverables did not have stand-alone value. The Company determined that the license deliverable granted under the Agreement does not have standalone value given the highly specific nature of the small molecules to be identified for use with Juno’s genetically-engineered T-cell immunotherapies. The Company concluded that there is one single unit of accounting, and the arrangement consideration will be recognized in the same manner as the final deliverable, which is the research services. As such, the upfront payment of $5.0 million was recorded as deferred revenue and is being recognized over the initial four-year research term under the Agreement. With respect to the $8.0 million payment for the Company’s common stock, the Company determined that the common stock purchase price of $8.00 per share represented a premium of $3.40 per share. This premium represents arrangement consideration and therefore the aggregate premium of $3.4 million was recorded as deferred revenue and is being recorded as revenue ratably over the initial four-year research term. The remaining $4.6 million consideration that represents the purchase of common stock was recorded as the issuance of common stock in shareholders’ equity. Pursuant to the collaboration’s research plan under the Agreement, the Company is responsible for screening and identifying small molecule modulators of immunological cells, while Juno will be responsible for the development and commercialization of engineered T-cell immunotherapies incorporating the Company’s modulators. As the Company is principally responsible for the performance of the research services under the Agreement, revenue is recognized on a gross basis for such services when earned. Billings for research services will be recognized as deferred revenue until earned. Total revenue recognized under the Agreement for the years ended December 31, 2017, 2016, and 2015 was $4.1 million, $4.4 million, and $2.4 million, respectively. As of December 31, 2017, aggregate deferred revenue related to the Agreement was $2.8 million. Under the Agreement, the Company has granted Juno an exclusive worldwide license to certain of its intellectual property, including its intellectual property arising under the collaboration, to make, use, sell and otherwise exploit genetically-engineered T-cell immunotherapies using or incorporating small molecule modulators directed against certain designated tumor-associated antigen targets, subject to the selection of a target by Juno. The Company has retained exclusive rights to such intellectual property, including its intellectual property arising under the collaboration, for all other purposes, including its use outside of those targets selected by Juno. The Company is eligible under the Agreement to receive selection fees for each tumor-associated antigen target selected by Juno and bonus selection fees based on the aggregate number of tumor-associated antigen targets selected by Juno. In accordance with ASC 605-28, Revenue Recognition — Milestone Method, the Company determined that such contingent payments do not constitute milestone payments and will not be accounted for under the milestone method of revenue recognition. The events leading to these payments do not meet the definition of a milestone under ASU 2010-17 because the achievement of these events depends on Juno’s performance and selections. Any revenue from these contingent selection payments would be subject to an allocation of arrangement consideration and would be recognized over any remaining period of performance obligation, if any, relating to the collaboration. In connection with each Juno therapy that uses or incorporates the Company’s small molecule modulators, Juno has agreed to pay the Company non-refundable, non-creditable milestone payments totaling up to approximately $51.0 million in the aggregate per therapy upon the achievement of various clinical, regulatory and commercial milestones. Additionally, in connection with the third Juno therapy and the fifth Juno therapy that uses or incorporates the Company’s small molecule modulators, Juno has agreed to pay the Company additional non-refundable, non-creditable bonus milestone payments totaling up to approximately $116.0 million and $137.5 million, respectively, in the aggregate, per therapy upon the achievement of various clinical, regulatory, and commercial milestones. In accordance with ASU 2010-17, the Company determined that these contingent payments meet the definition of a milestone under ASU 2010-17, and that the milestones are substantive given that the milestones are commensurate with the Company’s performance, relate solely to the Company’s past performance, and are reasonable relative to other deliverables and payments under the Agreement. Accordingly, the milestones under the Agreement will be accounted for as revenue on the achievement date, if any. Beginning on the date of the first commercial sale (in each country) for each Juno therapy that uses or incorporates the Company’s small molecule modulators, and continuing until the later of: i) the expiration of the last valid patent claim, ii) ten years after such first commercial sale, or iii) the expiration of all data and other regulatory exclusivity periods afforded each therapy, Juno has agreed to pay the Company royalties in the low single-digits on net sales of each Juno therapy that uses or incorporates the Company’s small molecule modulators. The Agreement will end on the date that no further payments are due under the Agreement.

Short-term Investments

12 Months Ended

Dec. 31, 2017

Short Term Investments [Abstract]

Short-term Investments

3. Short-term Investments

The Company invests portions of excess cash in United States treasuries with maturities ranging from six to twelve months from the purchase date. These debt securities are classified as short-term investments in the accompanying consolidated balance sheets and are accounted for as available-for-sale securities.

The following table summarizes the Company’s short-term investments accounted for as available-for-sale securities as of December 31, 2017 and 2016 (in thousands):

	Maturity (in years)	Amortized Cost		Unrealized Losses			Unrealized Gains		Estimated Fair Value
December 31, 2017
U.S. Treasury debt securities	1 or less		12,000		(3	)		—		11,997
Total		$	12,000	$	(3	)	$	—	$	11,997

December 31, 2016
U.S. Treasury debt securities	1 or less		3,504		(1	)		—		3,503
Total		$	3,504	$	(1	)	$	—	$	3,503

The Company reviewed its investment holdings as of December 31, 2017 and determined that the unrealized losses were not other-than-temporary unrealized losses because the Company does not intend to sell the underlying securities prior to maturity and it is not more likely than not that the Company will be required to sell these securities before the recovery of their amortized cost basis. During the years ended December 31, 2017 and 2016, the Company did not recognize any impairment or gains or losses on sales of available-for-sale securities.

Property and Equipment

12 Months Ended

Dec. 31, 2017

Property Plant And Equipment [Abstract]

Property and Equipment

4. Property and Equipment

Property and equipment consist of the following (in thousands):

	December 31,
	2017			2016
Furniture and fixtures	$	508		$	324
Computer and office equipment		527			318
Software		103			103
Leasehold improvements—building		180			180
Scientific equipment		6,371			4,858
Property and equipment, gross		7,689			5,783
Less accumulated depreciation and amortization		(5,139	)		(4,204	)
Property and equipment, net	$	2,550		$	1,579

Depreciation expense related to property and equipment was $1.0 million, $0.9 million, and $0.7 million, for the years ended December 31, 2017, 2016, and 2015, respectively. No material gains or losses on the disposal of property and equipment have been recorded for the years ended December 31, 2017, 2016, and 2015.

Accrued Expenses, Long-Term Debt, Commitments and Contingencies

12 Months Ended

Dec. 31, 2017

Accrued Expenses Long Term Debt Commitments And Contingencies Disclosure [Abstract]

Accrued Expenses, Long-Term Debt, Commitments and Contingencies

5. Accrued Expenses, Long-Term Debt, Commitments and Contingencies

Accrued Expenses

Current accrued expenses consist of the following (in thousands):

	December 31, 2017		December 31, 2016
Accrued payroll and other employee benefits	$	1,761	$	1,505
Accrued clinical trial related costs		3,323		1,043
Accrued other		2,170		1,409
Accrued expenses	$	7,254	$	3,957

Long-term accrued expenses consist primarily of accruals for the final payment fees associated with our long-term debt.

Long-Term Debt

Long-term debt and unamortized discount balances are as follows (in thousands):

	December 31, 2017			December 31, 2016
Long-term debt	$	15,000		$	10,765
Less debt issuance costs and discount, net of current portion		(192	)		(7	)
Long-term debt, net of long-term portion of debt issuance costs and discount		14,808			10,758
Less current portion of long-term debt		—			(8,257	)
Long-term debt, net	$	14,808		$	2,501
Current portion of long-term debt	$	—		$	8,257
Less current portion of debt issuance costs and discount		—			(70	)
Current portion of long-term debt, net	$	—		$	8,187

SVB Loan Amendment

On July 14, 2017 (the First Amendment Effective Date), the Company entered into the First Amendment (the SVB Loan Amendment) to the Amended and Restated Loan and Security Agreement (the Restated LSA) between the Company and Silicon Valley Bank (SVB) dated July 30, 2014. The SVB Loan Amendment amends the Restated LSA.

Pursuant to the SVB Loan Amendment, SVB extended an additional term loan to the Company on July 14, 2017 in the principal amount of $15.0 million (2017 Term Loan), a portion of which was applied to repay in full the Company’s existing outstanding debt with SVB under the Restated LSA, which included outstanding principal, accrued interest, and final payment fees. Following such repayment in full of the Company’s existing outstanding debt with SVB under the Restated LSA, cash proceeds to the Company from the remaining portion of the 2017 Term Loan were $7.5 million. The net proceeds of the 2017 Term Loan are expected to be used for working capital purposes, including the advancement of the Company’s clinical and research programs.

The 2017 Term Loan matures on January 1, 2022 (the Term Loan Maturity Date) and bears interest at a floating per annum rate equal to the greater of (i) 3.50% above the Prime Rate (as defined in the SVB Loan Amendment) or (ii) 7.25%; provided, however, that in no event shall such interest rate exceed 8.25%. Interest is payable on a monthly basis on the first day of each month. The interest rate as of December 31, 2017 was 8.00%.

From August 1, 2017 through January 1, 2019 (the Interest-only Period), the Company is required to make monthly payments of interest only. Thereafter, the Company is required to repay the principal, plus monthly payments of accrued interest, in 36 equal monthly installments based on a 36-month amortization schedule. Notwithstanding the foregoing, subject to the achievement of a product development milestone by the Company before the expiration of the above-described Interest-only Period, at the Company’s election (i) the Interest-only Period shall be extended from January 1, 2019 through and including to July 1, 2019 and (ii) the Company shall thereafter repay the principal, plus monthly payments of accrued interest, in 30 equal monthly installments based on a 30-month amortization schedule.

The Company’s final payment, due on the Term Loan Maturity Date, shall include all outstanding principal and accrued and unpaid interest under the 2017 Term Loan, plus a 7.5%, or $1.1 million, final payment fee. This final payment fee is accrued as interest expense over the term of the 2017 Term Loan and recorded in accrued expenses.

In connection with the SVB Loan Amendment, the Company issued to SVB on the First Amendment Effective Date a fully exercisable warrant (the 2017 Warrant) to purchase up to an aggregate of 91,463 shares of the Company’s common stock, subject to adjustment, at an exercise price equal to $3.28 per share. The 2017 Warrant expires in July 2024. The aggregate fair value of the 2017 Warrant was determined to be $0.2 million using the Black-Scholes option pricing model and was recorded as a debt discount on the 2017 Term Loan. This debt discount is amortized to interest expense over the term of the 2017 Term Loan using the effective interest method. The Company determined the effective interest rate of the 2017 Term Loan to be 10.2% as of the First Amendment Effective Date.

The Company determined the repayment of the Restated LSA and issuance of the 2017 Term Loan was a debt extinguishment, and accounted for the 2017 Term Loan at fair value as of the First Amendment Effective Date, accordingly. During the year ended December 31, 2017, the Company recorded a loss on debt extinguishment of $0.1 million, which was primarily related to the unaccrued amount of the final payment fee under the Restated LSA that was paid in connection with the 2017 Term Loan.

The Company is required under its loan agreement with SVB to maintain its deposit and securities accounts with SVB and to comply with various operating covenants and default clauses. A breach of any of these covenants or clauses could result in a default under the agreement, which would cause all of the outstanding indebtedness under the facility to become immediately due and payable. The Company is in compliance with all such covenants and clauses.

For the year ended December 31, 2017, the Company recorded $0.8 million in aggregate interest expense related to the 2017 Term Loan.

Restated LSA

On July 30, 2014, the Company entered into the Restated LSA with SVB, collateralized by substantially all of the Company’s assets, excluding certain intellectual property. Pursuant to the Restated LSA, SVB agreed to make loans to the Company in an aggregate principal amount of up to $20.0 million, comprised of (i) a $10.0 million term loan, funded at the closing date (Term A Loan) and (ii) subject to the achievement of a specified clinical milestone, additional term loans totaling up to $10.0 million in the aggregate, which were available until December 31, 2014 (Term B Loan). On December 24, 2014, the Company elected to draw on the full $10.0 million under a Term B Loan.

The Term A Loan and the Term B Loan were scheduled to mature on January 1, 2018 and June 1, 2018, respectively.

The Company was required to make a final payment fee of 7.5%, equaling $0.8 million, of the funded amount for each of the Term A Loan and Term B Loan on the respective maturity dates. These final payment fees were accrued as interest expense over the terms of the loans and recorded in accrued expenses.

In connection with the funding of the Term B Loan, the Company issued SVB and one of its affiliates fully-exercisable warrants to purchase an aggregate of 98,039 shares of the Company’s common stock (the 2014 Warrants) at an exercise price of $4.08 per share. The 2014 Warrants expire in December 2021 and remain outstanding as of December 31, 2017.

For the years ended December 2017, 2016, and 2015, the Company recorded $0.5 million, $1.6 million and $2.2 million, respectively, in aggregate interest expense related to the Term A and Term B Loans.

Warrants to purchase 36,074 shares of the Company’s common stock at a weighted average exercise price of $7.21 per share issued in connection with a prior debt agreement between the Company and SVB in 2009 remain outstanding as of December 31, 2017, with 5,305 and 30,769 of such warrants having expiration dates in January 2019 and August 2021, respectively.

Facility Lease

The Company leases certain office and laboratory space, comprising approximately 48,000 square feet, under a non-cancelable operating lease through June 2023. The lease is subject to additional charges for common area maintenance and other costs. In connection with the lease, the Company entered into a cash-collateralized irrevocable standby letter of credit in the amount of $0.1 million. As of December 31, 2017, future minimum payments under the operating lease are $12.9 million.

In January 2015, the Company entered into a sublease for additional laboratory space. The sublease was accounted for as an operating lease and expired in September 2017. No future payments remain under the sublease.

Aggregate contractual rent expense was $2.3 million, $1.3 million, and $1.2 million for the years ended December 31, 2017, 2016, and 2015, respectively.

License Agreements

The Company has entered into exclusive license agreements with certain academic institutions and universities pursuant to which the Company acquired certain intellectual property. Pursuant to each agreement, as consideration for an exclusive license to the intellectual property, the Company paid a license fee, reimbursed the institution for historical patent costs and, in certain instances, issued the institution shares of restricted common stock. Additionally, under each agreement, the institution is generally eligible to receive future consideration including, but not limited to, annual maintenance fees, royalties, milestone payments and sublicensing fees. Each of the license agreements is generally cancelable by the Company, given appropriate prior written notice. Minimum annual payments to maintain these cancelable licenses total an aggregate of $0.2 million.

Commitments

Future minimum payments under the long-term debt and the non-cancelable operating leases as of December 31, 2017 are as follows (in thousands):

	Long–Term Debt			Operating Leases		Total
Years Ending December 31,
2018		1,215			2,116		3,331
2019		5,645			2,278		7,923
2020		5,661			2,346		8,007
2021		5,253			2,417		7,670
2022		1,545			2,489		4,034
2023		—			1,282		1,282
Total	$	19,319		$	12,928	$	32,247
Less interest		(3,194	)
Less additional payments due upon maturity		(1,125	)
Less unamortized debt discount and debt issuance costs		(192	)
Long–term debt, net of current portion	$	14,808

The Company’s long-term debt bears interest at a floating per annum rate equal to the greater of (i) 3.50% above the Prime Rate (as defined in the SVB Loan Amendment) or (ii) 7.25%; provided, however, that in no event shall such interest rate exceed 8.25%. The amounts in the table above assume payment at the current interest rate, which is subject to change. The amounts in the above table also assume the Interest-only Period to be from August 1, 2017 through January 1, 2019.

Convertible Preferred Stock and Stockholders' Equity

12 Months Ended

Dec. 31, 2017

Convertible Preferred Stock And Stockholders Deficit Disclosure [Abstract]

Convertible Preferred Stock and Stockholders’ Equity

6. Convertible Preferred Stock and Stockholders’ Equity

Convertible Preferred Stock

In November 2016, the Company completed a private placement of stock in which investors, certain of which are affiliated with the directors and officers of the Company, purchased convertible preferred stock and common stock of the Company (the November 2016 Placement). The Company issued 2,819,549 shares of non-voting Class A Convertible Preferred Stock (the Class A Preferred) at $13.30 per share, each of which is convertible into five shares of common stock upon certain conditions defined in the Certificate of Designation of Preferences, Rights and Limitations of the Class A Preferred filed with the Delaware Secretary of State on November 22, 2016 (the CoD). The Class A Preferred were purchased exclusively by entities affiliated with Redmile Group, LLC (collectively, Redmile). The terms of the CoD prohibited Redmile from converting the Class A Preferred into shares of the Company’s common stock if, as a result of conversion, Redmile, together with its affiliates, would own more than 9.99% of the Company’s common stock then issued and outstanding (the Redmile Percentage Limitation), which percentage could change at Redmile’s election upon 61 days’ notice to the Company to i) any other number less than or equal to 19.99% or (ii) subject to approval of the Company’s stockholders to the extent required in accordance with the NASDAQ Global Market rules, any number in excess of 19.99%. On May 2, 2017, the Company’s stockholders approved the issuance of up to an aggregate of 14,097,745 shares of common stock upon the conversion of the outstanding shares of Class A Preferred. As a result, Redmile has the right to increase the Redmile Percentage Limitation to any percentage in excess of 19.99% at its election. The Company also issued 7,236,837 shares of common stock at $2.66 per share as part of the November 2016 Placement. Gross proceeds from the November 2016 Placement were $56.7 million, and after giving effect to costs related to placement, net proceeds were $54.9 million.

The rights of the Class A Preferred issued in November 2016 are set forth in the CoD. The Class A Preferred are non-voting shares and have a stated par value of $0.001 per share and are convertible into five shares of the Company’s common stock at a conversion price of $2.66 per share, which was the fair value of the Company’s common stock on the date of issuance. Holders of the Class A Preferred have the same dividend rights as holders of the Company’s common stock. Additionally, the liquidation preferences of the Class A Preferred are pari passu among holders of the Company’s common stock and holders of the Class A Preferred, pro rata based on the number of shares held by each such holder (treated for this purpose as if the Class A Preferred had been converted to common stock).

The Company evaluated the Class A Preferred for liability or equity classification under ASC 480, Distinguishing Liabilities from Equity, and determined that equity treatment was appropriate because the Class A Preferred did not meet the definition of the liability instruments defined thereunder for convertible instruments. Specifically, the Class A Preferred are not mandatorily redeemable and do not embody an obligation to buy back the shares outside of the Company’s control in a manner that could require the transfer of assets. Additionally, the Company determined that the Class A Preferred would be recorded as permanent equity, not temporary equity, based on the guidance of ASC 480 given that they are not redeemable for cash or other assets (i) on a fixed or determinable date, (ii) at the option of the holder, and (iii) upon the occurrence of an event that is not solely within control of the Company.

The Company has also evaluated the Class A Preferred in accordance with the provisions of ASC 815, Derivatives and Hedging, including the consideration of embedded derivatives requiring bifurcation from the equity host. Based on this assessment, the Company determined that the conversion option is clearly and closely related to the equity host, and thus, bifurcation is not required.

The issuance of convertible preferred stock could generate a beneficial conversion feature (BCF), which arises when a debt or equity security is issued with an embedded conversion option that is beneficial to the investor (or in-the-money) at inception because the conversion option has an effective strike price that is less than the market price of the underlying stock on the commitment date. The Class A Preferred have an effective conversion price of $2.66 per common share, which was equal to the market price of the Company’s stock on the commitment date. Therefore, no BCF is present.

The Company also entered into a registration rights agreement (the Registration Rights Agreement) with certain of the purchasers in the November 2016 Placement, excluding those purchasers affiliated with the Company’s directors and officers, requiring the Company to register for the resale of the relevant shares. The Company registered all of the relevant shares issued in the November 2016 Placement for resale on a Form S-3 filed with the SEC, as required under the Registration Rights Agreement, and the registration statement was declared effective in January 2017.

Description of Securities

Dividends

As of December 31, 2017, the Board of Directors of the Company has not declared any dividends.

2007 Equity Incentive Plan, 2013 Stock Option and Incentive Plan, and Inducement Equity Plan

The Company adopted an Equity Incentive Plan in 2007 (the 2007 Plan) under which, as amended in August 2013, 2,423,072 shares of common stock were reserved for issuance to employees, nonemployee directors and consultants of the Company. The 2007 Plan provides for the grant of incentive stock options, non-statutory stock options, rights to purchase restricted stock, stock appreciation rights, dividend equivalents, stock payments, and restricted stock units to eligible recipients. In connection with the issuance of restricted common stock, the Company maintains a repurchase right where shares of restricted common stock are released from such repurchase right over a period of time of continued service by the recipient. Effective upon the completion of the Company’s IPO, the board of directors determined not to grant any further awards under the 2007 Plan.

On August 28, 2013, the Company’s board of directors and stockholders approved and adopted the 2013 Stock Option and Incentive Plan (the 2013 Plan and collectively with the 2007 Plan, the Plans). The 2013 Plan became effective immediately prior to the Company’s IPO. Under the 2013 Plan, the Company may grant stock options, stock appreciation rights, restricted stock, restricted stock units and other awards to individuals who are then employees, officers, directors or consultants of the Company or its subsidiaries. A total of 1,020,000 shares of common stock were initially reserved for issuance under the 2013 Plan, and in May 2017, stockholders approved an additional 2,500,000 shares of common stock for issuance under the 2013 Plan. The shares issuable pursuant to awards granted under the 2013 Plan will be authorized, but unissued shares. The shares of common stock underlying any awards from the 2013 Plan and the 2007 Plan that are forfeited, cancelled, held back upon exercise or settlement of an award to satisfy the exercise price or tax withholding, reacquired by us prior to vesting, satisfied without any issuance of common stock, expire or are otherwise terminated (other than by exercise) under the 2013 Plan and the 2007 Plan will be added back to the shares of common stock available for issuance under the 2013 Plan.

In addition, the number of shares of stock available for issuance under the 2013 Plan will be automatically increased each January 1 by 4% of the outstanding number of shares of the Company’s common stock on the immediately preceding December 31 or such lesser number as determined by the compensation committee of the Company’s board of directors.

Recipients of stock options under the Plans shall be eligible to purchase shares of the Company’s common stock at an exercise price equal to no less than the estimated fair value of such stock on the date of grant. Under the Plans, stock options generally vest 25% on the first anniversary of the original vesting date, with the balance vesting monthly over the remaining three years, or vest monthly over four years, unless they contain specific performance and/or market-based vesting provisions. The maximum term of stock options granted under the Plans is ten years.

Restricted stock units under the 2013 Plan generally vest 50% on the second anniversary of the grant date, and 50% on the fourth anniversary of the grant date.

Inducement Plan

On May 10, 2016, the Company’s board of directors approved the Fate Therapeutics, Inc. Inducement Equity Plan (the Inducement Plan), the purpose of which is to enable the Company to grant equity awards to induce highly-qualified prospective officers and employees who are not employed by the Company to accept employment with the Company. Under the Inducement Plan, the Company may grant non-qualified stock options and restricted stock units. A total of 500,000 shares of common stock were initially reserved for issuance under the Inducement Plan, and in January 2018 an additional 400,000 shares of common stock were reserved for issuance under the Inducement Plan. The shares of common stock underlying any awards from the Inducement Plan that are forfeited, cancelled, held back upon exercise or settlement of an award to satisfy the exercise price or tax withholding, reacquired by us prior to vesting, satisfied without any issuance of common stock, expire or are otherwise terminated (other than by exercise) under the Inducement Plan will be added back to the shares of common stock available for issuance under the Inducement Plan.

Employee Stock Purchase Plan

On September 13, 2013, the Company’s board of directors approved and adopted the 2013 Employee Stock Purchase Plan (the ESPP). A total of 729,000 shares of common stock were initially reserved for issuance under the ESPP. In addition, the number of shares of stock available for issuance under the ESPP will be automatically increased each January 1, beginning on January 1, 2015, by the lesser of (i) 2% of the outstanding number of shares of the Company’s common stock on the immediately preceding December 31, (ii) 450,000 shares, or (iii) such lesser number as determined by the compensation committee of the Company’s board of directors.

No purchases have been made to date under the ESPP.

Stock Options and Restricted Stock Unit Awards

Stock Options. The following table summarizes stock option activity and related information under all equity plans for the years ended December 31, 2017, 2016 and 2015:

	Options			Weighted Average Exercise Price Per Share		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value (in 000s)
Outstanding at December 31, 2014		2,425,969		$	3.83		8.06	$	4,839
Granted		1,345,744			5.23
Exercised		(227,215	)		1.85
Cancelled		(957,024	)		4.20
Outstanding at December 31, 2015		2,587,474		$	4.59		6.92	$	1,486
Granted		2,245,240			2.62
Exercised		(136,368	)		1.44
Cancelled		(785,996	)		3.58
Outstanding at December 31, 2016		3,910,350		$	3.77		8.28	$	682
Granted		2,522,920			3.09
Exercised		(83,220	)		2.79
Cancelled		(892,007	)		3.50
Outstanding at December 31, 2017		5,458,043		$	3.52		7.99	$	14,754
Options vested and expected to vest at December 31, 2017		5,300,033		$	3.53		8.02	$	14,267
Options exercisable at December 31, 2017		2,479,556		$	3.92		7.16	$	5,972

As of December 31, 2017, 2016 and 2015, the outstanding options included 73,600, 73,600, and 14,769, respectively, of performance-based options for which the achievement of the performance-based vesting provisions was determined not to be probable. The aggregate grant date fair value of these options at December 31, 2017, 2016 and 2015, was $0.1 million, $0.1 million and $0.1 million, respectively.

For the years ended December 31, 2017, 2016, and 2015, the Company granted its employees and directors 2.5 million, 2.2 million and 1.2 million stock options, respectively, at a weighted-average grant date fair value per share equal to $2.29, $1.80 and $3.54, respectively.

As of December 31, 2017, 2016 and 2015, the unrecognized compensation cost related to outstanding options (excluding those with unachieved performance- based conditions) was $5.8 million, $4.9 million and $3.8 million, respectively, which was expected to be recognized as expense over approximately 2.6 years, 2.6 years and 2.8 years, respectively.

The total intrinsic value, which is the amount by which the exercise price was exceeded by the price of the Company’s common stock on the date of exercise, of stock options exercised during the year ended December 31, 2017 was $0.1 million. Total cash received upon the exercise of stock options was $0.2 million for the year ended December 31, 2017.

Restricted Stock Units. The following table summarizes Restricted Stock Unit activity and related information under all equity plans for the years ended December 31, 2017, 2016 and 2015:

	Number of Restricted Stock Units			Weighted Average Grant Date Fair Value Per Share		Weighted Average Remaining Vesting Period		Aggregate Intrinsic Value (in 000s)
Outstanding at December 31, 2014		—		$	—		—	$	—
Granted		525,250			4.89
Vested		—			—
Cancelled		—			—
Outstanding at December 31, 2015		525,250		$	4.89		3.80	$	1,770
Granted		—			—
Vested		—			—
Cancelled		—			—
Outstanding at December 31, 2016		525,250		$	4.89		2.80	$	1,318
Granted		—			—
Vested		(225,125	)		4.89
Cancelled		(87,500	)		4.89
Outstanding at December 31, 2017		212,625		$	4.89		1.80	$	1,299
Restricted Stock Units expected to vest at December 31, 2017		212,625		$	4.89		1.80	$	1,299

As of December 31, 2017, 2016, and 2015, the unrecognized compensation cost related to outstanding restricted stock units was $0.9 million, $1.8 million, and $2.4 million respectively, which was expected to be recognized as expense over approximately 1.8 years, 2.8 years, and 3.8 years respectively.

Stock-Based Compensation Expense

The allocation of stock-based compensation for all stock awards is as follows (in thousands):

	Years Ended December 31,
	2017		2016		2015
Research and development	$	2,095	$	1,802	$	1,241
General and administrative		1,511		1,382		1,159
Total stock-based compensation expense	$	3,606	$	3,184	$	2,400

Employee Stock Option Grants. The weighted-average assumptions used in the Black-Scholes option pricing model to determine the fair value of the employee stock option grants were as follows:

	Years Ended December 31,
	2017			2016			2015
Risk–free interest rate		2.0	%		1.6	%		1.6	%
Expected volatility		90	%		80	%		81	%
Expected term (in years)		6.0			6.0			6.0
Expected dividend yield		0.0	%		0.0	%		0.0	%

Risk-free interest rate. The Company bases the risk-free interest rate assumption on observed interest rates appropriate for the expected term of the stock option grants.

Expected dividend yield. The Company bases the expected dividend yield assumption on the fact that it has never paid cash dividends and has no present intention to pay cash dividends.

Expected volatility. Due to the Company’s limited operating history and lack of company-specific historical or implied volatility, the expected volatility assumption is based on historical volatilities of a peer group of similar companies whose share prices are publicly available. The peer group was developed based on companies in the biotechnology industry.

Expected term. The expected term represents the period of time that options are expected to be outstanding. As the Company does not have sufficient historical exercise behavior, it determines the expected life assumption using the simplified method, which is an average of the contractual term of the option and its vesting period.

Non-Employee Stock Option Grants. The weighted-average assumptions used in the Black-Scholes option pricing model to determine the fair value of the non-employee stock option grants were as follows:

	Years Ended December 31,
	2017			2016			2015
Risk–free interest rate		2.1	%		1.5	%		0.8	%
Expected volatility		87	%		83	%		72	%
Expected term (in years)		8.5			6.4			2.7
Expected dividend yield		0.0	%		0.0	%		0.0	%

Warrants to Purchase Common Stock in Connection with Debt Issuance

As a result of the financing of the Loan Amendment on July 14, 2017, the Company issued SVB fully-exercisable warrants to purchase an aggregate of 91,463 shares of the Company’s common stock at an exercise price of $3.28 per share. The warrants expire in July 2024. See Note 5 of the Notes to the Consolidated Financial Statements for additional information on the debt issuance.

The fair value of the warrants was determined to be $0.2 million, which was recorded to additional paid-in capital as a debt discount. The weighted- average assumptions used in the Black-Scholes option pricing model to determine the fair value of the warrants issued were as follows:

	As of July 14, 2017
Risk–free interest rate		2.1	%
Expected volatility		88	%
Expected term (in years)		7.0
Expected dividend yield		0.0	%

Common Stock Reserved for Future Issuance

Common stock reserved for future issuance is as follows:

	December 31,
	2017		2016
Common stock warrants		225,756		134,113
Convertible preferred stock (if converted)		14,097,745		14,097,745
Common stock options		5,458,043		3,910,350
Restricted stock units		212,625		525,250
Awards available under the 2013 Plan		3,572,112		760,065
Awards available under the Inducement Plan		100,000		300,000
Employee stock purchase plan		729,000		729,000
		24,395,281		20,456,523

Income Taxes

12 Months Ended

Dec. 31, 2017

Income Tax Disclosure [Abstract]

Income Taxes

7. Income Taxes

The following is a reconciliation of the Company’s expected federal income tax provision (benefit) to the actual income tax provision (in thousands):

	Years Ended December 31,
	2017			2016			2015
Tax computed at federal statutory rate	$	(14,603	)	$	(11,377	)	$	(10,198	)
State tax, net of federal tax benefit		(1,315	)		(2,089	)		2,226
Permanent differences		795			292			43
Stock compensation		539			968			213
R&D tax credits		(2,934	)		(971	)		(594	)
Reserve for uncertain tax positions		1,326			2,076			1,733
Tax attribute limitation		—			54			2,727
Tax Cuts and Jobs Act		25,280			—			—
Other		46			(74	)		(37	)
Valuation allowance		(9,134	)		11,121			3,887
Income tax expense	$	—		$	—		$	—

Significant components of the Company’s deferred tax assets are summarized as follows (in thousands):

	As of December 31,
	2017			2016
Deferred tax assets:
Section 59e amortization	$	14,365		$	17,699
Net operating losses		27,699			35,049
R&D tax credits		5,421			2,175
Depreciation and amortization		581			1,020
Deferred revenue		594			1,677
Stock compensation		876			1,222
Other		96			(76	)
Deferred tax assets		49,632			58,766
Valuation allowance		(49,632	)		(58,766	)
Net deferred tax assets	$	—		$	—

A valuation allowance of $49.6 million and $58.8 million at December 31, 2017 and 2016, respectively, has been established to offset the deferred tax assets, as realization of such assets is uncertain.

At December 31, 2017, the Company had federal and California net operating loss (NOL) carryforwards of $121.2 million and $120.8 million, respectively, which may be available to offset future taxable income. The federal and California NOL carryforwards begin to expire in 2027 and 2028, respectively, unless previously utilized. At December 31, 2017, the Company had federal and California research and development (R&D) credit carryforwards of $5.7 million and $4.2 million, respectively. The federal R&D tax credit carryforwards will begin to expire in 2035 unless previously utilized. The California R&D credit carryforwards will carry forward indefinitely.

Under Sections 382 and 383 of the Internal Revenue Code of 1986, as amended, (the Code), substantial changes in the Company’s ownership may limit the amount of net operating loss and research and development credit carryforwards that could be used annually in the future to offset taxable income. The tax benefits related to future utilization of federal and state net operating loss carryforwards, credit carryforwards, and other deferred tax assets may be limited or lost if cumulative changes in ownership exceeds 50% within any three-year period. The Company completed a study to assess whether an ownership change, as defined by Section 382 of the Code, had occurred from the Company’s formation through December 31, 2015. Based upon this study, the Company determined that several ownership changes had occurred. Accordingly, the Company reduced its deferred tax assets related to the federal NOL carryforwards and the federal R&D credit carryforwards that are anticipated to expire unused as a result of these ownership changes. These tax attributes were excluded from deferred tax assets with a corresponding reduction of the valuation allowance with no net effect on income tax expense or the effective tax rate. The Company updated the study through December 31, 2017 and concluded there were no ownership changes during 2016 and 2017. Future ownership changes may further limit the Company’s ability to utilize its remaining tax attributes.

The Company files income tax returns in the United States and California, and has historically filed income tax returns in Canada. The Company currently has no years under examination by any jurisdiction; however, the Company is subject to income tax examination by federal, Californian and Canadian tax authorities for years beginning in 2014, 2013, and 2013, respectively. However, to the extent allowed by law, the taxing authorities may have the right to examine prior periods where NOLs and tax credits were generated and carried forward, and make adjustments up to the amount of the carryforwards.

The change in the Company’s unrecognized tax benefits is summarized as follows (in thousands):

Balance at December 31, 2014	$	1,820
Increase related to current year tax positions		374
Increase related to prior year tax positions		2,624
Decrease related to prior year tax positions		(949	)
Balance at December 31, 2015	$	3,869
Increase related to current year tax positions		2,268
Increase related to prior year tax positions		1,625
Decrease related to prior year tax positions		(32	)
Balance at December 31, 2016	$	7,730
Increase related to current year tax positions		4,077
Increase related to prior year tax positions		6
Decrease related to prior year tax positions		(13	)
Balance at December 31, 2017	$	11,800

The Company does not anticipate that the amount of unrecognized tax benefits as of December 31, 2017 will significantly change within the next twelve months. Due to the valuation allowance recorded against the Company’s deferred tax assets, none of the total unrecognized tax benefits as of December 31, 2017 would reduce the effective tax rate if recognized. The Company has not recognized interest or penalties in its consolidated statements of operations and comprehensive loss since inception.

The Company adopted ASU No. 2016-09 on January 1, 2017. ASU 2016-09 simplifies how several aspects of share-based payments are accounted for and presented in the financial statements. The Company had excess tax benefits for which a benefit could not be previously recognized of $0.1 million. Due to the full valuation allowance on the deferred tax assets, there was no impact to the Company’s consolidated financial statements as a result of the adoption.

The Tax Cuts and Jobs Act (the Act) was enacted on December 22, 2017. The Act reduces the US federal corporate tax rate from 34% to 21%. The reduction in the rate caused the Company to remeasure its deferred tax assets and liabilities at December 31, 2017. This resulted in a provisional reduction of the Company’s deferred tax balance by $25.3 million, offset by an equal change in the valuation allowance.

On December 22, 2017, SEC Staff Accounting Bulletin No. 118 (SAB 118) was issued to address the application of US GAAP in situations when a registrant does not have the necessary information available, prepared, or analyzed (including computations) in reasonable detail to complete the accounting for certain income tax effect of the Act. In accordance with SAB 118, the Company has determined that $25.3 million of the deferred tax expense recorded in connection with remeasuring certain deferred tax assets and liabilities is a provisional amount and reasonable estimate at December 31, 2017. Additional work is necessary to do a more detailed analysis. Any subsequent adjustment to these amounts will be recorded to current tax expense in the period the analysis is complete.

Selected Quarterly Financial Data

12 Months Ended

Dec. 31, 2017

Quarterly Financial Information Disclosure [Abstract]

Selected Quarterly Financial Data

9. Selected Quarterly Financial Data

(in thousands, except per share data) (unaudited)	First Quarter			Second Quarter			Third Quarter			Fourth Quarter
2017
Revenues	$	1,027		$	1,026		$	1,026		$	1,027
Total operating expenses		10,998			10,596			11,366			13,271
Net loss		(10,126	)		(9,645	)		(10,684	)		(12,497	)
Basic and diluted net loss per common share	$	(0.24	)	$	(0.23	)	$	(0.26	)	$	(0.29	)
2016
Revenues	$	1,322		$	1,027		$	1,026		$	1,027
Total operating expenses		9,238			9,031			9,415			8,681
Net loss		(8,377	)		(8,408	)		(8,737	)		(7,940	)
Basic and diluted net loss per common share	$	(0.29	)	$	(0.29	)	$	(0.27	)	$	(0.21	)

Organization and Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2017

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Public Equity Offerings

Private Placement of Common Stock and Convertible Preferred Stock

Private Placements of Common Stock and Convertible Preferred Stock

Use of Estimates

Principles of Consolidation

Segment Reporting

Fair Value of Financial Instruments

Level 1: Observable inputs such as quoted prices in active markets;

Level 2: Inputs, other than the quoted prices in active markets, that are observable either directly or indirectly; and

Level 3: Unobservable inputs in which there is little or no market data, which require the reporting entity to develop its own assumptions.

			Fair Value Measurements at Reporting Date Using
	Total		Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
As of December 31, 2017:
Cash equivalents	$	88,952	$	88,952	$	—	$	—
U.S. Treasury debt securities		11,997		11,997		—		—
Total assets	$	100,949	$	100,949	$	—	$	—

As of December 31, 2016:
Cash equivalents	$	88,609	$	88,609	$	—	$	—
U.S. Treasury debt securities		3,503		3,503		—		—
Total assets	$	92,112	$	92,112	$	—	$	—

None of the Company’s non-financial assets or liabilities is recorded at fair value on a non-recurring basis. No transfers between levels have occurred during the periods presented.

As of December 31, 2017 and 2016, the Company had no material liabilities measured at fair value on a recurring basis.

Cash, Cash Equivalents and Restricted Cash

	December 31, 2017		December 31, 2016		December 31, 2015
Cash and cash equivalents	$	88,952	$	88,609	$	64,809
Restricted cash		122		122		122
Total cash, cash equivalents, and restricted cash shown in the statement of cash flows	$	89,074	$	88,731	$	64,931

Amounts included in restricted cash represent security deposits required to secure the Company’s credit card limit and its facilities lease.

Short-Term Investments

Concentration of Credit Risk

Property and Equipment

Impairment of Long-Lived Assets

Deferred Rent

Revenue Recognition

Revenue from government grants is recorded when reimbursable expenses are incurred under the grant in accordance with the terms of the grant award.

Research and Development Costs

All research and development costs are expensed as incurred.

Patent Costs

Costs related to filing and pursuing patent applications are recorded as general and administrative expense and expensed as incurred since recoverability of such expenditures is uncertain.

Stock-Based Compensation

Convertible Preferred Stock

Income Taxes

Comprehensive Loss

Net Loss Per Common Share

Potentially dilutive securities not included in the calculation of diluted net loss per common share because to do so would be anti-dilutive are as follows (in common stock equivalent shares):

	As of December 31,
	2017		2016		2015
Warrants for common stock		225,756		134,113		134,113
Common stock options		5,458,043		3,910,350		2,587,474
Restricted stock units		212,625		525,250		525,250
Series A convertible preferred stock (if converted)		14,097,745		14,097,745		—
		19,994,169		18,667,458		3,246,837

Recent Accounting Pronouncements

Going Concern Assessment

Organization and Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2017

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Schedule of Assets Measured at Fair Value on Recurring Basis

The following table presents the Company’s assets which were measured at fair value on a recurring basis as of December 31, 2017 and 2016 (in thousands):

			Fair Value Measurements at Reporting Date Using
	Total		Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
As of December 31, 2017:
Cash equivalents	$	88,952	$	88,952	$	—	$	—
U.S. Treasury debt securities		11,997		11,997		—		—
Total assets	$	100,949	$	100,949	$	—	$	—

As of December 31, 2016:
Cash equivalents	$	88,609	$	88,609	$	—	$	—
U.S. Treasury debt securities		3,503		3,503		—		—
Total assets	$	92,112	$	92,112	$	—	$	—

Reconciliation of Cash, Cash Equivalents, and Restricted Cash

	December 31, 2017		December 31, 2016		December 31, 2015
Cash and cash equivalents	$	88,952	$	88,609	$	64,809
Restricted cash		122		122		122
Total cash, cash equivalents, and restricted cash shown in the statement of cash flows	$	89,074	$	88,731	$	64,931

Schedule of Anti-Dilutive Securities not Included in Calculation of Diluted Net Loss Per Common Share

Potentially dilutive securities not included in the calculation of diluted net loss per common share because to do so would be anti-dilutive are as follows (in common stock equivalent shares):

	As of December 31,
	2017		2016		2015
Warrants for common stock		225,756		134,113		134,113
Common stock options		5,458,043		3,910,350		2,587,474
Restricted stock units		212,625		525,250		525,250
Series A convertible preferred stock (if converted)		14,097,745		14,097,745		—
		19,994,169		18,667,458		3,246,837

Short-term Investments (Tables)

12 Months Ended

Dec. 31, 2017

Short Term Investments [Abstract]

Summary of Short-term Investments

The following table summarizes the Company’s short-term investments accounted for as available-for-sale securities as of December 31, 2017 and 2016 (in thousands):

	Maturity (in years)	Amortized Cost		Unrealized Losses			Unrealized Gains		Estimated Fair Value
December 31, 2017
U.S. Treasury debt securities	1 or less		12,000		(3	)		—		11,997
Total		$	12,000	$	(3	)	$	—	$	11,997

December 31, 2016
U.S. Treasury debt securities	1 or less		3,504		(1	)		—		3,503
Total		$	3,504	$	(1	)	$	—	$	3,503

- Definition

Tabular disclosure of available-for-sale securities which includes, but is not limited to, changes in the cost basis and fair value, fair value and gross unrealized gain (loss), fair values by type of security, contractual maturity and classification, amortized cost basis, contracts to acquire securities to be accounted for as available-for-sale, debt maturities, transfers to trading, change in net unrealized holding gain (loss) net of tax, continuous unrealized loss position fair value, aggregate losses qualitative disclosures, other than temporary impairment (OTTI) losses or other disclosures related to available for sale securities.

+ References + Details

Property and Equipment (Tables)

12 Months Ended

Dec. 31, 2017

Property Plant And Equipment [Abstract]

Schedule of property and equipment

Property and equipment consist of the following (in thousands):

	December 31,
	2017			2016
Furniture and fixtures	$	508		$	324
Computer and office equipment		527			318
Software		103			103
Leasehold improvements—building		180			180
Scientific equipment		6,371			4,858
Property and equipment, gross		7,689			5,783
Less accumulated depreciation and amortization		(5,139	)		(4,204	)
Property and equipment, net	$	2,550		$	1,579

Accrued Expenses, Long-Term Debt, Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2017

Accrued Expenses Long Term Debt Commitments And Contingencies Disclosure [Abstract]

Schedule of accrued expenses

Current accrued expenses consist of the following (in thousands):

	December 31, 2017		December 31, 2016
Accrued payroll and other employee benefits	$	1,761	$	1,505
Accrued clinical trial related costs		3,323		1,043
Accrued other		2,170		1,409
Accrued expenses	$	7,254	$	3,957

Schedule of long-term debt and unamortized discount balances

Long-term debt and unamortized discount balances are as follows (in thousands):

	December 31, 2017			December 31, 2016
Long-term debt	$	15,000		$	10,765
Less debt issuance costs and discount, net of current portion		(192	)		(7	)
Long-term debt, net of long-term portion of debt issuance costs and discount		14,808			10,758
Less current portion of long-term debt		—			(8,257	)
Long-term debt, net	$	14,808		$	2,501
Current portion of long-term debt	$	—		$	8,257
Less current portion of debt issuance costs and discount		—			(70	)
Current portion of long-term debt, net	$	—		$	8,187

Schedule of future minimum payments under the long-term debt and the non-cancelable operating leases

Future minimum payments under the long-term debt and the non-cancelable operating leases as of December 31, 2017 are as follows (in thousands):

	Long–Term Debt			Operating Leases		Total
Years Ending December 31,
2018		1,215			2,116		3,331
2019		5,645			2,278		7,923
2020		5,661			2,346		8,007
2021		5,253			2,417		7,670
2022		1,545			2,489		4,034
2023		—			1,282		1,282
Total	$	19,319		$	12,928	$	32,247
Less interest		(3,194	)
Less additional payments due upon maturity		(1,125	)
Less unamortized debt discount and debt issuance costs		(192	)
Long–term debt, net of current portion	$	14,808

- Definition

Tabular disclosure of long-debt instruments or arrangements, including identification, terms, features, collateral requirements and other information necessary to a fair presentation. These are debt arrangements that originally required repayment more than twelve months after issuance or greater than the normal operating cycle of the entity, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22)
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
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-URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

Reference 4: 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),(f))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 5: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
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Reference 6: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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-Section 50
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Reference 7: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 505
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Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
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Reference 9: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
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Reference 10: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
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-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21521-112644

+ Details

Convertible Preferred Stock and Stockholders' Equity (Tables)

12 Months Ended

Dec. 31, 2017

Stockholders' Equity

Summary of stock option activity and related information under all equity plans

The following table summarizes stock option activity and related information under all equity plans for the years ended December 31, 2017, 2016 and 2015:

	Options			Weighted Average Exercise Price Per Share		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value (in 000s)
Outstanding at December 31, 2014		2,425,969		$	3.83		8.06	$	4,839
Granted		1,345,744			5.23
Exercised		(227,215	)		1.85
Cancelled		(957,024	)		4.20
Outstanding at December 31, 2015		2,587,474		$	4.59		6.92	$	1,486
Granted		2,245,240			2.62
Exercised		(136,368	)		1.44
Cancelled		(785,996	)		3.58
Outstanding at December 31, 2016		3,910,350		$	3.77		8.28	$	682
Granted		2,522,920			3.09
Exercised		(83,220	)		2.79
Cancelled		(892,007	)		3.50
Outstanding at December 31, 2017		5,458,043		$	3.52		7.99	$	14,754
Options vested and expected to vest at December 31, 2017		5,300,033		$	3.53		8.02	$	14,267
Options exercisable at December 31, 2017		2,479,556		$	3.92		7.16	$	5,972

Summary of restricted stock unit activity and related information under all equity plans

The following table summarizes Restricted Stock Unit activity and related information under all equity plans for the years ended December 31, 2017, 2016 and 2015:

	Number of Restricted Stock Units			Weighted Average Grant Date Fair Value Per Share		Weighted Average Remaining Vesting Period		Aggregate Intrinsic Value (in 000s)
Outstanding at December 31, 2014		—		$	—		—	$	—
Granted		525,250			4.89
Vested		—			—
Cancelled		—			—
Outstanding at December 31, 2015		525,250		$	4.89		3.80	$	1,770
Granted		—			—
Vested		—			—
Cancelled		—			—
Outstanding at December 31, 2016		525,250		$	4.89		2.80	$	1,318
Granted		—			—
Vested		(225,125	)		4.89
Cancelled		(87,500	)		4.89
Outstanding at December 31, 2017		212,625		$	4.89		1.80	$	1,299
Restricted Stock Units expected to vest at December 31, 2017		212,625		$	4.89		1.80	$	1,299

Schedule of common stock reserved for future issuance

Common stock reserved for future issuance is as follows:

	December 31,
	2017		2016
Common stock warrants		225,756		134,113
Convertible preferred stock (if converted)		14,097,745		14,097,745
Common stock options		5,458,043		3,910,350
Restricted stock units		212,625		525,250
Awards available under the 2013 Plan		3,572,112		760,065
Awards available under the Inducement Plan		100,000		300,000
Employee stock purchase plan		729,000		729,000
		24,395,281		20,456,523

Warrant

Stockholders' Equity

Schedule of weighted-average assumptions used to determine the fair value of stock option grants

The weighted- average assumptions used in the Black-Scholes option pricing model to determine the fair value of the warrants issued were as follows:

	As of July 14, 2017
Risk–free interest rate		2.1	%
Expected volatility		88	%
Expected term (in years)		7.0
Expected dividend yield		0.0	%

Employee Stock Option

Stockholders' Equity

Schedule of weighted-average assumptions used to determine the fair value of stock option grants

The weighted-average assumptions used in the Black-Scholes option pricing model to determine the fair value of the employee stock option grants were as follows:

	Years Ended December 31,
	2017			2016			2015
Risk–free interest rate		2.0	%		1.6	%		1.6	%
Expected volatility		90	%		80	%		81	%
Expected term (in years)		6.0			6.0			6.0
Expected dividend yield		0.0	%		0.0	%		0.0	%

Non Employee Stock Option

Stockholders' Equity

Schedule of weighted-average assumptions used to determine the fair value of stock option grants

The weighted-average assumptions used in the Black-Scholes option pricing model to determine the fair value of the non-employee stock option grants were as follows:

	Years Ended December 31,
	2017			2016			2015
Risk–free interest rate		2.1	%		1.5	%		0.8	%
Expected volatility		87	%		83	%		72	%
Expected term (in years)		8.5			6.4			2.7
Expected dividend yield		0.0	%		0.0	%		0.0	%

Stock Option And Incentive Plan 2007 And 2013

Stockholders' Equity

Summary of outstanding restricted stock awards granted under the plan

The allocation of stock-based compensation for all stock awards is as follows (in thousands):

	Years Ended December 31,
	2017		2016		2015
Research and development	$	2,095	$	1,802	$	1,241
General and administrative		1,511		1,382		1,159
Total stock-based compensation expense	$	3,606	$	3,184	$	2,400

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2017

Income Tax Disclosure [Abstract]

Schedule of reconciliation of the Company's expected federal income tax provision (benefit) to the actual income tax provision

The following is a reconciliation of the Company’s expected federal income tax provision (benefit) to the actual income tax provision (in thousands):

	Years Ended December 31,
	2017			2016			2015
Tax computed at federal statutory rate	$	(14,603	)	$	(11,377	)	$	(10,198	)
State tax, net of federal tax benefit		(1,315	)		(2,089	)		2,226
Permanent differences		795			292			43
Stock compensation		539			968			213
R&D tax credits		(2,934	)		(971	)		(594	)
Reserve for uncertain tax positions		1,326			2,076			1,733
Tax attribute limitation		—			54			2,727
Tax Cuts and Jobs Act		25,280			—			—
Other		46			(74	)		(37	)
Valuation allowance		(9,134	)		11,121			3,887
Income tax expense	$	—		$	—		$	—

Summary of significant components of the Company's deferred tax assets

Significant components of the Company’s deferred tax assets are summarized as follows (in thousands):

	As of December 31,
	2017			2016
Deferred tax assets:
Section 59e amortization	$	14,365		$	17,699
Net operating losses		27,699			35,049
R&D tax credits		5,421			2,175
Depreciation and amortization		581			1,020
Deferred revenue		594			1,677
Stock compensation		876			1,222
Other		96			(76	)
Deferred tax assets		49,632			58,766
Valuation allowance		(49,632	)		(58,766	)
Net deferred tax assets	$	—		$	—

Summary of the change in the Company's unrecognized tax benefits

The change in the Company’s unrecognized tax benefits is summarized as follows (in thousands):

Balance at December 31, 2014	$	1,820
Increase related to current year tax positions		374
Increase related to prior year tax positions		2,624
Decrease related to prior year tax positions		(949	)
Balance at December 31, 2015	$	3,869
Increase related to current year tax positions		2,268
Increase related to prior year tax positions		1,625
Decrease related to prior year tax positions		(32	)
Balance at December 31, 2016	$	7,730
Increase related to current year tax positions		4,077
Increase related to prior year tax positions		6
Decrease related to prior year tax positions		(13	)
Balance at December 31, 2017	$	11,800

Selected Quarterly Financial Data (Tables)

12 Months Ended

Dec. 31, 2017

Quarterly Financial Information Disclosure [Abstract]

Schedule of selected quarterly financial data

(in thousands, except per share data) (unaudited)	First Quarter			Second Quarter			Third Quarter			Fourth Quarter
2017
Revenues	$	1,027		$	1,026		$	1,026		$	1,027
Total operating expenses		10,998			10,596			11,366			13,271
Net loss		(10,126	)		(9,645	)		(10,684	)		(12,497	)
Basic and diluted net loss per common share	$	(0.24	)	$	(0.23	)	$	(0.26	)	$	(0.29	)
2016
Revenues	$	1,322		$	1,027		$	1,026		$	1,027
Total operating expenses		9,238			9,031			9,415			8,681
Net loss		(8,377	)		(8,408	)		(8,737	)		(7,940	)
Basic and diluted net loss per common share	$	(0.29	)	$	(0.29	)	$	(0.27	)	$	(0.21	)

Organization and Summary of Significant Accounting Policies (Details) $ / shares in Units, $ in Thousands	1 Months Ended				12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares shares	Nov. 30, 2016 USD ($) $ / shares shares	Aug. 31, 2016 USD ($) $ / shares shares	May 31, 2015 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) item $ / shares shares	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($) shares
Equity transactions
Gross proceeds from issuance of shares							$ 4,580
Net proceeds from issuance of shares after related cash costs					$ 205	$ 186	$ 420
Segment Reporting
Number of operating segments \| item					1
Common Stock
Equity transactions
Shares sold \| shares					10,953,750		6,900,000
Public Equity Offerings
Equity transactions
Shares sold \| shares	10,953,750			6,900,000
Share issue price (in dollars per share) \| $ / shares	$ 4.20			$ 5.00	$ 4.20
Gross proceeds from issuance of shares	$ 46,000			$ 34,500
Net proceeds from issuance of shares after related cash costs	43,000			$ 32,100
Costs related to equity offering	3,000
Costs related to equity offering not paid	$ 200				$ 200
Private Placement \| Non-Voting Class A Preferred Stock
Equity transactions
Share issue price (in dollars per share) \| $ / shares		$ 13.30
Preferred stock, issued shares \| shares		2,819,549
Preferred stock convertible into common stock, shares \| shares		5
Private Placement \| Common Stock
Equity transactions
Shares sold \| shares		7,236,837	5,250,000
Share issue price (in dollars per share) \| $ / shares		$ 2.66	$ 1.96
Gross proceeds from issuance of shares			$ 10,300
Net proceeds from issuance of shares after related cash costs		$ 54,900	$ 10,200
Gross proceeds from issuance of shares		$ 56,700

Organization and Summary of Significant Accounting Policies (Details 2) - Fair Value Measurements Recurring - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	$ 88,952	$ 88,609
Total assets	100,949	92,112
U.S. Treasury debt securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
U.S. Treasury debt securities	11,997	3,503
Quoted prices in Active Market for Identical Assets (Level 1)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	88,952	88,609
Total assets	100,949	92,112
Quoted prices in Active Market for Identical Assets (Level 1) \| U.S. Treasury debt securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
U.S. Treasury debt securities	$ 11,997	$ 3,503

Organization and Summary of Significant Accounting Policies (Details 3) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Transfer of assets from level 1 to level 2	$ 0
Transfer of assets from level 2 to level 1	0
Transfer of liabilities from level 1 to level 2	0
Transfer of liabilities from level 2 to level 1	0
Fair Value Measurements Nonrecurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Non-financial assets	0
Non-financial liabilities	0
Fair Value Measurements Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liabilities measured at fair value	$ 0	$ 0

Organization and Summary of Significant Accounting Policies (Details 4) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Cash and cash equivalents	$ 88,952	$ 88,609	$ 64,809
Restricted cash	122	122	122
Total cash, cash equivalents, and restricted cash shown in the statement of cash flows	$ 89,074	$ 88,731	$ 64,931	$ 49,223

Organization and Summary of Significant Accounting Policies (Details 5)	12 Months Ended
	Dec. 31, 2017
Furniture And Fixtures \| Minimum
Property and equipment
Estimated useful lives	2 years
Furniture And Fixtures \| Maximum
Property and equipment
Estimated useful lives	5 years
Computer Equipment \| Minimum
Property and equipment
Estimated useful lives	2 years
Computer Equipment \| Maximum
Property and equipment
Estimated useful lives	5 years
Scientific Equipment \| Minimum
Property and equipment
Estimated useful lives	2 years
Scientific Equipment \| Maximum
Property and equipment
Estimated useful lives	5 years
Office Equipment \| Minimum
Property and equipment
Estimated useful lives	2 years
Office Equipment \| Maximum
Property and equipment
Estimated useful lives	5 years

Organization and Summary of Significant Accounting Policies (Details 6) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue Recognition [Abstract]
Four criteria needed for Company to recognize revenue	(i) persuasive evidence that an agreement exists; (ii) delivery of the products and/or services has occurred; (iii) the selling price is fixed or determinable; and (iv) collectability is reasonably assured.
Uncertain tax position description	more than 50 percent
Anti-dilutive securities (in shares)	19,994,169	18,667,458	3,246,837
Series A Convertible Preferred Stock (if converted)
Revenue Recognition [Abstract]
Anti-dilutive securities (in shares)	14,097,745	14,097,745
Employee Stock Option
Revenue Recognition [Abstract]
Anti-dilutive securities (in shares)	0	3,284	44,381
Warrants For Common Stock
Revenue Recognition [Abstract]
Anti-dilutive securities (in shares)	225,756	134,113	134,113
Stock Compensation Plan
Revenue Recognition [Abstract]
Anti-dilutive securities (in shares)	5,458,043	3,910,350	2,587,474
Restricted Stock Units (RSUs)
Revenue Recognition [Abstract]
Anti-dilutive securities (in shares)	212,625	525,250	525,250

Juno Collaboration and License Agreement (Details) - USD ($)		12 Months Ended
	May 04, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Collaboration agreement
Issuance of common stock				$ 4,580,000
Juno Therapeutics, Inc
Collaboration agreement
Common stock issued	1,000,000
Research term	4 years
Minimum annual research payments receivable	$ 2,000,000
Extended term of research	2 years
Extension fee	$ 3,000,000
Minimum annual payments receivable for extended term of research	$ 4,000,000
Common stock premium percentage on trading price upon exercise of extension	120.00%
Trailing trading period considered for premium	30 days
Upfront payments recorded as deferred revenue	$ 5,000,000
Recognition period of deferred revenue	4 years
Proceeds from sale of common stock	$ 8,000,000
Premium on share price (in dollars per share)	$ 3.40
Aggregate premium on shares issued recorded as deferred revenue	$ 3,400,000
Issuance of common stock	$ 4,600,000
Term of royalties payable after first commercial sale	10 years
Juno Therapeutics, Inc \| Maximum
Collaboration agreement
Common stock issued upon exercise of extension	$ 10,000,000
Aggregate milestone payments	51,000,000
Juno Therapeutics, Inc \| Maximum \| Third Juno Product
Collaboration agreement
Additional aggregate milestone payments	116,000,000
Juno Therapeutics, Inc \| Maximum \| Fifth Juno Product
Collaboration agreement
Additional aggregate milestone payments	137,500,000
Juno Therapeutics, Inc \| Collaborative Arrangement
Collaboration agreement
Proceeds from non refundable upfront fee	$ 5,000,000
Share Price	$ 8.00
Revenue recognized		$ 4,100,000	$ 4,400,000	$ 2,400,000
Aggregate deferred revenue		$ 2,800,000

Short-term Investments (Details) - USD ($)	12 Months Ended
Short-term Investments (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale securities, impairment	$ 0
Available-for-sale securities, gains (losses) on sales	$ 0	$ 0
U.S. Treasury debt securities
Schedule Of Available For Sale Securities [Line Items]
Short term investments, maturity start range	6 months
Short term investments, maturity end range	12 months

Short-term Investments (Details 2) - USD ($) $ in Thousands	12 Months Ended
Short-term Investments (Details 2) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 12,000	$ 3,504
Unrealized Losses	(3)	(1)
Estimated Fair Value	$ 11,997	$ 3,503
U.S. Treasury debt securities
Schedule Of Available For Sale Securities [Line Items]
Maturity (in years)	1 or less	1 or less
Amortized Cost	$ 12,000	$ 3,504
Unrealized Losses	(3)	(1)
Estimated Fair Value	$ 11,997	$ 3,503

Property and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
Property and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property and equipment
Property and equipment, gross	$ 7,689	$ 5,783
Less accumulated depreciation and amortization	(5,139)	(4,204)
Property, Plant and Equipment, Net	2,550	1,579
Depreciation expense	971	881	$ 687
Furniture And Fixtures
Property and equipment
Property and equipment, gross	508	324
Computer And Office Equipment
Property and equipment
Property and equipment, gross	527	318
Software And Software Development Costs
Property and equipment
Property and equipment, gross	103	103
Leasehold Improvements
Property and equipment
Property and equipment, gross	180	180
Scientific Equipment
Property and equipment
Property and equipment, gross	$ 6,371	$ 4,858

Accrued Expenses, Long-Term Debt, Commitments and Contingencies (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current accrued expenses
Accrued payroll and other employee benefits	$ 1,761	$ 1,505
Accrued clinical trial related costs	3,323	1,043
Accrued other	2,170	1,409
Accrued expenses	$ 7,254	$ 3,957

Accrued Expenses, Long-Term Debt, Commitments and Contingencies (Details 2) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Debt Disclosure [Abstract]
Long-term debt	$ 15,000	$ 10,765
Less debt issuance costs and discount, net of current portion	(192)	(7)
Long-term debt, net of long-term portion of debt issuance costs and discount	14,808	10,758
Less current portion of long-term debt		(8,257)
Long-term debt, net	$ 14,808	2,501
Current portion of long-term debt		8,257
Less current portion of debt issuance costs and discount		(70)
Current portion of long-term debt, net		$ 8,187

Accrued Expenses, Long-Term Debt, Commitments and Contingencies (Details 3)		12 Months Ended
	Jul. 14, 2017 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) ft² $ / shares shares	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 24, 2014 USD ($)	Jul. 30, 2014 USD ($)
Current accrued expenses
Loss on extinguishment of debt		$ (118,000)
Cash-collateralized irrevocable standby letter of credit		100,000
Future minimum payments		12,928,000
Rent expense		2,300,000	$ 1,300,000	$ 1,200,000
Minimum annual payments to maintain cancelable licenses		$ 200,000
Non-Cancelable Operating Lease
Current accrued expenses
Lease expiration period		2023-06
Leased space of office and laboratory \| ft²		48,000
Future minimum payments		$ 12,900,000
Sublease Lease
Current accrued expenses
Future minimum payments		$ 0
Sublease expiration date		2017-09
Term B Loan \| Warrant
Current accrued expenses
Fair values of warrants issued	$ 200,000
SVB Loan Amendment
Current accrued expenses
First amendment effective date		Jul. 14, 2017
Debt instrument, variable rate description		The Company’s long-term debt bears interest at a floating per annum rate equal to the greater of (i) 3.50% above the Prime Rate (as defined in the SVB Loan Amendment) or (ii) 7.25%; provided, however, that in no event shall such interest rate exceed 8.25%.
Debt instrument interest only payment period		August 1, 2017 through January 1, 2019
SVB Loan Amendment \| Greater of 7.25%
Current accrued expenses
Debt instrument, floating rate		7.25%
SVB Loan Amendment \| Minimum \| Prime Rate \| Greater of 3.50% Above Prime Rate
Current accrued expenses
Debt instrument, floating rate		3.50%
SVB Loan Amendment \| Maximum
Current accrued expenses
Debt instrument, floating rate		8.25%
SVB Loan Amendment \| 2017 Term Loan
Current accrued expenses
Principal amount	15,000,000
Cash proceeds from remaining portion of debt	$ 7,500,000
Debt instrument, maturity date	Jan. 01, 2022
Debt instrument, variable rate description		The 2017 Term Loan matures on January 1, 2022 (the Term Loan Maturity Date) and bears interest at a floating per annum rate equal to the greater of (i) 3.50% above the Prime Rate (as defined in the SVB Loan Amendment) or (ii) 7.25%; provided, however, that in no event shall such interest rate exceed 8.25%.
Debt instrument, interest payment terms		Interest is payable on a monthly basis on the first day of each month.
Debt instrument, frequency of periodic payment		monthly
Interest rate (as a percent)		8.00%
Debt instrument interest only payment period		August 1, 2017 through January 1, 2019
Number of equal monthly installments to repay principal and accrued interest	36 months
Debt instrument extended interest only payment period		January 1, 2019 through and including to July 1, 2019
Final payment fee		7.50%
Final payment fee (amounts in dollars)		$ 1,100,000
Effective interest rate	10.20%
Loss on extinguishment of debt		(100,000)
Aggregate interest expense		800,000
SVB Loan Amendment \| 2017 Term Loan \| 2017 Warrant
Current accrued expenses
Exercise price (in dollars per share) \| $ / shares	$ 3.28
Warrants expiration date	2024-07
Fair values of warrants issued	$ 200,000
SVB Loan Amendment \| 2017 Term Loan \| Product Development Milestone Achievement
Current accrued expenses
Number of equal monthly installments to repay principal and accrued interest	30 months
SVB Loan Amendment \| 2017 Term Loan \| Minimum \| Greater of 7.25%
Current accrued expenses
Debt instrument, floating rate	7.25%
SVB Loan Amendment \| 2017 Term Loan \| Minimum \| Prime Rate \| Greater of 3.50% Above Prime Rate
Current accrued expenses
Debt instrument, floating rate	3.50%
SVB Loan Amendment \| 2017 Term Loan \| Maximum
Current accrued expenses
Debt instrument, floating rate	8.25%
SVB Loan Amendment \| 2017 Term Loan \| Maximum \| 2017 Warrant
Current accrued expenses
Warrants to purchase shares of common stock issued on conversion \| shares	91,463
Amended And Restated Loan And Security Agreement
Current accrued expenses
Aggregate interest expense		$ 500,000	$ 1,600,000	$ 2,200,000
Amended And Restated Loan And Security Agreement \| Warrants Expiration Period January 2019
Current accrued expenses
Warrants to purchase shares of common stock issued on conversion \| shares		5,305
Warrants expiration date		2019-01
Amended And Restated Loan And Security Agreement \| Warrants Expiration Period August 2021
Current accrued expenses
Warrants to purchase shares of common stock issued on conversion \| shares		30,769
Warrants expiration date		2021-08
Amended And Restated Loan And Security Agreement \| Warrant
Current accrued expenses
Warrants to purchase shares of common stock issued on conversion \| shares		36,074
Exercise price (in dollars per share) \| $ / shares		$ 7.21
Amended And Restated Loan And Security Agreement \| Maximum
Current accrued expenses
Principal amount						$ 20,000,000
Amended And Restated Loan And Security Agreement \| Term A Loan
Current accrued expenses
Principal amount						10,000,000
Debt instrument, maturity date		Jan. 01, 2018
Final payment fee		7.50%
Final payment fee (amounts in dollars)		$ 800,000
Amended And Restated Loan And Security Agreement \| Term B Loan
Current accrued expenses
Debt instrument, maturity date		Jun. 01, 2018
Final payment fee		7.50%
Final payment fee (amounts in dollars)		$ 800,000
Amended And Restated Loan And Security Agreement \| Term B Loan \| 2014 Warrants
Current accrued expenses
Warrants to purchase shares of common stock issued on conversion \| shares		98,039
Exercise price (in dollars per share) \| $ / shares		$ 4.08
Warrants expiration date		2021-12
Amended And Restated Loan And Security Agreement \| Term B Loan \| Maximum
Current accrued expenses
Principal amount						$ 10,000,000
Amended And Restated Loan And Security Agreement \| Term B Loan Tranche 1 \| Minimum
Current accrued expenses
Principal amount					$ 10,000,000

Accrued Expenses, Long-Term Debt, Commitments and Contingencies (Details 4) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Long-Term Debt
Years Ending December 31, 2018	$ 1,215
Years Ending December 31, 2019	5,645
Years Ending December 31, 2020	5,661
Years Ending December 31, 2021	5,253
Years Ending December 31, 2022	1,545
Long-term debt	19,319
Less interest	(3,194)
Less additional payments due upon maturity	(1,125)
Less unamortized debt discount and debt issuance costs	(192)
Long–term debt, net of current portion	14,808	$ 2,501
Operating Leases
Years Ending December 31, 2018	2,116
Years Ending December 31, 2019	2,278
Years Ending December 31, 2020	2,346
Years Ending December 31, 2021	2,417
Years Ending December 31, 2022	2,489
Years Ending December 31, 2023	1,282
Total	12,928
Total
Years Ending December 31, 2018	3,331
Years Ending December 31, 2019	7,923
Years Ending December 31, 2020	8,007
Years Ending December 31, 2021	7,670
Years Ending December 31, 2022	4,034
Years Ending December 31, 2023	1,282
Total	$ 32,247

Convertible Preferred Stock and Stockholders' Equity (Details) - USD ($)			1 Months Ended		12 Months Ended			52 Months Ended
	Jul. 14, 2017	Sep. 13, 2013	Nov. 30, 2016	Aug. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Jan. 31, 2018	May 31, 2017	May 02, 2017	May 10, 2016	Dec. 31, 2014	Aug. 31, 2013	Aug. 28, 2013
Convertible preferred stock
Net proceeds from issuance of shares after related cash costs					$ 205,000	$ 186,000	$ 420,000
Preferred stock, par value (in dollars per share)					$ 0.001	$ 0.001		$ 0.001
Dividends declared					$ 0
Shares reserved for issuance					24,395,281	20,456,523		24,395,281
Stock purchases under the plan (in shares)								0
SVB Loan Amendment
Convertible preferred stock
Warrants to purchase shares of common stock issued on conversion	91,463
Employee And Non Employee Stock Option
Convertible preferred stock
Total intrinsic value of stock options exercised					$ 100,000
Cash received upon exercise of options (in dollars per share)					$ 200,000
Employee Stock
Convertible preferred stock
Shares reserved for issuance		729,000			729,000	729,000		729,000
Additional shares authorized (as a percent)		2.00%
Additional shares authorized		450,000
Restricted Stock Units (RSUs)
Convertible preferred stock
Shares reserved for issuance					212,625	525,250		212,625
Expected recognition period of unrecognized compensation cost					1 year 9 months 18 days	2 years 9 months 18 days	3 years 9 months 18 days
Unrecognized compensation cost related to unvested restricted shares					$ 900,000	$ 1,800,000	$ 2,400,000	$ 900,000
Equity Incentive Plan 2007
Convertible preferred stock
Shares authorized														2,423,072
Stock Option And Incentive Plan 2013
Convertible preferred stock
Shares reserved for issuance										2,500,000					1,020,000
Additional shares authorized (as a percent)															4.00%
Stock Option And Incentive Plan 2013 \| Restricted Stock \| Second Year Anniversary Vesting Date
Convertible preferred stock
Stock options generally vested on anniversary grant date (as a percent)					50.00%
Stock Option And Incentive Plan 2013 \| Restricted Stock \| Fourth Year Anniversary Vesting Date
Convertible preferred stock
Stock options generally vested on anniversary grant date (as a percent)					50.00%
Stock Option And Incentive Plan 2007 And 2013 \| Employee And Non Employee Stock Option
Convertible preferred stock
Stock options generally vested on anniversary grant date (as a percent)					25.00%
General vesting period after first year					3 years
Monthly vesting period					4 years
Term of stock awards					10 years
Outstanding options					5,458,043	3,910,350	2,587,474	5,458,043					2,425,969
Granted					2,522,920	2,245,240	1,345,744
Unrecognized compensation cost related to outstanding options					$ 5,800,000	$ 4,900,000	$ 3,800,000	$ 5,800,000
Expected recognition period of unrecognized compensation cost					2 years 7 months 6 days	2 years 7 months 6 days	2 years 9 months 18 days
Stock Option And Incentive Plan 2007 And 2013 \| Employee And Non Employee Stock Option \| Vesting Based On Performance
Convertible preferred stock
Outstanding options					73,600	73,600	14,769	73,600
Aggregate grant date fair value					$ 100,000	$ 100,000	$ 100,000
Stock Option And Incentive Plan 2007 And 2013 \| Employee and Directors Stock Option
Convertible preferred stock
Granted					2,500,000	2,200,000	1,200,000
Weighted-average grant date fair value per share of employee options granted					$ 2.29	$ 1.80	$ 3.54
Inducement Plan
Convertible preferred stock
Shares reserved for issuance					100,000	300,000		100,000				500,000
Inducement Plan \| Subsequent Event
Convertible preferred stock
Additional shares reserved for issuance									400,000
Common Stock
Convertible preferred stock
Common stock issued					10,953,750		6,900,000
Warrant
Convertible preferred stock
Shares reserved for issuance					225,756	134,113		225,756
Warrant \| SVB Loan Amendment
Convertible preferred stock
Exercise price (in dollars per share)	$ 3.28
Warrant \| Term B Loan
Convertible preferred stock
Fair values of warrants issued	$ 200,000
Non-Voting Class A Preferred Stock
Convertible preferred stock
Preferred stock, par value (in dollars per share)			$ 0.001
Conversion price			$ 2.66
Non-Voting Class A Preferred Stock \| Redmile Group, LLC and Affiliates
Convertible preferred stock
Terms of conversion					The Class A Preferred were purchased exclusively by entities affiliated with Redmile Group, LLC (collectively, Redmile). The terms of the CoD prohibited Redmile from converting the Class A Preferred into shares of the Company’s common stock if, as a result of conversion, Redmile, together with its affiliates, would own more than 9.99% of the Company’s common stock then issued and outstanding (the Redmile Percentage Limitation), which percentage could change at Redmile’s election upon 61 days’ notice to the Company to i) any other number less than or equal to 19.99% or (ii) subject to approval of the Company’s stockholders to the extent required in accordance with the NASDAQ Global Market rules, any number in excess of 19.99%. On May 2, 2017, the Company’s stockholders approved the issuance of up to an aggregate of 14,097,745 shares of common stock upon the conversion of the outstanding shares of Class A Preferred. As a result, Redmile has the right to increase the Redmile Percentage Limitation to any percentage in excess of 19.99% at its election.
Non-Voting Class A Preferred Stock \| Maximum
Convertible preferred stock
Number of shares to be issued upon conversion											14,097,745
Non-Voting Class A Preferred Stock \| Maximum \| Redmile Group, LLC and Affiliates
Convertible preferred stock
Percentage of common stock ownership upon preferred stock conversion			9.99%
Preferred shares converted into common stock percentage of ownership change upon notice			19.99%
Private Placement \| Common Stock
Convertible preferred stock
Share issue price (in dollars per share)			$ 2.66	$ 1.96
Common stock issued			7,236,837	5,250,000
Net proceeds from issuance of shares after related cash costs			$ 54,900,000	$ 10,200,000
Gross proceeds from issuance of shares			$ 56,700,000
Private Placement \| Non-Voting Class A Preferred Stock
Convertible preferred stock
Preferred stock, issued shares			2,819,549
Share issue price (in dollars per share)			$ 13.30
Number of shares to be issued upon conversion			5

Convertible Preferred Stock and Stockholders' Equity (Details 2) - Stock Option And Incentive Plan 2007 And 2013 - Employee And Non Employee Stock Option - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Number of Options
Balance at the beginning of the period	3,910,350	2,587,474	2,425,969
Granted	2,522,920	2,245,240	1,345,744
Exercised	(83,220)	(136,368)	(227,215)
Cancelled	(892,007)	(785,996)	(957,024)
Balance at the end of the period	5,458,043	3,910,350	2,587,474	2,425,969
Vested and expected to vest at the end of the period	5,300,033
Exercisable at the end of the period	2,479,556
Weighted-Average Price
Balance at the beginning of the period	$ 3.77	$ 4.59	$ 3.83
Granted	3.09	2.62	5.23
Exercised	2.79	1.44	1.85
Cancelled	3.50	3.58	4.20
Balance at the end of the period	3.52	$ 3.77	$ 4.59	$ 3.83
Vested and expected to vest at the end of the period	3.53
Exercisable at the end of the period	$ 3.92
Weighted Average Remaining Contractual Term
Outstanding at the end of the period	7 years 11 months 26 days	8 years 3 months 11 days	6 years 11 months 1 day	8 years 22 days
Vested and expected to vest at the end of the period	8 years 7 days
Exercisable at the end of the period	7 years 1 month 28 days
Aggregate Intrinsic Value
Outstanding at the end of the period	$ 14,754	$ 682	$ 1,486	$ 4,839
Vested and expected to vest at the end of the period	14,267
Exercisable at the end of the period	$ 5,972

Convertible Preferred Stock and Stockholders' Equity (Details 3) - Restricted Stock Units (RSUs) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Number of Restricted Stock Units
Balance at the beginning of the period	525,250	525,250
Granted			525,250
Vested	(225,125)
Cancelled	(87,500)
Balance at the end of the period	212,625	525,250	525,250
Expected to vest at the end of the period	212,625
Weighted-Average Grant Date Fair Value per Share
Balance at the beginning of the period	$ 4.89	$ 4.89
Granted			$ 4.89
Vested	4.89
Cancelled	4.89
Balance at the end of the period	4.89	$ 4.89	$ 4.89
Expected to vest at the end of the period	$ 4.89
Weighted Average Remaining Vesting Period
Outstanding at the end of the period	1 year 9 months 18 days	2 years 9 months 18 days	3 years 9 months 18 days
Expected to vest at the end of the period	1 year 9 months 18 days
Aggregate Intrinsic Value
Outstanding at the end of the period (in dollars)	$ 1,299	$ 1,318	$ 1,770
Restricted Stock Units expected to vest at December 31, 2017	$ 1,299

Convertible Preferred Stock and Stockholders' Equity (Details 4) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Convertible preferred stock
Total stock-based compensation expense	$ 3,606	$ 3,184	$ 2,400
Research And Development
Convertible preferred stock
Total stock-based compensation expense	2,095	1,802	1,241
General And Administrative
Convertible preferred stock
Total stock-based compensation expense	$ 1,511	$ 1,382	$ 1,159

Convertible Preferred Stock and Stockholders' Equity (Details 5)		12 Months Ended
	Jul. 14, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Warrant
Weighted-average assumptions to determine fair value of stock options
Risk–free interest rate	2.10%
Expected volatility	88.00%
Expected term (in years)	7 years
Expected dividend yield	0.00%
Employee Stock Option
Weighted-average assumptions to determine fair value of stock options
Risk–free interest rate		2.00%	1.60%	1.60%
Expected volatility		90.00%	80.00%	81.00%
Expected term (in years)		6 years	6 years	6 years
Expected dividend yield		0.00%	0.00%	0.00%
Non Employee Stock Option
Weighted-average assumptions to determine fair value of stock options
Risk–free interest rate		2.10%	1.50%	0.80%
Expected volatility		87.00%	83.00%	72.00%
Expected term (in years)		8 years 6 months	6 years 4 months 24 days	2 years 8 months 12 days
Expected dividend yield		0.00%	0.00%	0.00%

Convertible Preferred Stock and Stockholders' Equity (Details 6) - shares	Dec. 31, 2017	Dec. 31, 2016	May 10, 2016	Sep. 13, 2013
Weighted-average assumptions to determine fair value of stock options
Shares reserved for issuance	24,395,281	20,456,523
Convertible Preferred Stock (if converted)
Weighted-average assumptions to determine fair value of stock options
Shares reserved for issuance	14,097,745	14,097,745
Inducement Plan
Weighted-average assumptions to determine fair value of stock options
Shares reserved for issuance	100,000	300,000	500,000
Employee Stock Option
Weighted-average assumptions to determine fair value of stock options
Shares reserved for issuance	5,458,043	3,910,350
Stock Compensation Plan
Weighted-average assumptions to determine fair value of stock options
Shares reserved for issuance	3,572,112	760,065
Restricted Stock Units (RSUs)
Weighted-average assumptions to determine fair value of stock options
Shares reserved for issuance	212,625	525,250
Employee Stock
Weighted-average assumptions to determine fair value of stock options
Shares reserved for issuance	729,000	729,000		729,000
Warrant
Weighted-average assumptions to determine fair value of stock options
Shares reserved for issuance	225,756	134,113

Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
Income Taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of the Company's expected federal income tax provision (benefit) to the actual income tax provision
Tax computed at federal statutory rate	$ (14,603)	$ (11,377)	$ (10,198)
State tax, net of federal tax benefit	(1,315)	(2,089)	2,226
Permanent differences	795	292	43
Stock compensation	539	968	213
R&D tax credits	(2,934)	(971)	(594)
Reserve for uncertain tax positions	1,326	2,076	1,733
Tax attribute limitation		54	2,727
Tax Cuts and Jobs Act	25,280
Other	46	(74)	(37)
Valuation allowance	(9,134)	11,121	$ 3,887
Deferred tax assets:
Section 59e amortization	14,365	17,699
Net operating losses	27,699	35,049
R&D tax credits	5,421	2,175
Depreciation and amortization	581	1,020
Deferred revenue	594	1,677
Stock compensation	876	1,222
Other	96	(76)
Deferred tax assets	49,632	58,766
Valuation allowance	$ (49,632)	$ (58,766)

Income Taxes (Details 2) - USD ($) $ in Thousands	12 Months Ended
Income Taxes (Details 2) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Operating Loss Carryforwards [Line Items]
Valuation allowance	$ 49,632	$ 58,766
U.S. Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	$ 121,200
Net operating loss carryforward expiration year	2027
California
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	$ 120,800
Net operating loss carryforward expiration year	2028

Income Taxes (Details 3) $ in Millions	12 Months Ended
Income Taxes (Details 3) $ in Millions	Dec. 31, 2017 USD ($)
Tax Credit Carryforward [Line Items]
Cumulative changes in ownership percentage	50.00%
Period of change in ownership	3 years
U.S. Federal \| Research and development
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	$ 5.7
Tax credit carryforward expiration year	2035
California \| Research and development
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	$ 4.2

Income Taxes (Details 4) - USD ($)	12 Months Ended
Income Taxes (Details 4) - USD ($)	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in the Company's unrecognized tax benefits
Balance at the beginning of the period	$ 7,730,000	$ 3,869,000	$ 1,820,000
Increase related to current year tax positions	4,077,000	2,268,000	374,000
Increase related to prior year tax positions	6,000	1,625,000	2,624,000
Decrease related to prior year tax positions	(13,000)	(32,000)	(949,000)
Balance at the end of the period	11,800,000	$ 7,730,000	$ 3,869,000
Income Taxes
Unrecognized tax benefit	0
Unrecognized tax benefits that, if recognized, would reduce the effective tax rate	$ 0

Income Taxes (Details 5) - USD ($) $ in Thousands	12 Months Ended
Income Taxes (Details 5) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Contingency [Line Items]
Excess tax benefit previously not recognized		$ 11,800	$ 7,730	$ 3,869	$ 1,820
Corporate tax rate		34.00%
Tax cuts and jobs act of 2017 incomplete accounting change in tax rate deferred tax liability provisional income tax benefit		$ 25,300
Remeasurement of deferred tax assets and liabilities		25,300
Scenario Plan
Income Tax Contingency [Line Items]
Corporate tax rate	21.00%
ASU No. 2016-09
Income Tax Contingency [Line Items]
Excess tax benefit previously not recognized		$ 100

Selected Quarterly Financial Data (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 Financial Information Disclosure [Abstract]
Revenues	$ 1,027	$ 1,026	$ 1,026	$ 1,027	$ 1,027	$ 1,026	$ 1,027	$ 1,322
Total operating expenses	13,271	11,366	10,596	10,998	8,681	9,415	9,031	9,238	$ 46,231	$ 36,365	$ 30,213
Net loss	$ (12,497)	$ (10,684)	$ (9,645)	$ (10,126)	$ (7,940)	$ (8,737)	$ (8,408)	$ (8,377)	$ (42,952)	$ (33,462)	$ (29,992)
Basic and diluted net loss per common share (in dollars per share)	$ (0.29)	$ (0.26)	$ (0.23)	$ (0.24)	$ (0.21)	$ (0.27)	$ (0.29)	$ (0.29)	$ (1.02)	$ (1.05)	$ (1.18)

Employee Benefits	12 Months Ended
Employee Benefits	Dec. 31, 2017
Compensation And Retirement Disclosure [Abstract]
Employee Benefits	8. Employee Benefits Effective January 1, 2009, the Company adopted a defined contribution 401(k) plan for employees who are at least 21 years of age. Employees are eligible to participate in the plan beginning on the first day of the calendar quarter following date of hire. Under the terms of the plan, employees may make voluntary contributions as a percent of compensation. No matching contributions have been made by the Company since the adoption of the 401(k) plan.

Subsequent Events	12 Months Ended
Subsequent Events	Dec. 31, 2017
Subsequent Events [Abstract]
Subsequent Events	10. Subsequent Events On February 22, 2018, the California Institute for Regenerative Medicine (CIRM) announced that it awarded the Company a $4.0 million grant to advance the Company’s FT516 product candidate into a first-in-human clinical trial. FT516 is being developed by the Company as an off-the-shelf engineered NK cell cancer immunotherapy.

Employee Benefits (Details) - USD ($)		12 Months Ended
Employee Benefits (Details) - USD ($)	Jan. 02, 2009	Dec. 31, 2017
Compensation And Retirement Disclosure [Abstract]
Minimum age of employees	21 years
Matching contributions under 401(k) plan		$ 0

Subsequent Events (Details) $ in Millions	Feb. 22, 2018 USD ($)
California Institute for Regenerative Medicine \| FT516 \| Subsequent Event
Subsequent Event [Line Items]
Grants for first-in-human clinical trial	$ 4.0