Mirati Therapeutics (MRTX) 10-K28 Feb 192018 FY Annual reportFinancial data

Company Profile

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
Document and Entity Information - USD ($) $ in Millions	Dec. 31, 2018	Feb. 22, 2019	Jun. 30, 2018
Document and Entity Information
Entity Registrant Name	MIRATI THERAPEUTICS, INC.
Entity Central Index Key	1,576,263
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	FY
Entity Filer Category	Large Accelerated Filer
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Shell Company	false
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Common Stock, Shares Outstanding		35,267,601
Entity Public Float			$ 1,189

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets
Cash and cash equivalents	$ 32,694	$ 107,703
Short-term investments	190,096	43,134
Other current assets	3,870	4,922
Total current assets	226,660	155,759
Property and equipment, net	473	525
Other long-term assets	1,321	962
Total assets	228,454	157,246
Current liabilities
Accounts payable and accrued liabilities	25,775	13,644
Deferred revenue	371	0
Total current liabilities	26,146	13,644
Deferred revenue and other liabilities	732	314
Total liabilities	26,878	13,958
Stockholders' equity
Preferred stock, $0.001 par value, 10,000,000 shares authorized; none issued and outstanding at both December 31, 2018 and December 31, 2017	0	0
Common stock, $0.001 par value; 100,000,000 authorized; 32,538,857 and 28,622,886 issued and outstanding at December 31, 2018 and December 31, 2017, respectively	33	29
Additional paid-in capital	751,109	594,407
Accumulated other comprehensive income	9,479	9,479
Accumulated deficit	(559,045)	(460,627)
Total stockholders' equity	201,576	143,288
Total liabilities and stockholders' equity	$ 228,454	$ 157,246

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, shares authorized	100,000,000	100,000,000
Common stock, shares issued	32,538,857	28,622,886
Common stock, shares outstanding	32,538,857	28,622,886

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue
Total revenue	$ 12,926,000	$ 0	$ 0
Expenses
Research and development	93,872,000	58,085,000	68,487,000
General and administrative	21,681,000	13,450,000	15,292,000
Total operating expenses	115,553,000	71,535,000	83,779,000
Loss from operations	(102,627,000)	(71,535,000)	(83,779,000)
Other income, net	4,209,000	1,105,000	661,000
Net loss	(98,418,000)	(70,430,000)	(83,118,000)
Unrealized gain (loss) on available-for-sale investments	0	54,000	25,000
Comprehensive loss	$ (98,418,000)	$ (70,484,000)	$ (83,143,000)
Basic and diluted net loss per share (USD per share)	$ (3.19)	$ (2.78)	$ (4.20)
Weighted average number of shares used in computing net loss per share, basic and diluted (in shares)	30,897,717	25,290,222	19,787,349

CONSOLIDATED STATEMENTS OF CHAN

CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock	Additional paid-in capital	Accumulated other comprehensive income (loss)	Accumulated deficit
Beginning balance (in shares) at Dec. 31, 2015		19,282,935
Beginning balance at Dec. 31, 2015	$ 118,176	$ 19	$ 415,232	$ 9,558	$ (306,633)
Increase (Decrease) in Stockholders' Equity
Net loss for the year	(83,118)				(83,118)
Share-based compensation expense	10,624		10,624
Issuance of common stock from ESPP	297		297
Issuance of common stock from ESPP (in shares)		28,483
Exercise of options for cash	240		240
Exercise of options for cash (in shares)		22,132
Exercise of warrants for cash	$ 2,115	$ 1	2,114
Exercise of warrants for cash (in shares)	313,756	313,756
Net exercise of warrants	$ 0
Net exercise of warrants (in shares)	603,545	289,789
Unrealized loss on investments	$ (25)			(25)
Ending balance (in shares) at Dec. 31, 2016		19,937,095
Ending balance at Dec. 31, 2016	48,309	$ 20	428,507	9,533	(389,751)
Increase (Decrease) in Stockholders' Equity
Net loss for the year	(70,430)				(70,430)
Share-based compensation expense	6,786		6,786
Issuance of common stock and warrants, net of issuance costs	153,530	$ 8	153,522
Issuance of common stock and warrants, net of issuance costs (shares)		7,941,688
Issuance of common stock from ESPP	144		144
Issuance of common stock from ESPP (in shares)		59,976
Exercise of options for cash	399		399
Exercise of options for cash (in shares)		45,573
Exercise of warrants for cash	$ 4,604	$ 1	4,603
Exercise of warrants for cash (in shares)	585,729	585,729
Net exercise of warrants	$ 0
Net exercise of warrants (in shares)	638,554	52,825
Unrealized loss on investments	$ (54)			(54)
Ending balance (in shares) at Dec. 31, 2017	28,622,886	28,622,886
Ending balance at Dec. 31, 2017	$ 143,288	$ 29	594,407	9,479	(460,627)
Increase (Decrease) in Stockholders' Equity
Net loss for the year	(98,418)				(98,418)
Share-based compensation expense	15,854		15,854
Issuance of common stock and warrants, net of issuance costs	130,663	$ 3	130,660
Issuance of common stock and warrants, net of issuance costs (shares)		3,162,500
Issuance of common stock from ESPP	442		442
Issuance of common stock from ESPP (in shares)		21,536
Exercise of options for cash	$ 9,747	$ 1	9,746
Exercise of options for cash (in shares)	731,935	731,935
Unrealized loss on investments	$ 0
Ending balance (in shares) at Dec. 31, 2018	32,538,857	32,538,857
Ending balance at Dec. 31, 2018	$ 201,576	$ 33	$ 751,109	$ 9,479	$ (559,045)

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating activities
Net loss	$ (98,418)	$ (70,430)	$ (83,118)
Non-cash adjustments reconciling net loss to operating cash flows
Depreciation of property and equipment	175	184	180
(Accretion of discount) amortization of premium on investments	(1,320)	(266)	7
Share-based compensation expense	15,854	6,786	10,624
Changes in operating assets and liabilities
Other current assets	1,052	(3,409)	254
Other long-term assets	(359)	3,606	(1,259)
Accounts payable, accrued liabilities, deferred revenue and other liabilities	12,920	(1,177)	5,295
Cash flows used in operating activities	(70,096)	(64,706)	(68,017)
Investing activities:
Purchases of short-term investments	(255,795)	(100,558)	(70,269)
Sales and maturities of short-term investments	110,152	91,988	108,720
Purchases of property and equipment	(122)	(81)	(196)
Cash flows provided by (used in) investing activities	(145,765)	(8,651)	38,255
Financing activities:
Proceeds from issuance of common stock and warrants, net of issuance costs	130,663	153,530	0
Proceeds from exercise of common stock options and warrants	9,747	5,003	2,355
Proceeds from issuances under employee stock purchase plan	442	144	297
Cash flows provided by financing activities	140,852	158,677	2,652
Increase (decrease) in cash and cash equivalents	(75,009)	85,320	(27,110)
Cash and cash equivalents, beginning of year	107,703	22,383	49,493
Cash and cash equivalents, end of year	$ 32,694	$ 107,703	$ 22,383

DESCRIPTION OF BUSINESS

DESCRIPTION OF BUSINESS	12 Months Ended
DESCRIPTION OF BUSINESS	Dec. 31, 2018
DESCRIPTION OF BUSINESS
DESCRIPTION OF BUSINESS	Description of Business Mirati Therapeutics, Inc. ("Mirati" or the "Company") is a clinical-stage oncology company developing product candidates to address the genetic and immunological promoters of cancer. The Company was incorporated under the laws of the State of Delaware on April 29, 2013 as Mirati Therapeutics, Inc. and is located in San Diego, California. The Company has a wholly owned subsidiary in Canada, MethylGene, Inc. ("MethylGene"), and operates in one business segment, primarily in the United States. The Company's common stock has been listed on the NASDAQ Global Select Market since June 5, 2018, and was previously listed on the NASDAQ Capital Market since July 15, 2013, under the ticker symbol "MRTX."

SUMMARY OF SIGNIFICANT ACCOUNTI

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	12 Months Ended
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	Dec. 31, 2018
Accounting Policies [Abstract]
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	Summary of Significant Accounting Policies Basis of Presentation These consolidated financial statements are prepared in accordance with accounting principles generally accepted in the United States ("U.S. GAAP"). These consolidated financial statements include the accounts of the Company and MethylGene. All significant inter-company transactions, balances and expenses have been eliminated upon consolidation. Mirati was incorporated under the laws of the State of Delaware on April 29, 2013. On May 8, 2013, the Company's Board of Directors approved and the Company entered into an arrangement agreement ("Arrangement") with MethylGene. Upon completion of the Arrangement, MethylGene became the Company's wholly-owned subsidiary. These consolidated financial statements are presented in United States ("U.S.") Dollars, which is also the functional currency of the Company. Use of Estimates The preparation of the Company's consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ materially from those estimates. Estimates and assumptions are reviewed quarterly. Any revisions to accounting estimates are recognized in the period in which the estimates are revised and in any future periods affected. Cash, Cash Equivalents and Short-term Investments Cash and cash equivalents consist of cash and highly liquid securities with original maturities at the date of acquisition of ninety days or less. Investments with an original maturity of more than ninety days are considered short-term investments and have been classified by management as available-for-sale. These investments are classified as current assets, even though the stated maturity date may be one year or more beyond the current balance sheet date, which reflects management’s intention to use the proceeds from sales of these securities to fund its operations, as necessary. Such investments are carried at fair value, with unrealized gains and losses included as a separate component of stockholders’ equity. Realized gains and losses from the sale of available-for-sale securities or the amounts, net of tax, reclassified out of accumulated other comprehensive income, if any, are determined on a specific identification basis. Concentration of Credit Risk The Company invests its excess cash in accordance with its investment policy. The Company's investments are comprised primarily of commercial paper and debt instruments of financial institutions, corporations, U.S. government-sponsored agencies and the U.S. Treasury. The Company mitigates credit risk by maintaining a diversified portfolio and limiting the amount of investment exposure as to institution, maturity and investment type. Financial instruments that potentially subject the Company to significant credit risk consist principally of cash equivalents and short-term investments. Revenue Recognition Effective January 1, 2017, the Company adopted Accounting Standards Codification, or ASC, Topic 606, Revenue from Contracts with Customers , using the full retrospective transition method. As the Company did not have any revenue contracts prior to the first quarter of 2018, an adjustment to prior periods under this method was not applicable. This standard applies to all contracts with customers, except for contracts that are within the scope of other standards, such as leases, insurance, collaboration arrangements and financial instruments. Under Topic 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration that the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of Topic 606, the entity performs the following five steps: (i) identify the contract(s) 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 in the contract; and (v) recognize revenue when (or as) the entity satisfies a performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. At contract inception, once the contract is determined to be within the scope of Topic 606, the Company assesses the goods or services promised within each contract and determines those that are performance obligations, and assesses whether each promised good or service is distinct. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied. Property and Equipment, Net Property and equipment is stated at historical cost less accumulated depreciation. Historical cost includes expenditures that are directly attributable to the acquisition of the items. All repairs and maintenance are charged to net loss during the financial period in which they are incurred. Depreciation of property and equipment is calculated using the straight-line method over the estimated useful lives of the assets, as follows: Computer equipment 3 years Office and other equipment 6 years Laboratory equipment 6 years Leasehold improvements The lesser of the lease term or the life of the asset Upon disposal or impairment of property and equipment, the cost and related accumulated depreciation is removed from the consolidated financial statements and the net amount, less any proceeds, is included in net loss. Impairment of Long-Lived Assets The Company reviews its long-lived assets for impairment whenever events or changes in circumstances indicate that the carrying value of an asset may not be recoverable. If such circumstances are determined to exist, an estimate of undiscounted future cash flows produced by the long-lived asset, including its eventual residual value, is compared to the carrying value to determine whether impairment exists. In the event that such cash flows are not expected to be sufficient to recover the carrying amount of the assets, the assets are written-down to their estimated fair values. Fair value is estimated through discounted cash flow models to project cash flows from the asset. The Company recognized no impairment charges for the years ended December 31, 2018 , 2017 and 2016 . Share-Based Compensation The Company measures and recognizes compensation expense for share-based payments based on estimated fair value, using the fair value of stock options granted using the Black-Scholes option-pricing model. The Black-Scholes option-pricing model requires the use of certain estimates and judgmental assumptions that affect the amount of share-based compensation expense recognized in the Company's consolidated financial statements. These assumptions include the historical volatility of the Company's stock price, expected term of the options, the risk-free interest rate and expected dividend yields. Share-based compensation is recognized using the graded accelerated vesting method. Research and Development Expenses Research and development expenditures are charged to net loss in the period in which they are incurred and are comprised of the following types of costs incurred in performing research and development activities: contract services for clinical trials and related clinical manufacturing costs, salaries and benefits including share-based compensation expense, costs for allocated facilities and depreciation of equipment and license fees paid in connection with our early discovery efforts. General and Administrative Expenses General and administrative expenses consist primarily of salaries and related benefits, including share-based compensation, related to our executive, finance, business development, legal, human resources and support functions. Other general and administrative expenses include professional fees for auditing, tax, consulting and patent-related services, rent and utilities and insurance. Income Taxes Income taxes have been accounted for using the asset and liability method. Under the asset and liability method, deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial carrying amounts of existing assets and liabilities and their respective tax bases and operating loss and tax credit carryforwards. Deferred tax assets and liabilities are measured using enacted tax rates applicable to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in net loss in the period that includes the enactment date. A valuation allowance against deferred tax assets is recorded if, based upon the weight of all available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized. For uncertain tax positions that meet "a more likely than not" threshold, the Company recognizes the benefit of uncertain tax positions in the consolidated financial statements. Segment Reporting Operating segments are components of a business where separate discrete financial information is available for evaluation by the chief operating decision-maker for purposes of making decisions regarding resource allocation and assessing performance. To date, the Company and the chief operating decision-maker has viewed its operations and managed its business as one segment operating primarily in the United States. Net Loss per Share Basic net loss per common share is calculated by dividing the net loss by the weighted-average number of common shares outstanding during the period, without consideration for potentially dilutive securities. Diluted net loss per share is computed by dividing the net loss attributable to common stockholders by the weighted-average number of common shares and potentially dilutive securities outstanding for the period. Common share equivalents outstanding, determined using the treasury stock method, are comprised of shares that may be issued under the Company’s stock option and warrant agreements. The following table presents the weighted average number of potentially dilutive securities not included in the calculation of diluted net loss per share due to the anti-dilutive effect of the securities: Year ended December 31, 2018 2017 2016 Common stock options 1,781,388 38,675 173,776 Common stock warrants 11,631,636 7,534,576 315,834 Total 13,413,024 7,573,251 489,610

RECENTLY ISSUED AND RECENTLY AD

RECENTLY ISSUED AND RECENTLY ADOPTED ACCOUNTING PRONOUNCEMENTS	12 Months Ended
	Dec. 31, 2018
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
RECENTLY ISSUED ACCOUNTING PRONOUNCEMENTS AND RECENTLY ADOPTED ACCOUNTING PRONOUNCEMENTS	Recently Issued and Recently Adopted Accounting Pronouncements From time to time, new accounting pronouncements are issued by the Financial Accounting Standards Board ("FASB") or other standard setting bodies that are adopted by the Company as of the specified effective date. Recently Adopted Accounting Pronouncements In June 2018, the FASB issued Accounting Standards Update, or ASU 2018-07, Compensation-Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting , which expands the scope of Topic 718 to include share-based payment transactions for acquiring goods and services from nonemployees. An entity should apply the requirements of Topic 718 to nonemployee awards except for specific guidance on inputs to an option pricing model and the attribution of cost (that is, the period of time over which share-based payment awards vest and the pattern of cost recognition over that period). The amendments specify that Topic 718 applies to all share-based payment transactions in which a grantor acquires goods or services to be used or consumed in a grantor’s own operations by issuing share-based payment awards. The amendments also clarify that Topic 718 does not apply to share-based payments used to effectively provide (1) financing to the issuer or (2) awards granted in conjunction with selling goods or services to customers as part of a contract accounted for under Topic 606. The Company early adopted ASU 2018-07 in the third quarter of 2018. The adoption of ASU 2018-07 did not have a material impact on the Company's consolidated financial statements or related financial statement disclosures. In May 2017, the FASB issued ASU 2017-09, Compensation-Stock Compensation (Topic 718): Scope of Modification Accounting , to provide clarity and reduce both 1) diversity in practice and 2) cost and complexity when applying the guidance in Topic 718 to a change in the terms or conditions of a share-based payment award. ASU 2017-09 provided guidance about which changes to the terms or conditions of a share-based payment award require an entity to apply modification accounting under Topic 718. The amendments in ASU 2017-09 were effective for fiscal and interim reporting periods in fiscal years beginning after December 15, 2017. The amendments in ASU 2017-09 should be applied prospectively to an award modified on or after the adoption date. Effective January 1, 2018, the Company adopted the provisions of ASU 2017-09. The adoption did not have a material impact on the Company's consolidated financial statements or related financial statement disclosures. In January 2016, the FASB issued ASU 2016-01, Financial Instruments - Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities . The new guidance enhanced the reporting model for financial instruments and includes amendments to address aspects of recognition, measurement, presentation and disclosure. The update to the standard was effective for public companies for interim and annual periods beginning after December 15, 2017. Effective January 1, 2018, the Company adopted the provisions of ASU 2016-01. The adoption did not have a material impact on the Company's consolidated financial statements or related financial statement disclosures. Recently Issued Accounting Pronouncements In November 2018, the FASB issued ASU 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606 . The amendments provide guidance on whether certain transactions between collaborative arrangement participants should be accounted for as revenue under ASC 606. It also specifically (i) addresses when the participant should be considered a customer in the content of a unit of account, (ii) adds unit-of-account guidance in ASC 808 to align with guidance with ASC 606, and (iii) precludes presenting revenue from a collaborative arrangement together with revenue recognized under ASC 606 if the collaborative arrangement participant is not a customer. The guidance in ASU 2018-18 is effective for fiscal years beginning after December 15, 2019, and interim periods within those fiscal years. Early adoption is permitted and should be applied retrospectively. The Company is currently evaluating this guidance to determine the impact it may have on its consolidated financial statements. In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement . The new guidance modifies the disclosure requirements on fair value measurements in Topic 820. The amendments in ASU 2018-13 are effective for all entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. The Company does not anticipate that the adoption of ASU 2018-13 will have a material impact on the Company's consolidated financial statements or related financial statement disclosures. In July 2017, the FASB issued ASU 2017-11, Earnings Per Share (Topic 260); Distinguishing Liabilities from Equity (Topic 480); Derivatives and Hedging (Topic 815): (Part I) Accounting for Certain Financial Instruments with Down Round Features, (Part II) Replacement of the Indefinite Deferral for Mandatorily Redeemable Financial Instruments of Certain Nonpublic Entities and Certain Mandatorily Redeemable Non-controlling Interests with a Scope Exception . The ASU allows companies to exclude a down round feature when determining whether a financial instrument (or embedded conversion feature) is considered indexed to the entity’s own stock. As a result, financial instruments (or embedded conversion features) with down round features may no longer be required to be classified as liabilities. A company will recognize the value of a down round feature only when it is triggered and the strike price has been adjusted downward. For equity-classified freestanding financial instruments, such as warrants, an entity will treat the value of the effect of the down round, when triggered, as a dividend and a reduction of income available to common shareholders in computing basic earnings per share. For convertible instruments with embedded conversion features containing down round provisions, entities will recognize the value of the down round as a beneficial conversion discount to be amortized to earnings. The guidance in ASU 2017-11 is effective for fiscal years beginning after December 15, 2018, and interim periods within those fiscal years. Early adoption is permitted, and the guidance is to be applied using a full or modified retrospective approach. The Company does not anticipate that the adoption of ASU 2017-11 will have a material impact on its consolidated financial statements unless a transaction occurs that would need to be evaluated under this guidance at which time the Company will assess the impact of this standard. In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) in order to increase transparency and comparability among organizations by recognizing lease assets and lease liabilities on the balance sheet for those leases classified as operating leases under previous GAAP. ASU 2016-02 requires a lessee to recognize a liability for lease payments (the lease liability) and a right-of-use asset (representing its right to use the underlying asset for the lease term) on the balance sheet. ASU 2016-02 is effective for fiscal years beginning after December 15, 2018 (including interim periods within those periods) using a modified retrospective approach and early adoption is permitted. In July 2018, the FASB issued ASU 2018-11, Leases (Topic 842): Targeted Improvements , provides entities an optional transition method to apply the new guidance as of the adoption date, rather than as of the earliest period presented. In transition, entities may also elect a package of practical expedients that must be applied in its entirety to all leases commencing before the effective date, unless the lease was modified, to not reassess (a) the existence of a lease, (b) lease classification or (c) determination of initial direct costs, which effectively allows entities to carryforward accounting conclusions under previous U.S. GAAP. The Company plans to adopt the standard on January 1, 2019, using the optional transition method to apply the new guidance as of January 1, 2019, rather than as of the earliest period presented, and elect the package of practical expedients described above. The Company currently believes the most significant changes will be related to the recognition of new right-of-use assets and lease liabilities on the Company's consolidated balance sheet for real estate operating leases, and expects to recognize lease liabilities ranging from $300,000 to $400,000 on January 1, 2019, with corresponding right-of-use assets of approximately the same amount based on the present value of the remaining minimum rental payments.

INVESTMENTS

INVESTMENTS	12 Months Ended
INVESTMENTS	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
INVESTMENTS	Investments The following tables summarize the Company's short-term investments (in thousands): As of December 31, 2018 Maturity Amortized cost Gross unrealized gains Gross unrealized losses Estimated fair value Corporate debt securities 1 year or less $ 111,933 $ 26 $ (43 ) $ 111,916 Commercial paper 1 year or less 74,433 — (24 ) 74,409 U.S. Treasury bills 1 year or less 3,771 — — 3,771 $ 190,137 $ 26 $ (67 ) $ 190,096 As of December 31, 2017 Maturity Amortized cost Gross unrealized gains Gross unrealized losses Estimated fair value Corporate debt securities 1 year or less $ 24,264 $ — $ (23 ) $ 24,241 Commercial paper 1 year or less 18,912 — (19 ) 18,893 $ 43,176 $ — $ (42 ) $ 43,134 Unrealized gains and losses on available-for-sale securities are included as a component of comprehensive loss. At December 31, 2018 , the Company did not have any securities in material unrealized loss positions. The Company reviews its investments to identify and evaluate investments that have an indication of possible other-than-temporary impairment. Factors considered in determining whether a loss is other-than-temporary include the length of time and extent to which fair value has been less than the cost basis, the financial condition and near-term prospects of the investee, and the Company’s intent and ability to hold the investment for a period of time sufficient to allow for any anticipated recovery in market value. The Company does not intend to sell any investments prior to recovery of their amortized cost basis for any investments in an unrealized loss position.

FAIR VALUE MEASUREMENTS

FAIR VALUE MEASUREMENTS	12 Months Ended
FAIR VALUE MEASUREMENTS	Dec. 31, 2018
Fair Value Disclosures [Abstract]
FAIR VALUE MEASUREMENTS	Fair Value Measurements The Company has certain financial assets and liabilities recorded at fair value which have been classified as Level 1 or 2 within the fair value hierarchy as described in the accounting standards for fair value measurements. The Company has no financial assets or liabilities recorded at fair value which have been classified as Level 3. The authoritative guidance for fair value measurements defines fair value as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or the most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Market participants are buyers and sellers in the principal market that are (i) independent, (ii) knowledgeable, (iii) able to transact, and (iv) willing to transact. The guidance prioritizes the inputs used in measuring fair value into the following hierarchy: • Level 1- Quoted prices (unadjusted) in active markets for identical assets or liabilities; • Level 2- Inputs other than quoted prices included within Level 1 that are either directly or indirectly observable; and • Level 3- Unobservable inputs in which little or no market activity exists, therefore requiring an entity to develop its own assumptions about the assumptions that market participants would use in pricing. The following table summarizes the assets and liabilities measured at fair value on a recurring basis (in thousands): December 31, 2018 Total Level 1 Level 2 Level 3 Assets Cash and cash equivalents: Cash $ 3,731 $ 3,731 $ — $ — Money market funds 28,963 28,963 — — Total cash and cash equivalents 32,694 32,694 — — Short-term investments: U.S. Treasury bills 3,771 3,771 — — Corporate debt securities 111,916 — 111,916 — Commercial paper 74,409 — 74,409 — Total short-term investments 190,096 3,771 186,325 — Total $ 222,790 $ 36,465 $ 186,325 $ — December 31, 2017 Total Level 1 Level 2 Level 3 Assets Cash and cash equivalents: Cash $ 1,026 $ 1,026 $ — $ — Money market funds 106,677 106,677 — — Total cash and cash equivalents 107,703 107,703 — — Short-term investments: Corporate debt securities 24,241 — 24,241 — Commercial paper 18,893 — 18,893 — Total short-term investments 43,134 — 43,134 — Total $ 150,837 $ 107,703 $ 43,134 $ — The Company’s investments in Level 1 assets are valued based on publicly available quoted market prices for identical securities as of December 31, 2018 and 2017 . The Company determines the fair value of Level 2 related securities with the aid of valuations provided by third parties using proprietary valuation models and analytical tools. These valuation models and analytical tools use market pricing or prices for similar instruments that are both objective and publicly available, including matrix pricing or reported trades, benchmark yields, broker/dealer quotes, issuer spreads, two-sided markets, benchmark securities, bids and/or offers. There were no transfers between fair value measurement levels for the years ended December 31, 2018 and 2017 .

OTHER CURRENT ASSETS AND LONG-T

OTHER CURRENT ASSETS AND LONG-TERM ASSETS	12 Months Ended
OTHER CURRENT ASSETS AND LONG-TERM ASSETS	Dec. 31, 2018
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
OTHER CURRENT ASSETS AND LONG-TERM ASSETS	Other Current Assets and Other Long-Term Assets Other current assets consisted of the following (in thousands): December 31, 2018 2017 Prepaid expenses $ 1,261 $ 3,085 Deposits and other receivables 1,841 1,600 Interest receivables 768 237 $ 3,870 $ 4,922 The other long-term assets balance as of December 31, 2018 consists of $1.3 million in deposits paid in conjunction with the Company's research and development activities compared to $1.0 million as of December 31, 2017 .

PROPERTY AND EQUIPMENT

PROPERTY AND EQUIPMENT	12 Months Ended
PROPERTY AND EQUIPMENT	Dec. 31, 2018
Property, Plant and Equipment [Abstract]
PROPERTY AND EQUIPMENT	Property and Equipment, Net Property and equipment, net consisted of the following (in thousands): December 31, 2018 2017 Computer equipment $ 201 $ 329 Office and other equipment 260 301 Laboratory equipment 729 643 Leasehold improvements 63 63 Gross property and equipment 1,253 1,336 Less: Accumulated depreciation (780 ) (811 ) Property and equipment, net $ 473 $ 525 The Company incurred depreciation expense of $0.2 million during the years ended December 31, 2018 , 2017 and 2016 , respectively.

ACCOUNTS PAYABLE AND ACCRUED LI

ACCOUNTS PAYABLE AND ACCRUED LIABILITIES	12 Months Ended
ACCOUNTS PAYABLE AND ACCRUED LIABILITIES	Dec. 31, 2018
Payables and Accruals [Abstract]
ACCOUNTS PAYABLE AND ACCRUED LIABILITIES	Accounts Payable and Accrued Liabilities Accounts payable and accrued liabilities consisted of the following (in thousands): December 31, 2018 2017 Accounts payable $ 8,531 $ 4,344 Accrued clinical expense 10,154 5,412 Accrued development and other expense 1,243 867 Accrued compensation and benefits 5,847 3,021 $ 25,775 $ 13,644 The long-term liabilities balance of $0.7 million as of December 31, 2018 consisted of $0.1 million in deferred revenue and $0.6 million in other liabilities. The long-term liabilities of $0.3 million as of December 31, 2017 consisted of $0.3 million in other liabilities.

BEIGENE AGREEMENT

BEIGENE AGREEMENT	12 Months Ended
BEIGENE AGREEMENT	Dec. 31, 2018
Research and Development [Abstract]
BEIGENE AGREEMENT	9. BeiGene Agreement Terms of Agreement On January 7, 2018, the Company and BeiGene Ltd, ("BeiGene") entered into a Collaboration and License Agreement (the “Agreement”), pursuant to which the Company and BeiGene agreed to collaboratively develop sitravatinib in Asia (excluding Japan and certain other countries), Australia and New Zealand (the “Licensed Territory”). Under the Agreement, the Company granted BeiGene an exclusive license to develop, manufacture and commercialize sitravatinib in the Licensed Territory, with Mirati retaining exclusive rights for the development, manufacture and commercialization of sitravatinib outside the Licensed Territory. As consideration for the rights granted to BeiGene under the Agreement, BeiGene paid the Company a non-refundable, non-creditable up-front fee of $10.0 million . BeiGene is also required to make milestone payments to the Company of up to an aggregate of $123.0 million upon the first achievement of specified clinical, regulatory and sales milestones. The Agreement additionally provides that BeiGene is obligated to pay to the Company royalties at tiered percentage rates ranging from mid-single digits to twenty percent on annual net sales of licensed products in the Licensed Territory, subject to reduction under specified circumstances. The Agreement also provides that the Company will supply BeiGene with sitravatinib for use in BeiGene’s development activities in the Licensed Territory. The Agreement will terminate upon the expiration of the last royalty term for the licensed products, which is the latest of (i) the date of expiration of the last valid patent claim related to the licensed products under the Agreement, (ii) 10 years after the first commercial sale of a licensed product and (iii) the expiration of any regulatory exclusivity as to a licensed product. BeiGene may terminate the Agreement at any time by providing 60 days prior written notice to the Company. Either party may terminate the Agreement upon a material breach by the other party that remains uncured following 60 days after the date of written notice of such breach or upon certain bankruptcy events. In addition, the Company may terminate the Agreement upon written notice to BeiGene under specified circumstances if BeiGene challenges the licensed patent rights. Revenue Recognition The Company evaluated the Agreement under Topic 606. In determining the appropriate amount of revenue to be recognized as the Company fulfills its obligations under the Agreement, the Company performed the following steps: (i) identified the promised goods or services in the contract; (ii) determined whether the promised goods or services are performance obligations including whether they are distinct in the context of the contract; (iii) measured the transaction price, including any constraints on variable consideration; (iv) allocated the transaction price to the performance obligations; and (v) recognized revenue when (or as) the Company satisfied each performance obligation. The Company determined the transaction price is equal to the up-front fee of $10.0 million . The transaction price was allocated to the performance obligations on the basis of the relative stand-alone selling price estimated for each performance obligation. In estimating the stand-alone selling price for each performance obligation, the Company developed assumptions that require judgment and included forecasted revenues, expected development timelines, discount rates, probabilities of technical and regulatory success and costs for manufacturing clinical supplies. A description of the performance obligations identified under the Agreement, as well as the amount of revenue allocated to each performance obligation, follows: Licenses of Intellectual Property. The license to the Company’s intellectual property, bundled with the associated know-how, represents a distinct performance obligation. The license and associated know-how was transferred to BeiGene during the year ended December 31, 2018 , therefore the Company recognized the full revenue related to this performance obligation in the amount of $9.5 million during the year ended December 31, 2018 as license and collaboration revenues in its condensed consolidated statements of operations and comprehensive loss. Manufacturing Supply Services. The Company's initial obligation to supply sitravatinib for clinical development in the Licensed Territory represents a distinct performance obligation. As such, the Company deferred $0.5 million of revenue related to the manufacturing supply services, and will recognize revenue when BeiGene obtains control of the goods, upon delivery, and which began in late 2018 and will continue into 2020. The Company and BeiGene share equally in the external costs of supplying sitravatinib for clinical development in the Licensed Territory for a limited period of time. The Company records these cost-sharing payments due from BeiGene as license and collaboration revenues. The Company recognized $0.5 million for this performance obligation for the year ended December 31, 2018 , of which $0.1 million relates to recognition from the deferred revenue balance, and $0.4 million relates to cost-sharing payments due from BeiGene during 2018 . At December 31, 2018 , $0.3 million of cost-sharing receivable from BeiGene has been recorded in other current assets on the consolidated balance sheets. Milestone Payments. During the year ended December 31, 2018 , the Company recognized $3.0 million in connection with a milestone payment from BeiGene for initiating the first clinical trial in the Licensed Territory and recorded the milestone payment as license and collaboration revenues. The next clinical development milestone is for BeiGene initiating the first pivotal clinical trial in the Licensed Territory upon which the Company will be paid a $5.0 million milestone payment. The Company is also entitled to certain regulatory milestone payments which are paid upon receipt of regulatory approvals within the Licensed Territory. The Company determined that as of December 31, 2018 , the remaining potential milestone payments are probable of significant revenue reversal as their achievement is highly dependent on factors outside the Company's control. Therefore, these payments have been fully constrained and are therefore not recognized as revenue. At the end of each subsequent reporting period, the Company will re-evaluate the probability of achievement of each milestone and any related constraint, and if necessary, adjust its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect the reported amount of license and collaboration revenues in the period of adjustment. Royalties. As the license is deemed to be the predominant item to which sales-based royalties relate, the Company will recognize revenue when the related sales occur. No royalty revenue was recognized during the year ended December 31, 2018 . The following table presents a summary of the activity in the Company's contract liabilities during the year ended December 31, 2018 (in thousands): Opening balance, January 1, 2018 $ — Revenue from performance obligations satisfied during reporting period 52 Payments received in advance (533 ) Closing Balance, December 31, 2018 $ (481 ) The closing balance as of December 31, 2018 includes $0.4 million in deferred revenue within current liabilities, and $0.1 million in deferred revenue and other liabilities.

STOCKHOLDERS' EQUITY

STOCKHOLDERS' EQUITY	12 Months Ended
STOCKHOLDERS' EQUITY	Dec. 31, 2018
Stockholders' Equity Note [Abstract]
STOCKHOLDERS' EQUITY	Stockholders' Equity Common Stock The following shares were reserved for future issuance: December 31, 2018 Common stock options outstanding and available for future grant 5,574,901 Warrants to purchase common stock 11,817,912 Employee Stock Purchase Plan 157,360 17,550,173 Sale of Common Stock In June 2018, the Company sold 3,162,500 shares of its common stock at a public offering price of $38.85 per share and sold warrants to purchase up to 421,650 shares of its common stock at a public offering price of $38.849 per warrant. After deducting underwriter discounts and offering expenses, the Company received net proceeds from the transaction of $130.7 million . These warrants were recorded as a component of stockholders’ equity within additional paid-in capital. Per their terms, the outstanding warrants to purchase shares of common stock may not be exercised if certain holders' ownership of the Company’s common stock would exceed 9.99 percent for certain holders, and 19.99 percent for other holders, following such exercise. In November 2017, the Company sold 2,938,986 shares of its common stock at a public offering price of $13.00 per share and sold warrants to purchase up to 4,137,999 shares of its common stock at a public offering price of $12.999 per warrant share. After deducting underwriter discounts and offering expenses, the Company received net proceeds from the transaction of $86.7 million . In January 2017, the Company sold 5,002,702 shares of its common stock at a public offering price of $5.60 per share and sold warrants to purchase up to 7,258,263 shares of its common stock at a public offering price of $5.599 per warrant share. After deducting underwriter discounts and offering expenses, the Company received net proceeds from the transaction of $66.8 million . In all cases, the public offering price for the warrants was equal to the public offering price of the common stock, less the $0.001 per share exercise price of each warrant. These warrants were recorded as a component of stockholders’ equity within additional paid-in capital. Per their terms, the outstanding warrants to purchase shares of common stock may not be exercised if the holder’s ownership of the Company’s common stock would exceed 9.99 percent for certain holders, and 19.99 percent for other holders, following such exercise. Warrants As of December 31, 2018 , the following warrants for common stock were issued and outstanding: Issue date Expiration date Exercise price Number of warrants outstanding January 11, 2017 None $ 0.001 7,258,263 November 20, 2017 None $ 0.001 4,137,999 June 11, 2018 None $ 0.001 421,650 11,817,912 During the year ended December 31, 2018 , no warrants were exercised. During the year ended December 31, 2017 , warrants for 52,825 shares of the Company's common stock were exercised via cashless exercises and 585,729 shares were exercised for cash, generating proceeds of $4.6 million and resulting in the issuance of 638,554 shares of common stock. During the year ended December 31, 2016 , warrants for 289,789 shares of the Company's common stock were exercised via cashless exercises and 313,756 shares were exercised for cash, generating proceeds of $2.1 million and resulting in the issuance of 603,545 shares of common stock.

SHARE-BASED COMPENSATION

SHARE-BASED COMPENSATION	12 Months Ended
SHARE-BASED COMPENSATION	Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
SHARE-BASED COMPENSATION	Share-Based Compensation Equity Incentive Plan The Company has in place a stock option plan (the "Stock Option Plan") for the benefit of employees, directors, officers and consultants of the Company. In May 2013 our Board of Directors adopted the 2013 Equity Incentive Plan (the "2013 Plan"). The 2013 Plan was approved by our stockholders in connection with the Arrangement. Our Board of Directors and stockholders approved an amendment to the 2013 Plan in 2017 to, among other things, increase the aggregate number of shares of common stock authorized for issuance under the 2013 Plan by 2.9 million shares. The 2013 Plan is a continuation of and successor to the Stock Option Plan and no further grants will be made under the Stock Option Plan. As of December 31, 2018 , there were approximately 1.9 million stock options available to be issued. To date, share-based compensation awards under both the Stock Option Plan and the 2013 Plan consist of incentive and non-qualified stock options. Stock options granted under each of the plans must have an exercise price equal to at least 100% of the fair market value of our common stock on the date of grant and generally vest over four years. The Stock Option Plan has a contractual term of seven years and the 2013 Plan has contractual terms ranging from seven to ten years. The following table summarizes our stock option activity and related information for the year ended December 31, 2018 : Number of options Weighted average exercise price Weighted-Average Remaining Contractual Term (years) Aggregate Intrinsic Value (millions) Balance outstanding as of December 31, 2017 3,304,181 $ 13.67 Granted 1,261,693 $ 31.84 Exercised (731,935 ) $ 13.32 Canceled/forfeited (150,153 ) $ 22.83 Balance outstanding as of December 31, 2018 3,683,786 $ 19.59 7.3 $ 85.6 Options exercisable at December 31, 2018 1,910,058 $ 16.05 6.1 $ 50.4 The total intrinsic value of stock options exercised was $18.8 million , $0.3 million and $0.3 million for the years ended December 31, 2018 , 2017 , and 2016 , respectively. The Company received total cash of $9.7 million , $0.4 million and $0.2 million for the exercise of options for the years ended December 31, 2018 , 2017 and 2016 , respectively. The total fair value of options vested during the years ended December 31, 2018 , 2017 and 2016 was $8.2 million , $10.6 million and $8.6 million , respectively. Upon option exercise, the Company issues new shares of its common stock. Total share-based compensation expense by statement of operations and comprehensive loss classification is presented below (in thousands): Year ended December 31, 2018 2017 2016 Research and development expense $ 7,232 $ 3,192 $ 5,461 General and administrative expense 8,622 3,594 5,163 $ 15,854 $ 6,786 $ 10,624 For the years ended December 31, 2018 , 2017 and 2016 , no share-based compensation expense was capitalized and there were no recognized tax benefits associated with the share-based compensation charge. The fair value of options granted is estimated at the date of grant using the Black-Scholes option pricing model. Forfeitures are accounted for as incurred as reversal of any share-based compensation expense related to options that will not vest. The assumptions used for the specified reporting periods and the resulting estimates of weighted-average estimated fair value per share of options granted during those periods are as follows: Year Ended December 31, 2018 2017 2016 Risk-free interest rate 2.6% 2.1% 1.5% Dividend yield —% —% —% Volatility factor 94.3% 96.0% 101.7% Expected term (in years) 6.0 6.0 6.0 Weighted average estimated fair value per share $24.39 $4.17 $13.32 Risk-Free Interest Rate - The risk-free interest rate is the rate for periods equal to the expected term of the stock option based on U.S. Treasury zero-coupon bonds. Dividend Yield - The dividend yield is based on the Company’s history and expectation of dividend payouts. The Company has not paid, and does not intend to pay dividends. Volatility Factor - The expected volatility assumption was determined by examining the historical volatility of the Company's stock. Expected Term - The expected term represents the weighted average period the stock options are expected to be outstanding. The Company uses the simplified method for estimating the expected term as provided by the Securities and Exchange Commission. The simplified method calculates the expected term as the average time-to-vesting and the contractual life of the options. The Company believes this methodology is appropriate given the Company's limited history as a U.S. public company. The total compensation cost not yet recognized as of December 31, 2018 related to non-vested option awards was $17.1 million which will be recognized over a weighted-average period of 1.2 years. 2013 Employee Stock Purchase Plan In May 2013, the Company's Board of Directors adopted the ESPP. The ESPP was approved by the Company's stockholders in connection with the Arrangement. In December 2014, the ESPP became effective and the first purchase period began. The ESPP permits eligible employees to make payroll deductions to purchase up to $25,000 of the Company’s common stock on regularly scheduled purchase dates at a discount. Offering periods under the ESPP are not more than six months in duration and shares are purchased at 85% of the lower of the closing price for the Company’s common stock on the first day of the offering period or the date of purchase. The ESPP initially authorized the issuance of 300,000 shares of the Company’s common stock pursuant to rights granted to employees for their payroll deductions. As of December 31, 2018 , 142,640 shares have been issued out of the plan.

EMPLOYEE BENEFIT PLAN

EMPLOYEE BENEFIT PLAN	12 Months Ended
EMPLOYEE BENEFIT PLAN	Dec. 31, 2018
Retirement Benefits [Abstract]
EMPLOYEE BENEFIT PLAN	Employee Benefit Plan The Company has a defined contribution 401(k) plan (the "Plan") for all employees. Employees are eligible to participate in the Plan if they are at least 21 years of age or older. Under the terms of the Plan, employees may make voluntary contributions as a percentage of compensation. The Company matches up to 4% of an employee's contributions, subject to a limit of $2,500 per year. Expense associated with the Company's matching contribution totaled $0.1 million for the years ended December 31, 2018 , 2017 and 2016 , respectively.

INCOME TAXES

INCOME TAXES	12 Months Ended
INCOME TAXES	Dec. 31, 2018
Income Tax Disclosure [Abstract]
INCOME TAXES	Income Taxes The Company had no federal income tax expense and immaterial state tax expense for the years ended December 31, 2018 , 2017 and 2016 . The differences between the effective income tax rate and the statutory tax rates during the years ended 2018 , 2017 and 2016 are as follows (in thousands): Year Ended December 31, 2018 2017 2016 Net loss before tax $ (98,418 ) $ (70,430 ) $ (83,118 ) Statutory combined U.S. federal and state tax rate 21.00 % 34.00 % 34.00 % Statutory federal and state taxes (20,668 ) (23,946 ) (28,260 ) Increase (decrease) in taxes recoverable resulting from: Effect of change in valuation allowance 25,959 (4,154 ) 28,446 Non-deductible share-based compensation 884 695 1,247 Tax deductions for share-based compensation (2,924 ) (80 ) (12 ) Tax credits (5,130 ) (2,563 ) (2,906 ) Share issue costs - temporary difference — — (78 ) Write off of Methylgene US Inc. net operating loss — 307 — Differential in income tax rates of foreign subsidiary (1 ) (169 ) 261 Change in tax rate — 303 — Tax Cuts and Jobs Act — 28,569 — Uncertain tax positions 1,283 646 3,921 Return to provision and other true-ups 375 394 (2,619 ) Other differences 222 (2 ) — Income tax benefit $ — $ — $ — Deferred Tax The following table summarizes the significant components of our deferred tax assets (in thousands): December 31, 2018 2017 Deferred tax assets: Tangible and intangible depreciable assets $ 8,123 $ 17,927 Stock compensation 6,515 4,976 Provisions 1,020 611 Net operating loss carry forwards 74,721 44,598 Capital loss carryforward 178 83 Canada scientific research and experimental development expenditures 5,467 5,467 U.S. research and development tax credits 10,613 7,016 Total gross deferred tax assets 106,637 80,678 Less valuation allowance (106,637 ) (80,678 ) Net deferred tax assets $ — $ — Total valuation allowance increased by $26.0 million for the year ended December 31, 2018 . The Company has established a full valuation allowance against its deferred tax assets as of December 31, 2018 due to the uncertainty surrounding the realization of such assets as evidenced by the cumulative losses from operations through December 31, 2018 . For Canadian federal income tax purposes, the Company's Canadian federal scientific research and experimental development expenditures amounted to $19.9 million at December 31, 2018 , 2017 and 2016 and for provincial income tax purposes amounted to $21.6 million at December 31, 2018 , 2017 and 2016 . As operations in Canada ceased during 2014, no expenditures were incurred for the years ended December 31, 2018 , 2017 and 2016 . These expenditures are available to reduce future taxable income and have an unlimited carry forward period. Scientific research and development expenditures are subject to verification by the taxation authorities, and accordingly, these amounts may vary by a material amount. In addition, the Company has research and development tax credit carryforwards for U.S. federal and state income tax purposes as of December 31, 2018 of $10.6 million and $4.7 million , respectively. The federal credits will begin to expire in 2033 unless utilized and the state credits have an indefinite life. At December 31, 2018 , the Company's net operating loss carry forwards ("NOLs") for U.S. federal and state income taxes were $254.6 million and $77.3 million , respectively and the Company's NOLs for Canadian federal and provincial income tax purposes were $79.9 million and $79.2 million , respectively. The NOLs are available to offset future taxable income from both U.S. federal and state tax sources, as well as Canadian federal and provincial tax sources and the tax benefits of which have not been recognized in the consolidated financial statements. The NOLs expire as follows (in thousands): US Canada Federal State Federal Provincial Expires in: 2030 $ — $ — $ 5,907 $ 5,985 2031 — — 7,059 7,066 2032 — — 13,308 12,433 2033 2,225 2,232 18,623 19,385 2034 7,276 22,162 32,401 31,809 2035 53,359 52,950 1,084 1,084 2036 23,379 — 777 777 2037 65,509 — 697 697 2038 — — 7 $ 7 Does not expire 102,863 — — $ — $ 254,611 $ 77,344 $ 79,863 $ 79,243 The future utilization of the U.S. federal and state NOL carryforwards to offset future taxable income may be subject to an annual limitation as a result of ownership changes that may have occurred previously or may occur in the future. The Tax Reform Act of 1986 (the "1986 Act") limits a company's ability to utilize certain tax credit carryforwards and net operating loss carryforwards in the event of a cumulative change in ownership in excess of 50% as defined in the Act. During 2017, the Company completed a study to assess whether an ownership change, as identified by Section 382 of the 1986 Act, had occurred from the Company’s formation through December 31, 2017. The results of the study have been extended until December 31, 2018. Based upon the study, the Company determined an ownership change had occurred during 2017, causing the annual utilization of the NOL and credit carryforwards to be limited. The Company does not believe any of the NOL and credit carryforwards generated through December 31, 2018 would expire solely as a result of annual limitations on the utilization of those attributes. The Canadian Federal and Provincial Tax Acts maintain similar rules in the case of acquisition of control, which may limit the utilization of tax attributes. The Company files income tax returns in the U.S. (federal and state) and Canada (federal and provincial). The Company’s U.S. operations have not been audited for any open taxation years. The Company has experienced losses for U.S. tax purposes and therefore, the taxation authorities may review any loss year, if and when the losses are utilized. For Canadian tax purposes, the Company remains subject to federal and provincial audit for the December 31, 2014 and subsequent taxable years. Where tax years remain open, the Company considers it reasonably possible that issues may be raised or tax positions agreed to with the taxation authorities, which may result in increases or decreases of the balance of non-refundable investment tax credits ("ITCs") and NOLs. However, an estimate of such increases and decreases cannot be currently made. A reconciliation of the beginning and ending amounts of unrecognized tax positions are as follows (in thousands): Federal Provincial/State December 31, December 31, 2018 2017 2016 2018 2017 2016 Unrecognized tax positions, beginning of year $ 1,693 $ 1,095 $ 509 $ 7,556 $ 7,333 $ 2,274 Gross increase — current period tax positions 924 588 598 454 227 195 Gross decrease — prior period tax positions — — (9 ) — (3 ) — Gross increase — prior period tax positions — 11 — — — 4,866 Expiration of statute of limitations — (1 ) (3 ) — (1 ) (2 ) Unrecognized tax positions, end of year $ 2,617 $ 1,693 $ 1,095 $ 8,010 $ 7,556 $ 7,333 If recognized, none of the unrecognized tax positions would impact the Company's income tax benefit or effective tax rate as long as the Company's deferred tax assets remain subject to a full valuation allowance. The Company does not expect any significant increases or decreases to the Company's unrecognized tax positions within the next 12 months. The Company recognizes interest and penalties related to unrecognized tax benefits in income tax expense. The Company had no accrual for interest or penalties on tax matters as of December 31, 2018 and 2017 and the Company had no ongoing tax audits as of December 31, 2018 . On December 22, 2017, the President of the United States signed into law the Tax Cuts and Jobs Act (the "TCJA"). The TCJA made significant changes to U.S. tax laws, including, but not limited to, the following: (a) reducing the federal corporate tax rate from 35% to 21%; (b) eliminating the federal corporate alternative minimum tax ("AMT") and changing how existing AMT credits can be realized; and (c) eliminating several business deductions and credits, including deductions for certain executive compensation in excess of $1 million. As a result of the rate reduction, the Company reduced the deferred tax asset balance as of December 31, 2017 by $28.6 million . Due to the Company's full valuation allowance position, there was no net impact on the Company's income tax provision for the year ended December 31, 2017 as the reduction in the deferred tax asset balance was fully offset by a corresponding decrease in the valuation allowance. In December 2017, the SEC issued Staff Accounting Bulletin No. 118 ("SAB 118"), which provides guidance on accounting for the income tax effects of the TCJA. SAB 118 provides a measurement period that should not extend beyond one year from the TCJA enactment date for companies to complete the accounting related to the accounting relating to the TCJA under Accounting Standards Codification Topic 740, "Income Taxes" ("ASC 740"). In accordance with SAB 118, a company must reflect the income tax effects of those aspects of the TCJA for which the accounting under ASC 740 is complete. To the extent that a company's accounting for TCJA-related income tax effects is incomplete, but the company is able to determine a reasonable estimate, it must record a provisional estimate in its financial statements. If a company cannot determine a provisional estimate to be included in its financial statements, it should continue to apply ASC 740 on the basis of the provisions of the tax laws that were in effect before the enactment of the TCJA. The Company has completed its evaluation of the potential impacts of the TCJA on its December 31, 2018 financial statements and there is no impact on the income tax provision due to the full valuation allowance as of December 31, 2018 .

INVESTMENT TAX CREDITS

INVESTMENT TAX CREDITS	12 Months Ended
INVESTMENT TAX CREDITS	Dec. 31, 2018
INVESTMENT TAX CREDITS
INVESTMENT TAX CREDITS	Investment Tax Credits In prior years, the Company was entitled to claim Canadian federal and provincial ITCs for eligible scientific research and development expenditures. The Company recorded ITCs based on management's best estimates of the amount to be recovered and ITCs claimed are subject to audit by the taxation authorities and accordingly, may vary by a material amount. The Company has not recorded federal or provincial ITCs since the year ended December 31, 2013, as the primary operations of the Company were moved from Canada to San Diego, California in early 2014. The Company's non-refundable Canadian federal ITCs as of December 31, 2018 are $3.9 million and relate to scientific research and development expenditures, which may be utilized to reduce Canadian federal income taxes payable in future years. The benefits of the non-refundable Canadian federal ITCs have not been recognized in the financial statements and will be recorded as a reduction of tax expense when realized. The non-refundable investment tax credits expire as follows (in thousands): Federal ITC Expires in: 2030 $ 764 2031 1,000 2032 1,125 2033 1,018 $ 3,907

COMMITMENTS AND CONTINGENCIES

COMMITMENTS AND CONTINGENCIES	12 Months Ended
COMMITMENTS AND CONTINGENCIES	Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
COMMITMENTS AND CONTINGENCIES	Commitments and Contingencies On June 24, 2014, the Company entered into a lease agreement for completed office and laboratory space located in San Diego, California. The office space under the lease is the Company's corporate headquarters. The lease commenced in two phases (in July 2014 and March 2015) at a combined total initial monthly rent of $24,100 per month. The leased property is subject to a 3% annual rent increase following availability that results in the Company recording deferred rent over the term of the lease. In addition to such base monthly rent, the Company is obligated to pay certain customary amounts for its share of operating expenses and facility amenities. The original lease provided for expiration on January 31, 2018. On March 23, 2017, the Company entered into a First Amendment to Lease Agreement to amend the original lease agreement and to extend the term of the original lease for one year through January 31, 2019. On April 5, 2018, the Company entered into a Second Amendment to Lease Agreement to extend the lease term through January 31, 2020. Subsequently, on August 2, 2018, the Company entered into a Third Amendment to Lease Agreement to expand the size of the existing space for an additional base rent of $4,000 per month. All other terms and covenants from the original lease agreement remain unchanged. Future minimum payments required under the lease are summarized as follows (in thousands): Year Ending December 31: 2019 $ 363 2020 30 Thereafter — Total minimum lease payments $ 393 Total lease expense for the years ended December 31, 2018 , 2017 and 2016 was $0.8 million , $0.7 million , and $0.8 million , respectively.

QUARTERLY FINANCIAL DATA

QUARTERLY FINANCIAL DATA	12 Months Ended
QUARTERLY FINANCIAL DATA	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Quarterly Financial Data	Selected Quarterly Financial Data (Unaudited) The following is a summary of the quarterly results of the Company for the years ended December 31, 2018 and 2017 (unaudited, in thousands, except for per share data): Three Months Ended Year Ended 3/31/18 6/30/18 9/30/18 12/31/18 December 31, 2018 License and collaboration revenues $ 9,467 $ — $ — $ 3,459 $ 12,926 Loss from operations (15,346 ) (28,670 ) (28,939 ) (29,672 ) $ (102,627 ) Net loss (14,709 ) (27,869 ) (27,568 ) (28,272 ) (98,418 ) Basic and diluted net loss per share $ (0.51 ) $ (0.94 ) $ (0.85 ) $ (0.87 ) $ (3.19 ) Three Months Ended Year Ended 3/31/17 6/30/17 9/30/17 12/31/17 December 31, 2017 Loss from operations $ (18,090 ) $ (18,616 ) $ (16,601 ) $ (18,228 ) $ (71,535 ) Net loss (17,846 ) (18,339 ) (16,350 ) (17,895 ) (70,430 ) Basic and diluted net loss per share $ (0.73 ) $ (0.74 ) $ (0.65 ) $ (0.67 ) $ (2.78 ) Net loss per share is computed independently for each of the quarters presented. Therefore, the sum of the quarterly per-share calculations will not necessarily equal the annual per share calculation.

SUBSEQUENT EVENT

SUBSEQUENT EVENT	12 Months Ended
SUBSEQUENT EVENT	Dec. 31, 2018
Subsequent Events [Abstract]
Subsequent Event	Subsequent Event Sale of Common Stock In January 2019, the Company sold 1,854,838 shares of its common stock at a public offering price of $62.00 per share. After deducting underwriter discounts, commissions and offering expenses, the Company received net proceeds from the transaction of $107.9 million .

SUMMARY OF SIGNIFICANT ACCOUN_2

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)	12 Months Ended
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)	Dec. 31, 2018
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation These consolidated financial statements are prepared in accordance with accounting principles generally accepted in the United States ("U.S. GAAP"). These consolidated financial statements include the accounts of the Company and MethylGene. All significant inter-company transactions, balances and expenses have been eliminated upon consolidation. Mirati was incorporated under the laws of the State of Delaware on April 29, 2013. On May 8, 2013, the Company's Board of Directors approved and the Company entered into an arrangement agreement ("Arrangement") with MethylGene. Upon completion of the Arrangement, MethylGene became the Company's wholly-owned subsidiary. These consolidated financial statements are presented in United States ("U.S.") Dollars, which is also the functional currency of the Company.
Use of Estimates	Use of Estimates The preparation of the Company's consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ materially from those estimates. Estimates and assumptions are reviewed quarterly. Any revisions to accounting estimates are recognized in the period in which the estimates are revised and in any future periods affected.
Cash, Cash Equivalents, and Short-term Investments	Cash, Cash Equivalents and Short-term Investments Cash and cash equivalents consist of cash and highly liquid securities with original maturities at the date of acquisition of ninety days or less. Investments with an original maturity of more than ninety days are considered short-term investments and have been classified by management as available-for-sale. These investments are classified as current assets, even though the stated maturity date may be one year or more beyond the current balance sheet date, which reflects management’s intention to use the proceeds from sales of these securities to fund its operations, as necessary. Such investments are carried at fair value, with unrealized gains and losses included as a separate component of stockholders’ equity. Realized gains and losses from the sale of available-for-sale securities or the amounts, net of tax, reclassified out of accumulated other comprehensive income, if any, are determined on a specific identification basis.
Concentration of Credit Risk	Concentration of Credit Risk The Company invests its excess cash in accordance with its investment policy. The Company's investments are comprised primarily of commercial paper and debt instruments of financial institutions, corporations, U.S. government-sponsored agencies and the U.S. Treasury. The Company mitigates credit risk by maintaining a diversified portfolio and limiting the amount of investment exposure as to institution, maturity and investment type. Financial instruments that potentially subject the Company to significant credit risk consist principally of cash equivalents and short-term investments.
Revenue Recognition	Revenue Recognition Effective January 1, 2017, the Company adopted Accounting Standards Codification, or ASC, Topic 606, Revenue from Contracts with Customers , using the full retrospective transition method. As the Company did not have any revenue contracts prior to the first quarter of 2018, an adjustment to prior periods under this method was not applicable. This standard applies to all contracts with customers, except for contracts that are within the scope of other standards, such as leases, insurance, collaboration arrangements and financial instruments. Under Topic 606, an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration that the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of Topic 606, the entity performs the following five steps: (i) identify the contract(s) 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 in the contract; and (v) recognize revenue when (or as) the entity satisfies a performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. At contract inception, once the contract is determined to be within the scope of Topic 606, the Company assesses the goods or services promised within each contract and determines those that are performance obligations, and assesses whether each promised good or service is distinct. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied.
Property and Equipment, Net	Property and Equipment, Net Property and equipment is stated at historical cost less accumulated depreciation. Historical cost includes expenditures that are directly attributable to the acquisition of the items. All repairs and maintenance are charged to net loss during the financial period in which they are incurred. Depreciation of property and equipment is calculated using the straight-line method over the estimated useful lives of the assets, as follows: Computer equipment 3 years Office and other equipment 6 years Laboratory equipment 6 years Leasehold improvements The lesser of the lease term or the life of the asset Upon disposal or impairment of property and equipment, the cost and related accumulated depreciation is removed from the consolidated financial statements and the net amount, less any proceeds, is included in net loss.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets The Company reviews its long-lived assets for impairment whenever events or changes in circumstances indicate that the carrying value of an asset may not be recoverable. If such circumstances are determined to exist, an estimate of undiscounted future cash flows produced by the long-lived asset, including its eventual residual value, is compared to the carrying value to determine whether impairment exists. In the event that such cash flows are not expected to be sufficient to recover the carrying amount of the assets, the assets are written-down to their estimated fair values. Fair value is estimated through discounted cash flow models to project cash flows from the asset.
Share-Based Compensation	Share-Based Compensation The Company measures and recognizes compensation expense for share-based payments based on estimated fair value, using the fair value of stock options granted using the Black-Scholes option-pricing model. The Black-Scholes option-pricing model requires the use of certain estimates and judgmental assumptions that affect the amount of share-based compensation expense recognized in the Company's consolidated financial statements. These assumptions include the historical volatility of the Company's stock price, expected term of the options, the risk-free interest rate and expected dividend yields. Share-based compensation is recognized using the graded accelerated vesting method.
Research and Development Expenses	Research and Development Expenses Research and development expenditures are charged to net loss in the period in which they are incurred and are comprised of the following types of costs incurred in performing research and development activities: contract services for clinical trials and related clinical manufacturing costs, salaries and benefits including share-based compensation expense, costs for allocated facilities and depreciation of equipment and license fees paid in connection with our early discovery efforts
General and Administrative Expenses	General and Administrative Expenses General and administrative expenses consist primarily of salaries and related benefits, including share-based compensation, related to our executive, finance, business development, legal, human resources and support functions. Other general and administrative expenses include professional fees for auditing, tax, consulting and patent-related services, rent and utilities and insurance.
Income Taxes	Income Taxes Income taxes have been accounted for using the asset and liability method. Under the asset and liability method, deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial carrying amounts of existing assets and liabilities and their respective tax bases and operating loss and tax credit carryforwards. Deferred tax assets and liabilities are measured using enacted tax rates applicable to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in net loss in the period that includes the enactment date. A valuation allowance against deferred tax assets is recorded if, based upon the weight of all available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized. For uncertain tax positions that meet "a more likely than not" threshold, the Company recognizes the benefit of uncertain tax positions in the consolidated financial statements.
Segment Reporting	Segment Reporting Operating segments are components of a business where separate discrete financial information is available for evaluation by the chief operating decision-maker for purposes of making decisions regarding resource allocation and assessing performance. To date, the Company and the chief operating decision-maker has viewed its operations and managed its business as one segment operating primarily in the United States.
Net Loss per Share	Net Loss per Share Basic net loss per common share is calculated by dividing the net loss by the weighted-average number of common shares outstanding during the period, without consideration for potentially dilutive securities. Diluted net loss per share is computed by dividing the net loss attributable to common stockholders by the weighted-average number of common shares and potentially dilutive securities outstanding for the period. Common share equivalents outstanding, determined using the treasury stock method, are comprised of shares that may be issued under the Company’s stock option and warrant agreements.
Fair Value Measurements	The authoritative guidance for fair value measurements defines fair value as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or the most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Market participants are buyers and sellers in the principal market that are (i) independent, (ii) knowledgeable, (iii) able to transact, and (iv) willing to transact. The guidance prioritizes the inputs used in measuring fair value into the following hierarchy: • Level 1- Quoted prices (unadjusted) in active markets for identical assets or liabilities; • Level 2- Inputs other than quoted prices included within Level 1 that are either directly or indirectly observable; and • Level 3- Unobservable inputs in which little or no market activity exists, therefore requiring an entity to develop its own assumptions about the assumptions that market participants would use in pricing.

SUMMARY OF SIGNIFICANT ACCOUN_3

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Tables)	12 Months Ended
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Tables)	Dec. 31, 2018
Accounting Policies [Abstract]
Schedule of estimated useful lives of assets	Depreciation of property and equipment is calculated using the straight-line method over the estimated useful lives of the assets, as follows: Computer equipment 3 years Office and other equipment 6 years Laboratory equipment 6 years Leasehold improvements The lesser of the lease term or the life of the asset
Schedule of potentially dilutive securities not included in the calculation of diluted net loss per share because to do so would be anti-dilutive	The following table presents the weighted average number of potentially dilutive securities not included in the calculation of diluted net loss per share due to the anti-dilutive effect of the securities: Year ended December 31, 2018 2017 2016 Common stock options 1,781,388 38,675 173,776 Common stock warrants 11,631,636 7,534,576 315,834 Total 13,413,024 7,573,251 489,610

INVESTMENTS (Tables)

INVESTMENTS (Tables)	12 Months Ended
INVESTMENTS (Tables)	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Schedule of Available-for-sale and Trading Securities Reconciliation	The following tables summarize the Company's short-term investments (in thousands): As of December 31, 2018 Maturity Amortized cost Gross unrealized gains Gross unrealized losses Estimated fair value Corporate debt securities 1 year or less $ 111,933 $ 26 $ (43 ) $ 111,916 Commercial paper 1 year or less 74,433 — (24 ) 74,409 U.S. Treasury bills 1 year or less 3,771 — — 3,771 $ 190,137 $ 26 $ (67 ) $ 190,096 As of December 31, 2017 Maturity Amortized cost Gross unrealized gains Gross unrealized losses Estimated fair value Corporate debt securities 1 year or less $ 24,264 $ — $ (23 ) $ 24,241 Commercial paper 1 year or less 18,912 — (19 ) 18,893 $ 43,176 $ — $ (42 ) $ 43,134

FAIR VALUE MEASUREMENTS (Tables

FAIR VALUE MEASUREMENTS (Tables)	12 Months Ended
FAIR VALUE MEASUREMENTS (Tables)	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Fair Value, Assets Measured on Recurring Basis	The following table summarizes the assets and liabilities measured at fair value on a recurring basis (in thousands): December 31, 2018 Total Level 1 Level 2 Level 3 Assets Cash and cash equivalents: Cash $ 3,731 $ 3,731 $ — $ — Money market funds 28,963 28,963 — — Total cash and cash equivalents 32,694 32,694 — — Short-term investments: U.S. Treasury bills 3,771 3,771 — — Corporate debt securities 111,916 — 111,916 — Commercial paper 74,409 — 74,409 — Total short-term investments 190,096 3,771 186,325 — Total $ 222,790 $ 36,465 $ 186,325 $ — December 31, 2017 Total Level 1 Level 2 Level 3 Assets Cash and cash equivalents: Cash $ 1,026 $ 1,026 $ — $ — Money market funds 106,677 106,677 — — Total cash and cash equivalents 107,703 107,703 — — Short-term investments: Corporate debt securities 24,241 — 24,241 — Commercial paper 18,893 — 18,893 — Total short-term investments 43,134 — 43,134 — Total $ 150,837 $ 107,703 $ 43,134 $ —

OTHER CURRENT ASSETS AND LONG_2

OTHER CURRENT ASSETS AND LONG-TERM ASSETS (Tables)	12 Months Ended
OTHER CURRENT ASSETS AND LONG-TERM ASSETS (Tables)	Dec. 31, 2018
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Schedule of other current assets	Other current assets consisted of the following (in thousands): December 31, 2018 2017 Prepaid expenses $ 1,261 $ 3,085 Deposits and other receivables 1,841 1,600 Interest receivables 768 237 $ 3,870 $ 4,922

PROPERTY AND EQUIPMENT (Tables)

PROPERTY AND EQUIPMENT (Tables)	12 Months Ended
PROPERTY AND EQUIPMENT (Tables)	Dec. 31, 2018
Property, Plant and Equipment [Abstract]
Schedule of property and equipment	Property and equipment, net consisted of the following (in thousands): December 31, 2018 2017 Computer equipment $ 201 $ 329 Office and other equipment 260 301 Laboratory equipment 729 643 Leasehold improvements 63 63 Gross property and equipment 1,253 1,336 Less: Accumulated depreciation (780 ) (811 ) Property and equipment, net $ 473 $ 525

ACCOUNTS PAYABLE AND ACCRUED _2

ACCOUNTS PAYABLE AND ACCRUED LIABILITIES (Tables)	12 Months Ended
ACCOUNTS PAYABLE AND ACCRUED LIABILITIES (Tables)	Dec. 31, 2018
Payables and Accruals [Abstract]
Schedule of accounts payable and accrued liabilities	Accounts payable and accrued liabilities consisted of the following (in thousands): December 31, 2018 2017 Accounts payable $ 8,531 $ 4,344 Accrued clinical expense 10,154 5,412 Accrued development and other expense 1,243 867 Accrued compensation and benefits 5,847 3,021 $ 25,775 $ 13,644

BEIGENE AGREEMENT (Tables)

BEIGENE AGREEMENT (Tables)	12 Months Ended
BEIGENE AGREEMENT (Tables)	Dec. 31, 2018
Research and Development [Abstract]
Schedule of Activity in Contract Liabilities	The following table presents a summary of the activity in the Company's contract liabilities during the year ended December 31, 2018 (in thousands): Opening balance, January 1, 2018 $ — Revenue from performance obligations satisfied during reporting period 52 Payments received in advance (533 ) Closing Balance, December 31, 2018 $ (481 )

STOCKHOLDERS' EQUITY (Tables)

STOCKHOLDERS' EQUITY (Tables)	12 Months Ended
STOCKHOLDERS' EQUITY (Tables)	Dec. 31, 2018
Stockholders' Equity Note [Abstract]
Schedule of Shares Reserved for Future Issuance	The following shares were reserved for future issuance: December 31, 2018 Common stock options outstanding and available for future grant 5,574,901 Warrants to purchase common stock 11,817,912 Employee Stock Purchase Plan 157,360 17,550,173
Schedule of warrants issued and outstanding	As of December 31, 2018 , the following warrants for common stock were issued and outstanding: Issue date Expiration date Exercise price Number of warrants outstanding January 11, 2017 None $ 0.001 7,258,263 November 20, 2017 None $ 0.001 4,137,999 June 11, 2018 None $ 0.001 421,650 11,817,912

SHARE-BASED COMPENSATION (Table

SHARE-BASED COMPENSATION (Tables)	12 Months Ended
SHARE-BASED COMPENSATION (Tables)	Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Schedule of changes to the number of stock options granted by the Company and their weighted average exercise price	The following table summarizes our stock option activity and related information for the year ended December 31, 2018 : Number of options Weighted average exercise price Weighted-Average Remaining Contractual Term (years) Aggregate Intrinsic Value (millions) Balance outstanding as of December 31, 2017 3,304,181 $ 13.67 Granted 1,261,693 $ 31.84 Exercised (731,935 ) $ 13.32 Canceled/forfeited (150,153 ) $ 22.83 Balance outstanding as of December 31, 2018 3,683,786 $ 19.59 7.3 $ 85.6 Options exercisable at December 31, 2018 1,910,058 $ 16.05 6.1 $ 50.4
Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Table Text Block]	Total share-based compensation expense by statement of operations and comprehensive loss classification is presented below (in thousands): Year ended December 31, 2018 2017 2016 Research and development expense $ 7,232 $ 3,192 $ 5,461 General and administrative expense 8,622 3,594 5,163 $ 15,854 $ 6,786 $ 10,624
Schedule of assumptions used to estimate the fair value of options granted at the date of grant using the Black-Scholes option pricing model	The assumptions used for the specified reporting periods and the resulting estimates of weighted-average estimated fair value per share of options granted during those periods are as follows: Year Ended December 31, 2018 2017 2016 Risk-free interest rate 2.6% 2.1% 1.5% Dividend yield —% —% —% Volatility factor 94.3% 96.0% 101.7% Expected term (in years) 6.0 6.0 6.0 Weighted average estimated fair value per share $24.39 $4.17 $13.32

INCOME TAXES (Tables)

INCOME TAXES (Tables)	12 Months Ended
INCOME TAXES (Tables)	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Schedule of Effective Income Tax Reconciliation	The differences between the effective income tax rate and the statutory tax rates during the years ended 2018 , 2017 and 2016 are as follows (in thousands): Year Ended December 31, 2018 2017 2016 Net loss before tax $ (98,418 ) $ (70,430 ) $ (83,118 ) Statutory combined U.S. federal and state tax rate 21.00 % 34.00 % 34.00 % Statutory federal and state taxes (20,668 ) (23,946 ) (28,260 ) Increase (decrease) in taxes recoverable resulting from: Effect of change in valuation allowance 25,959 (4,154 ) 28,446 Non-deductible share-based compensation 884 695 1,247 Tax deductions for share-based compensation (2,924 ) (80 ) (12 ) Tax credits (5,130 ) (2,563 ) (2,906 ) Share issue costs - temporary difference — — (78 ) Write off of Methylgene US Inc. net operating loss — 307 — Differential in income tax rates of foreign subsidiary (1 ) (169 ) 261 Change in tax rate — 303 — Tax Cuts and Jobs Act — 28,569 — Uncertain tax positions 1,283 646 3,921 Return to provision and other true-ups 375 394 (2,619 ) Other differences 222 (2 ) — Income tax benefit $ — $ — $ —
Schedule of deferred tax	The following table summarizes the significant components of our deferred tax assets (in thousands): December 31, 2018 2017 Deferred tax assets: Tangible and intangible depreciable assets $ 8,123 $ 17,927 Stock compensation 6,515 4,976 Provisions 1,020 611 Net operating loss carry forwards 74,721 44,598 Capital loss carryforward 178 83 Canada scientific research and experimental development expenditures 5,467 5,467 U.S. research and development tax credits 10,613 7,016 Total gross deferred tax assets 106,637 80,678 Less valuation allowance (106,637 ) (80,678 ) Net deferred tax assets $ — $ —
Schedule of expiration of NOLs	The NOLs expire as follows (in thousands): US Canada Federal State Federal Provincial Expires in: 2030 $ — $ — $ 5,907 $ 5,985 2031 — — 7,059 7,066 2032 — — 13,308 12,433 2033 2,225 2,232 18,623 19,385 2034 7,276 22,162 32,401 31,809 2035 53,359 52,950 1,084 1,084 2036 23,379 — 777 777 2037 65,509 — 697 697 2038 — — 7 $ 7 Does not expire 102,863 — — $ — $ 254,611 $ 77,344 $ 79,863 $ 79,243
Schedule of reconciliation of the beginning and ending gross amounts of unrecognized tax positions	A reconciliation of the beginning and ending amounts of unrecognized tax positions are as follows (in thousands): Federal Provincial/State December 31, December 31, 2018 2017 2016 2018 2017 2016 Unrecognized tax positions, beginning of year $ 1,693 $ 1,095 $ 509 $ 7,556 $ 7,333 $ 2,274 Gross increase — current period tax positions 924 588 598 454 227 195 Gross decrease — prior period tax positions — — (9 ) — (3 ) — Gross increase — prior period tax positions — 11 — — — 4,866 Expiration of statute of limitations — (1 ) (3 ) — (1 ) (2 ) Unrecognized tax positions, end of year $ 2,617 $ 1,693 $ 1,095 $ 8,010 $ 7,556 $ 7,333

INVESTMENT TAX CREDITS (Tables)

INVESTMENT TAX CREDITS (Tables)	12 Months Ended
INVESTMENT TAX CREDITS (Tables)	Dec. 31, 2018
INVESTMENT TAX CREDITS
Schedule of non-refundable investment tax credits expiration	The non-refundable investment tax credits expire as follows (in thousands): Federal ITC Expires in: 2030 $ 764 2031 1,000 2032 1,125 2033 1,018 $ 3,907

COMMITMENTS AND CONTINGENCIES (

COMMITMENTS AND CONTINGENCIES (Tables)	12 Months Ended
COMMITMENTS AND CONTINGENCIES (Tables)	Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Schedule of future minimum annual payment required in the next year	Future minimum payments required under the lease are summarized as follows (in thousands): Year Ending December 31: 2019 $ 363 2020 30 Thereafter — Total minimum lease payments $ 393

QUARTERLY FINANCIAL DATA (Table

QUARTERLY FINANCIAL DATA (Tables)	12 Months Ended
QUARTERLY FINANCIAL DATA (Tables)	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Schedule of Quarterly Financial Data	The following is a summary of the quarterly results of the Company for the years ended December 31, 2018 and 2017 (unaudited, in thousands, except for per share data): Three Months Ended Year Ended 3/31/18 6/30/18 9/30/18 12/31/18 December 31, 2018 License and collaboration revenues $ 9,467 $ — $ — $ 3,459 $ 12,926 Loss from operations (15,346 ) (28,670 ) (28,939 ) (29,672 ) $ (102,627 ) Net loss (14,709 ) (27,869 ) (27,568 ) (28,272 ) (98,418 ) Basic and diluted net loss per share $ (0.51 ) $ (0.94 ) $ (0.85 ) $ (0.87 ) $ (3.19 ) Three Months Ended Year Ended 3/31/17 6/30/17 9/30/17 12/31/17 December 31, 2017 Loss from operations $ (18,090 ) $ (18,616 ) $ (16,601 ) $ (18,228 ) $ (71,535 ) Net loss (17,846 ) (18,339 ) (16,350 ) (17,895 ) (70,430 ) Basic and diluted net loss per share $ (0.73 ) $ (0.74 ) $ (0.65 ) $ (0.67 ) $ (2.78 ) Net loss per share is computed independently for each of the quarters presented. Therefore, the sum of the quarterly per-share calculations will not necessarily equal the annual per share calculation.

SUMMARY OF SIGNIFICANT ACCOUN_4

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details)	12 Months Ended
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details)	Dec. 31, 2018
Computer equipment
Property and equipment
Estimated useful lives	3 years
Office and other equipment
Property and equipment
Estimated useful lives	6 years
Laboratory equipment
Property and equipment
Estimated useful lives	6 years

SUMMARY OF SIGNIFICANT ACCOUN_5

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details 2) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Asset Impairment Charges	$ 0	$ 0	$ 0

SUMMARY OF SIGNIFICANT ACCOUN_6

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details 3) - shares	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2018	Dec. 31, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities not included in the calculation of diluted net loss per share (in shares)	489,610	13,413,024	7,573,251
Common stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities not included in the calculation of diluted net loss per share (in shares)	173,776	1,781,388	38,675
Common stock warrants
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities not included in the calculation of diluted net loss per share (in shares)	315,834	11,631,636	7,534,576

RECENTLY ISSUED AND RECENTLY _2

RECENTLY ISSUED AND RECENTLY ADOPTED ACCOUNTING PRONOUNCEMENTS RECENTLY ISSUED AND RECENTLY ADOPTED ACCOUNTING PRONOUNCEMENTS (Details) - Subsequent Event - Forecast - Accounting Standards Update 2016-02	Jan. 01, 2019USD ($)
Minimum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease liability	$ 300,000
Right-of-use assets	300,000
Maximum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease liability	400,000
Right-of-use assets	$ 400,000

INVESTMENTS (Details)

INVESTMENTS (Details) - USD ($) $ in Thousands	12 Months Ended
INVESTMENTS (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Available-for-Sale, Amortized cost	$ 190,137	$ 43,176
Available-for-Sale, Gross unrealized gains	26	0
Available-for-Sale, Gross unrealized losses	(67)	(42)
Available-for-Sale, Estimated fair value	$ 190,096	$ 43,134
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, maximum maturity period	1 year	1 year
Available-for-Sale, Amortized cost	$ 111,933	$ 24,264
Available-for-Sale, Gross unrealized gains	26	0
Available-for-Sale, Gross unrealized losses	(43)	(23)
Available-for-Sale, Estimated fair value	$ 111,916	$ 24,241
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, maximum maturity period	1 year	1 year
Available-for-Sale, Amortized cost	$ 74,433	$ 18,912
Available-for-Sale, Gross unrealized gains	0	0
Available-for-Sale, Gross unrealized losses	(24)	(19)
Available-for-Sale, Estimated fair value	74,409	$ 18,893
U.S. Treasury bills
Debt Securities, Available-for-sale [Line Items]
Available-for-Sale, Amortized cost	3,771
Available-for-Sale, Gross unrealized gains	0
Available-for-Sale, Gross unrealized losses	0
Available-for-Sale, Estimated fair value	$ 3,771

FAIR VALUE MEASUREMENTS (Detail

FAIR VALUE MEASUREMENTS (Details) - Recurring basis - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	$ 32,694	$ 107,703
Short-term investments	190,096	43,134
Assets, Fair Value Disclosure, Recurring (Deprecated 2018-01-31)	222,790	150,837
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	32,694	107,703
Short-term investments	3,771
Assets, Fair Value Disclosure, Recurring (Deprecated 2018-01-31)	36,465	107,703
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure		0
Short-term investments	186,325	43,134
Assets, Fair Value Disclosure, Recurring (Deprecated 2018-01-31)	186,325	43,134
U.S. Treasury bills
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	3,771
U.S. Treasury bills \| Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	3,771
U.S. Treasury bills \| Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	0
Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	111,916	24,241
Corporate debt securities \| Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	111,916	24,241
Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	74,409	18,893
Commercial paper \| Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term investments	74,409	18,893
Cash
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	3,731	1,026
Cash \| Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	3,731	1,026
Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	28,963	106,677
Money market funds \| Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	$ 28,963	$ 106,677

OTHER CURRENT ASSETS AND LONG_3

OTHER CURRENT ASSETS AND LONG-TERM ASSETS (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Prepaid expenses	$ 1,261	$ 3,085
Deposits and other receivables	1,841	1,600
Interest receivables	768	237
Other current assets	3,870	4,922
Other Assets, Noncurrent [Abstract]
Deposits paid for research and development	$ 1,300	$ 1,000

PROPERTY AND EQUIPMENT (Details

PROPERTY AND EQUIPMENT (Details) - USD ($) $ in Thousands	12 Months Ended
PROPERTY AND EQUIPMENT (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property and equipment
Property and equipment, gross	$ 1,253	$ 1,336
Less: Accumulated depreciation	(780)	(811)
Property and equipment, net	473	525
Depreciation expenses	175	184	$ 180
Computer equipment
Property and equipment
Property and equipment, gross	201	329
Office and other equipment
Property and equipment
Property and equipment, gross	260	301
Laboratory equipment
Property and equipment
Property and equipment, gross	729	643
Leasehold improvements
Property and equipment
Property and equipment, gross	$ 63	$ 63

ACCOUNTS PAYABLE AND ACCRUED _3

ACCOUNTS PAYABLE AND ACCRUED LIABILITIES (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Payables and Accruals [Abstract]
Accounts payable	$ 8,531	$ 4,344
Accrued clinical expense	10,154	5,412
Accrued development and other expense	1,243	867
Accrued compensation and benefits	5,847	3,021
Total accounts payable and accrued liabilities	25,775	13,644
Long-term liabilities	732	314
Noncurrent deferred revenue	100
Other liabilities	$ 600	$ 300

BEIGENE AGREEMENT - Narrative (

BEIGENE AGREEMENT - Narrative (Details) - Collaboration and License Agreement - USD ($)	Jan. 07, 2018	Dec. 31, 2018
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Up-front fee received	$ 10,000,000
Termination of contract, period after first commercial sale of product	10 years
Period required for notice of termination of contract	60 days
Payments received in advance		$ 533,000
Revenue from performance obligations satisfied during reporting period		52,000
Revenue from performance obligation expected to be earned	$ 123,000,000
Licenses of Intellectual Property
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue from performance obligation earned		9,500,000
Manufacturing Supply Services
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Payments received in advance		(500,000)
Revenue from performance obligations satisfied during reporting period		100,000
Revenue from performance obligation earned		400,000
Cost-sharing receivable		300,000
Milestone Payments
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue from performance obligation earned		3,000,000
Revenue from performance obligation expected to be earned		5,000,000
Royalties
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue		$ 0

BEIGENE AGREEMENT - Activity in

BEIGENE AGREEMENT - Activity in Contract Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue	$ 371	$ 0
Noncurrent deferred revenue	100
Collaboration and License Agreement
Movement in Deferred Revenue [Roll Forward]
Opening balance at beginning of period	0
Revenue from performance obligations satisfied during reporting period	52
Payments received in advance	(533)
Closing balance at end of period	(481)
Current deferred revenue	400
Noncurrent deferred revenue	$ 100

STOCKHOLDERS' EQUITY (Details)

STOCKHOLDERS' EQUITY (Details)	Dec. 31, 2018shares
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)	17,550,173
Warrants to purchase common stock
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)	11,817,912
Stock Option Plan
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)	5,574,901
Employee Stock Purchase Plan
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)	157,360

STOCKHOLDERS' EQUITY (Details 2

STOCKHOLDERS' EQUITY (Details 2) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended		12 Months Ended
	Nov. 30, 2017	Jan. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Nov. 21, 2012
Warrants - issued and outstanding
Proceeds from Issuance of Common Stock			$ 130,663	$ 153,530	$ 0
Number of warrants (in shares)			11,817,912
Number of warrants exercised (in shares)			0	52,825	289,789
Exercise of warrants for cash (in shares)				585,729	313,756
Proceeds from warrant exercises				$ 4,600	$ 2,100
Net exercise of warrants (in shares)				638,554	603,545
Common Stock
Warrants - issued and outstanding
Issuance of common stock, net of costs (in shares)			3,162,500	7,941,688
Exercise of warrants for cash (in shares)				585,729	313,756
Net exercise of warrants (in shares)				52,825	289,789
June 11, 2018 Warrants
Warrants - issued and outstanding
Exercise price (in dollars per share)	$ 38.849
Proceeds from Issuance of Common Stock	$ 130,700
Number of warrants (in shares)	421,650
June 11, 2018 Warrants \| Common Stock
Warrants - issued and outstanding
Issuance of common stock, net of costs (in shares)	3,162,500
Share Price	$ 38.85
November 20, 2017 Warrants
Warrants - issued and outstanding
Exercise price (in dollars per share)	$ 12.999
Proceeds from Issuance of Common Stock	$ 86,700
Number of warrants (in shares)	4,137,999
November 20, 2017 Warrants \| Common Stock
Warrants - issued and outstanding
Issuance of common stock, net of costs (in shares)	2,938,986
Share Price	$ 13
January 11, 2017 Warrants
Warrants - issued and outstanding
Exercise price (in dollars per share)		$ 5.599	$ 0.001
Proceeds from Issuance of Common Stock		$ 66,800
Number of warrants (in shares)		7,258,263
January 11, 2017 Warrants \| Common Stock
Warrants - issued and outstanding
Issuance of common stock, net of costs (in shares)		5,002,702
Share Price		$ 5.60
Private Placement \| June 11, 2018 Warrants
Warrants - issued and outstanding
Exercise price (in dollars per share)						$ 0.001
Number of warrants (in shares)	421,650
Private Placement \| November 20, 2017 Warrants
Warrants - issued and outstanding
Exercise price (in dollars per share)						$ 0.001
Number of warrants (in shares)	4,137,999
Private Placement \| January 11, 2017 Warrants
Warrants - issued and outstanding
Exercise price (in dollars per share)			$ 0.001
Number of warrants (in shares)			7,258,263
Certain Holders
Warrants - issued and outstanding
Common stock, ownership proportion, threshold (as a percent)			999000.00%	1999000.00%
Other Holders
Warrants - issued and outstanding
Common stock, ownership proportion, threshold (as a percent)			1999000.00%	999000.00%

SHARE-BASED COMPENSATION (Detai

SHARE-BASED COMPENSATION (Details) - USD ($)	12 Months Ended
SHARE-BASED COMPENSATION (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	May 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options available for issue	1,900,000
Exercise price required percentage of fair market value	100.00%
Award vesting period	4 years
Intrinsic value of options exercised	$ 18,800,000	$ 300,000	$ 300,000
Cash received from exercise of stock options	9,700,000	400,000	200,000
Fair value of options vested	8,200,000	10,600,000	8,600,000
Capitalized costs	0	0	0
Tax benefit from compensation expense	0	$ 0	$ 0
Compensation cost not yet recognized	$ 17,100,000
Compensation cost not yet recognized, period for recognition	1 year 2 months 12 days
Stock Option Plan \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option contractual terms	7 years
2013 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee Stock Purchase Plan, Shares Authorized for Purchase	$ 25,000
Employee Stock Purchase Plan, Purchase Discount, Percent	85.00%
Employee Stock Purchase Plan, Shares Reserved for Future Issuance				300,000
Issuance of common stock from ESPP (in shares)	142,640
2013 Plan \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option contractual terms	7 years
2013 Plan \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option contractual terms	10 years

SHARE-BASED COMPENSATION (Det_2

SHARE-BASED COMPENSATION (Details 2) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018USD ($)$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Balance, beginning of year (in shares) \| shares	3,304,181
Granted (in shares) \| shares	1,261,693
Exercise of options (in shares) \| shares	(731,935)
Canceled (in shares) \| shares	(150,153)
Balance, end of year (in shares) \| shares	3,683,786
Options exercisable, end of year (in shares) \| shares	1,910,058
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Balance, beginning of year (in dollars per share) \| $ / shares	$ 13.67
Granted (in dollars per share) \| $ / shares	31.84
Exercised (in dollars per share) \| $ / shares	13.32
Canceled (in dollars per share) \| $ / shares	22.83
Balance, end of year (in dollars per share) \| $ / shares	19.59
Options exercisable, end of year (in dollars per share) \| $ / shares	$ 16.05
Options outstanding, Weighted-Average Remaining Contractual Term (years)	7 years 3 months 18 days
Options exercisable, Weighted-Average Remaining Contractual Term (years)	6 years 1 month 6 days
Options outstanding, Aggregate Intrinsic Value \| $	$ 85.6
Options exercisable, Aggregate Intrinsic Value \| $	$ 50.4

SHARE-BASED COMPENSATION (Det_3

SHARE-BASED COMPENSATION (Details 3) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 15,854	$ 6,786	$ 10,624
Research and development expense
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense	7,232	3,192	5,461
General and administrative expense
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 8,622	$ 3,594	$ 5,163

SHARE-BASED COMPENSATION (Det_4

SHARE-BASED COMPENSATION (Details 4) - $ / shares	12 Months Ended
SHARE-BASED COMPENSATION (Details 4) - $ / shares	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Risk-free interest rate	2.60%	2.10%	1.50%
Dividend yield	0.00%	0.00%	0.00%
Volatility factor	94.30%	96.00%	101.70%
Expected term (in years)	6 years	6 years	6 years
Weighted average estimated fair value per share	$ 24.39	$ 4.17	$ 13.32

EMPLOYEE BENEFIT PLAN (Details)

EMPLOYEE BENEFIT PLAN (Details) - USD ($)	12 Months Ended
EMPLOYEE BENEFIT PLAN (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Retirement Benefits [Abstract]
Percentage of employee contributions matched	4.00%
Maximum employer contribution per employee	$ 2,500
Matching contribution amount	$ 100,000	$ 100,000	$ 100,000

INCOME TAXES (Details)

INCOME TAXES (Details)	12 Months Ended
INCOME TAXES (Details)	Dec. 31, 2018USD ($)ongoing_tax_audit	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Net loss before tax	$ (98,418,000)	$ (70,430,000)	$ (83,118,000)
Statutory combined US federal and state tax rate (2012 - statutory combined Canadian federal and provincial tax rate)	21.00%	34.00%	34.00%
Statutory federal and provincial taxes	$ (20,668,000)	$ (23,946,000)	$ (28,260,000)
Effect of change in valuation allowance	25,959,000	(4,154,000)	28,446,000
Non-deductible share-based compensation	884,000	695,000	1,247,000
Tax deductions for share-based compensation	(2,924,000)	(80,000)	(12,000)
Tax credits	(5,130,000)	(2,563,000)	(2,906,000)
Share issue costs - temporary difference	0	0	(78,000)
Write off of Methylgene US Inc. net operating loss	0	307,000	0
Differential in income tax rates of foreign subsidiary	(1,000)	(169,000)	261,000
Change in tax rate	0	303,000	0
Tax Cuts and Jobs Act	0	28,569,000	0
Uncertain tax positions	1,283,000	646,000	3,921,000
Return to provision and other true-ups	375,000	394,000	(2,619,000)
Other differences	222,000	(2,000)	0
Income tax expense (benefit)	0	0	0
Deferred tax asset
Tangible and intangible depreciable assets	8,123,000	17,927,000
Stock compensation	6,515,000	4,976,000
Provisions	1,020,000	611,000
Net operating loss carry forwards	74,721,000	44,598,000
Capital loss carryforward	178,000	83,000
Canada scientific research and experimental development expenditures	5,467,000	5,467,000
U.S. research and development tax credits	10,613,000	7,016,000
Total gross deferred tax assets	106,637,000	80,678,000
Less valuation allowance	(106,637,000)	(80,678,000)
Net deferred tax assets	0	0
Additional disclosures
Amount of increase in valuation allowance	(26,000,000)
Reconciliation of the beginning and ending gross amounts of unrecognized tax positions
Accrual for interest or penalties on tax matters	$ 0
Number of ongoing tax audits \| ongoing_tax_audit	0
Deferred tax asset, provisional income tax expense related to the TCJA		28,600,000
Domestic Tax Authority
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Income tax expense (benefit)	$ 0	0	0
Canadian Federal
Additional disclosures
Scientific research and experimental development expenses deductible for income tax purposes	19,900,000	19,900,000
Net operating loss carry forwards (NOLs)	79,863,000
Reconciliation of the beginning and ending gross amounts of unrecognized tax positions
Unrecognized tax positions, beginning of year	1,693,000	1,095,000	509,000
Gross increase — current period tax positions	924,000	588,000	598,000
Gross decrease — prior period tax positions	0	0	(9,000)
Gross increase — prior period tax positions	0	11,000	0
Expiration of statute of limitations	0	(1,000)	(3,000)
Unrecognized tax positions, end of year	2,617,000	1,693,000	1,095,000
Canadian Federal \| 2030
Additional disclosures
Net operating loss carry forwards (NOLs)	5,907,000
Canadian Federal \| 2031
Additional disclosures
Net operating loss carry forwards (NOLs)	7,059,000
Canadian Federal \| 2032
Additional disclosures
Net operating loss carry forwards (NOLs)	13,308,000
Canadian Federal \| 2033
Additional disclosures
Net operating loss carry forwards (NOLs)	18,623,000
Canadian Federal \| 2034
Additional disclosures
Net operating loss carry forwards (NOLs)	32,401,000
Canadian Federal \| 2035
Additional disclosures
Net operating loss carry forwards (NOLs)	1,084,000
Canadian Federal \| 2036
Additional disclosures
Net operating loss carry forwards (NOLs)	777,000
Canadian Federal \| 2037
Additional disclosures
Net operating loss carry forwards (NOLs)	697,000
Canadian Federal \| 2038
Additional disclosures
Net operating loss carry forwards (NOLs)	7,000
Canadian Federal \| Does not expire
Additional disclosures
Net operating loss carry forwards (NOLs)	0
Canadian Provincial
Additional disclosures
Scientific research and experimental development expenses deductible for income tax purposes	21,600,000	21,600,000
Net operating loss carry forwards (NOLs)	79,243,000
Reconciliation of the beginning and ending gross amounts of unrecognized tax positions
Unrecognized tax positions, beginning of year	7,556,000	7,333,000	2,274,000
Gross increase — current period tax positions	454,000	227,000	195,000
Gross decrease — prior period tax positions	0	(3,000)	0
Gross increase — prior period tax positions	0	0	4,866,000
Expiration of statute of limitations	0	(1,000)	(2,000)
Unrecognized tax positions, end of year	8,010,000	$ 7,556,000	$ 7,333,000
Canadian Provincial \| 2030
Additional disclosures
Net operating loss carry forwards (NOLs)	5,985,000
Canadian Provincial \| 2031
Additional disclosures
Net operating loss carry forwards (NOLs)	7,066,000
Canadian Provincial \| 2032
Additional disclosures
Net operating loss carry forwards (NOLs)	12,433,000
Canadian Provincial \| 2033
Additional disclosures
Net operating loss carry forwards (NOLs)	19,385,000
Canadian Provincial \| 2034
Additional disclosures
Net operating loss carry forwards (NOLs)	31,809,000
Canadian Provincial \| 2035
Additional disclosures
Net operating loss carry forwards (NOLs)	1,084,000
Canadian Provincial \| 2036
Additional disclosures
Net operating loss carry forwards (NOLs)	777,000
Canadian Provincial \| 2037
Additional disclosures
Net operating loss carry forwards (NOLs)	697,000
Canadian Provincial \| 2038
Additional disclosures
Net operating loss carry forwards (NOLs)	7,000
Canadian Provincial \| Does not expire
Additional disclosures
Net operating loss carry forwards (NOLs)	0
US Federal
Additional disclosures
Net operating loss carry forwards (NOLs)	254,611,000
US Federal \| 2030
Additional disclosures
Net operating loss carry forwards (NOLs)	0
US Federal \| 2031
Additional disclosures
Net operating loss carry forwards (NOLs)	0
US Federal \| 2032
Additional disclosures
Net operating loss carry forwards (NOLs)	0
US Federal \| 2033
Additional disclosures
Net operating loss carry forwards (NOLs)	2,225,000
US Federal \| 2034
Additional disclosures
Net operating loss carry forwards (NOLs)	7,276,000
US Federal \| 2035
Additional disclosures
Net operating loss carry forwards (NOLs)	53,359,000
US Federal \| 2036
Additional disclosures
Net operating loss carry forwards (NOLs)	23,379,000
US Federal \| 2037
Additional disclosures
Net operating loss carry forwards (NOLs)	65,509,000
US Federal \| 2038
Additional disclosures
Net operating loss carry forwards (NOLs)	0
US Federal \| Does not expire
Additional disclosures
Net operating loss carry forwards (NOLs)	102,863,000
US Federal \| Research Tax Credit Carryforward
Additional disclosures
Tax credit carryforward	10,600,000
US State
Additional disclosures
Net operating loss carry forwards (NOLs)	77,344,000
US State \| 2030
Additional disclosures
Net operating loss carry forwards (NOLs)	0
US State \| 2031
Additional disclosures
Net operating loss carry forwards (NOLs)	0
US State \| 2032
Additional disclosures
Net operating loss carry forwards (NOLs)	0
US State \| 2033
Additional disclosures
Net operating loss carry forwards (NOLs)	2,232,000
US State \| 2034
Additional disclosures
Net operating loss carry forwards (NOLs)	22,162,000
US State \| 2035
Additional disclosures
Net operating loss carry forwards (NOLs)	52,950,000
US State \| 2036
Additional disclosures
Net operating loss carry forwards (NOLs)	0
US State \| 2037
Additional disclosures
Net operating loss carry forwards (NOLs)	0
US State \| 2038
Additional disclosures
Net operating loss carry forwards (NOLs)	0
US State \| Does not expire
Additional disclosures
Net operating loss carry forwards (NOLs)	0
US State \| Research Tax Credit Carryforward
Additional disclosures
Tax credit carryforward	$ 4,700,000

INVESTMENT TAX CREDITS (Details

INVESTMENT TAX CREDITS (Details) - Canadian Federal $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)
INVESTMENT TAX CREDITS
Non-refundable ITCs related to scientific research and development expenditures	$ 3,907
2,030
INVESTMENT TAX CREDITS
Non-refundable ITCs related to scientific research and development expenditures	764
2,031
INVESTMENT TAX CREDITS
Non-refundable ITCs related to scientific research and development expenditures	1,000
2,032
INVESTMENT TAX CREDITS
Non-refundable ITCs related to scientific research and development expenditures	1,125
2,033
INVESTMENT TAX CREDITS
Non-refundable ITCs related to scientific research and development expenditures	$ 1,018

COMMITMENTS AND CONTINGENCIES_2

COMMITMENTS AND CONTINGENCIES (Details) - USD ($)	12 Months Ended
COMMITMENTS AND CONTINGENCIES (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Sep. 30, 2015
Future minimum annual payments
2,018	$ 363,000
2,019	30,000
Thereafter	0
Total minimum lease payments	393,000
Operating Leases, Rent Expense, Net [Abstract]
Lease expense	800,000	$ 700,000	$ 800,000
Building
Operating Leased Assets [Line Items]
Monthly lease rent	24,100
Annual rent increase (percent)				3.00%
Addition monthly base rent	$ 4,000

QUARTERLY FINANCIAL DATA (Detai

QUARTERLY FINANCIAL DATA (Details) - USD ($)	3 Months Ended								12 Months Ended
QUARTERLY FINANCIAL DATA (Details) - USD ($)	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
License and collaboration revenues	$ 3,459,000	$ 0	$ 0	$ 9,467,000					$ 12,926,000	$ 0	$ 0
Loss from operations	(29,672,000)	(28,939,000)	(28,670,000)	(15,346,000)	$ (18,228,000)	$ (16,601,000)	$ (18,616,000)	$ (18,090,000)	(102,627,000)	(71,535,000)	(83,779,000)
Net loss	$ (28,272,000)	$ (27,568,000)	$ (27,869,000)	$ (14,709,000)	$ (17,895,000)	$ (16,350,000)	$ (18,339,000)	$ (17,846,000)	$ (98,418,000)	$ (70,430,000)	$ (83,118,000)
Loss per share, basic and diluted (USD per share)	$ (0.87)	$ (0.85)	$ (0.94)	$ (0.51)	$ (0.67)	$ (0.65)	$ (0.74)	$ (0.73)	$ (3.19)	$ (2.78)	$ (4.20)

SUBSEQUENT EVENT (Details)

SUBSEQUENT EVENT (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	Jan. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 17, 2019
Subsequent Event [Line Items]
Proceeds from Issuance Initial Public Offering		$ 130,663	$ 153,530	$ 0
Common Stock \| Subsequent Event
Subsequent Event [Line Items]
Issuance of common stock, net of costs (in shares)	1,854,838
Public offering stock price (USD per share)					$ 62
Proceeds from Issuance Initial Public Offering	$ 107,900

Uncategorized Items - mrtx-2018

Label	Element	Value
Additional Paid-in Capital [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption	$ 446,000
Retained Earnings [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption	$ (446,000)