Y-mAbs Therapeutics (YMAB) 10-K 1 Mar 21 2020 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Feb. 22, 2021	Jun. 30, 2020
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2020
Document Transition Report	false
Entity File Number	001-38650
Entity Registrant Name	Y-mAbs Therapeutics, Inc.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	47-4619612
Entity Address, Address Line One	230 Park Avenue
Entity Address, Address Line Two	Suite 3350
Entity Address, City or Town	New York
Entity Address, State or Province	NY
Entity Address, Postal Zip Code	10169
City Area Code	646
Local Phone Number	885-8505
Title of 12(b) Security	Common Stock
Trading Symbol	YMAB
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
ICFR Auditor Attestation Flag	true
Entity Public Float			$ 990.1
Entity Common Stock, Shares Outstanding		43,526,254
Entity Central Index Key	0001722964
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Amendment Flag	false

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
CURRENT ASSETS
Cash and cash equivalents	$ 114,634	$ 207,136
Other current assets	7,729	4,819
Total current assets	122,363	211,955
Property and equipment, net	1,825	2,052
Operating lease right-of-use assets	4,569	1,989
Other assets	3,290	370
TOTAL ASSETS	132,047	216,366
LIABILITIES
Accounts payable	9,372	8,520
Accrued liabilities	8,197	4,550
Operating lease liabilities, current portion	1,966	516
Total current liabilities	19,535	13,586
Accrued milestone and royalty payments	2,695	1,921
Operating lease liabilities, long-term portion	2,013	1,714
Other liabilities	1,968	242
TOTAL LIABILITIES	26,211	17,463
Commitments and contingencies (Note 6)
STOCKHOLDERS' EQUITY
Preferred stock, $0.0001 par value, 5,500,000 shares authorized at December 31, 2020 and December 31, 2019; none issued at December 31, 2020 and December 31, 2019
Common stock, $0.0001 par value, 100,000,000 shares authorized at December 31, 2020 and December 31, 2019; 40,688,447 and 39,728,416 shares issued at December 31, 2020 and December 31, 2019, respectively	4	4
Additional paid in capital	391,558	364,712
Accumulated other comprehensive income / (loss)	(526)	50
Accumulated deficit	(285,200)	(165,863)
TOTAL STOCKHOLDERS' EQUITY	105,836	198,903
TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY	$ 132,047	$ 216,366

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2020	Dec. 31, 2019
Consolidated Balance Sheets
Preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Preferred stock, authorized shares	5,500,000	5,500,000
Preferred stock, shares issued	0	0
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock, authorized shares	100,000,000	100,000,000
Common stock, shares issued	40,688,447	39,728,416

Consolidated Statements of Net

Consolidated Statements of Net Loss and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Consolidated Statements of Net Loss and Comprehensive Loss
Revenue	$ 20,750	$ 0
Revenue from Contract with Customer, Product and Service [Extensible List]	Us-gaap:LicenseMember	Us-gaap:LicenseMember
OPERATING EXPENSES
Research and development	$ 93,697	$ 63,492
General and administrative	44,785	19,512
Royalties	2,203
Total operating expenses	140,685	83,004
Loss from operations	(119,935)	(83,004)
OTHER INCOME, NET
Interest and other income	598	1,976
NET LOSS	(119,337)	(81,028)
Other comprehensive income / (loss)
Foreign currency translation	(576)	43
COMPREHENSIVE LOSS	$ (119,913)	$ (80,985)
Net loss per share attributable to common stockholders, basic and diluted	$ (2.97)	$ (2.30)
Weighted average common shares outstanding, basic and diluted	40,118,537	35,183,488

Consolidated Statements of Chan

Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income/(Loss)	Accumulated Deficit	Total
Balance at the beginning of period at Dec. 31, 2018	$ 3	$ 225,352	$ 7	$ (84,835)	$ 140,527
Balance at the beginning of period (in shares) at Dec. 31, 2018	34,193,666
Increase (Decrease) in Stockholders' Equity
Issuance of common stock to investors, net of issuance costs	$ 1	134,703			134,704
Issuance of common stock to investors, net of issuance costs (in shares)	5,134,750
Issuance of common stock to non-employees (in shares)	400,000
Stock-based compensation expense		4,657			4,657
Foreign currency translation			43		43
Net loss				(81,028)	(81,028)
Balance at the end of period at Dec. 31, 2019	$ 4	364,712	50	(165,863)	198,903
Balance at the end of period (in shares) at Dec. 31, 2019	39,728,416
Increase (Decrease) in Stockholders' Equity
Exercise of stock options		2,004			$ 2,004
Exercise of stock options (in shares)	299,706				299,706
Issuance of common stock to non-employees		8,707			$ 8,707
Issuance of common stock to non-employees (in shares)	656,896
Stock-based compensation expense		16,135			16,135
Stock-based compensation expense (in shares)	3,429
Foreign currency translation			(576)		(576)
Net loss				(119,337)	(119,337)
Balance at the end of period at Dec. 31, 2020	$ 4	$ 391,558	$ (526)	$ (285,200)	$ 105,836
Balance at the end of period (in shares) at Dec. 31, 2020	40,688,447

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
CASH FLOWS FROM OPERATING ACTIVITIES
Net loss	$ (119,337)	$ (81,028)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	396	166
Stock-based compensation	16,135	4,657
Non-cash expense in connection with equity issuance to MSK/MIT	1,331
Non-cash expense in connection with equity issuance to inventors	7,376
Foreign currency transactions	(576)	43
Changes in assets and liabilities:
Other current assets	(3,848)	(1,158)
Other assets	(311)	(182)
Accounts payable	852	2,617
Accrued liabilities and other	6,751	1,388
NET CASH USED IN OPERATING ACTIVITIES	(91,231)	(73,497)
CASH FLOWS FROM INVESTING ACTIVITIES
Purchase of property and equipment	(175)	(1,965)
Loans to inventors	(2,610)
NET CASH USED IN INVESTING ACTIVITIES	(2,785)	(1,965)
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from issuance of common stock, net of issuance costs		134,704
Proceeds from exercised stock options	2,004
NET CASH PROVIDED BY FINANCING ACTIVITIES	2,004	134,704
Effect of exchange rates on cash and cash equivalents	(490)	23
NET DECREASE IN CASH AND CASH EQUIVALENTS	(92,502)	59,265
Cash and cash equivalents at the beginning of period	207,136	147,871
Cash and cash equivalents at the end of period	114,634	207,136
SUPPLEMENTAL DISCLOSURE OF NON-CASH ACTIVITIES
Property and equipment purchases in accounts payable		49
Right-of-use assets obtained in exchange for lease obligations	$ 2,679	$ 901

ORGANIZATION AND DESCRIPTION OF

ORGANIZATION AND DESCRIPTION OF BUSINESS	12 Months Ended
	Dec. 31, 2020
ORGANIZATION AND DESCRIPTION OF BUSINESS
ORGANIZATION AND DESCRIPTION OF BUSINESS	NOTE 1—ORGANIZATION AND DESCRIPTION OF BUSINESS Y-mAbs Therapeutics, Inc. (“we,” “us,” “our,” the “Company,” or “Y-mAbs”) is a commercial-stage biopharmaceutical company focused on the development and commercialization of novel, antibody based therapeutic products for the treatment of cancer. We are leveraging our proprietary antibody platforms and deep expertise in the field of antibodies to develop a broad portfolio of innovative medicines. The Company is headquartered in New York and was incorporated on April 30, 2015 under the laws of the State of Delaware.

BASIS OF PRESENTATION

BASIS OF PRESENTATION	12 Months Ended
	Dec. 31, 2020
BASIS OF PRESENTATION
BASIS OF PRESENTATION	NOTE 2—BASIS OF PRESENTATION The Company has incurred losses since inception. Operations of the Company are subject to certain risks and uncertainties, including, among others, uncertainty of drug candidate development; technological uncertainty; uncertainty regarding patents and proprietary rights; uncertainty in obtaining FDA approval in the United States and regulatory approval in other jurisdictions; marketing or sales capability or experience; uncertainty in getting adequate payer coverage and reimbursement; dependence on key personnel; compliance with government regulations and the need to obtain additional financing. The Company’s drug candidates currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel infrastructure and extensive compliance-reporting capabilities. The Company’s drug candidates are in various stages of development. DANYELZA (naxitamab-gqgk) was approved by the U.S. FDA in November 2020, but there can be no assurance that the Company’s other research and development will be successfully completed, that adequate protection for the Company’s intellectual property will be obtained, that any products developed will obtain necessary government regulatory approval or that any approved products will be commercially viable. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will generate significant revenue from product sales. The Company operates in an environment of rapid change in technology and substantial competition from pharmaceutical and biotechnology companies. The Company’s financial statements have been prepared on the basis of continuity of operations, realization of assets and the satisfaction of liabilities in the ordinary course of business. The Company has experienced negative cash flows from operations since inception and had an accumulated deficit of $285.2 million as of December 31, 2020 and $165.9 million as of December 31, 2019. Through December 31, 2020, the Company has funded its operations through proceeds from sales of shares of its common stock, including its initial public offering in September 2018 and its secondary public offering in November 2019. As discussed in Note 12—Subsequent Events, after December 31, 2020, the Company announced the closing of its public offering of 2,804,878 shares of its common stock, at a public offering price of $41.00 per share, which includes the exercise in full of the underwriters' option to purchase 365,853 additional shares of common stock. The aggregate gross proceeds to Y-mAbs, before deducting underwriting discounts and commissions and estimated offering expenses payable by the Company, were approximately $115 million. As of December 31, 2020, the Company had cash and cash equivalents of $114.6 million, and as of December 31, 2019 the Company had cash and cash equivalents of $207.1 million. As of the issuance date of the financial statements for the year ended December 31, 2020, the Company expects that its cash and cash equivalents at December 31, 2020 will be sufficient to fund its operating expenses and capital expenditure requirements through at least the next twelve months, irrespective of whether any additional product approvals are obtained. The Company may raise additional capital to fund future operations through the sale of its equity securities, incurring debt, entering into licensing or collaboration agreements with partners, grants or other sources of financing. Sufficient funds may not be available to the Company on attractive terms or at all when needed from equity or debt financing. If FDA approval for omburtamab does not occur or is significantly delayed, and the Company is unable to obtain additional financing from these or other sources when needed, it will likely be necessary to take other actions to enhance its liquidity position which may include significantly reducing the current rate of spending through delaying, scaling back current operations, or suspending certain research and development programs and other operational programs. The accompanying consolidated financial statements reflect the accounts of the Company and its wholly-owned subsidiary and have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”). All intercompany balances and transactions have been eliminated.

SUMMARY OF SIGNIFICANT ACCOUNTI

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	12 Months Ended
	Dec. 31, 2020
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	NOTE 3—SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of expenses during the reporting period. Significant estimates and assumptions reflected in these financial statements include, but are not limited to, the accrual for research and development expenses, the accrual of milestone and royalty payments, and the valuation of stock options. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. Actual results could differ from those estimates. The full extent to which the COVID-19 pandemic will directly or indirectly impact our business, results of operations and financial condition, including expenses, manufacturing, clinical trials, research and development costs and employee-related amounts, will depend on future developments that are highly uncertain, including as a result of new information that may emerge concerning COVID-19 and the actions taken to contain it or treat COVID-19, as well as the economic impact on local, regional, national and international markets. We have made estimates of the impact of COVID-19 within our financial statements and there may be changes to those estimates in future periods. Actual results may differ from these estimates. Cash and Cash Equivalents The Company considers all highly liquid instruments with original maturities of three months or less from date of purchase to be cash equivalents. All cash and cash equivalents are held in highly rated securities including a Treasury money market fund which is unrestricted as to withdrawal or use. To date, the Company has not experienced any losses on its cash and cash equivalents. The carrying amount of cash and cash equivalents approximates its fair value due to its short-term and liquid nature. We maintain cash balances in excess of insured limits. We do not anticipate any losses with respect to such cash balances. Fair Value Measurements Certain assets and liabilities are carried at fair value under GAAP. ​ • Level 1 — Unadjusted quoted prices for identical assets or liabilities in active markets; • Level 2 — Inputs other than quoted prices in active markets for identical assets and liabilities that are observable either directly or indirectly for substantially the full term of the asset or liability; and • Level 3 — Unobservable inputs for the asset or liability, which include management's own assumption about the assumptions market participants would use in pricing the asset or liability, including assumptions about risk. Cash equivalents held in money market funds are valued using other significant observable inputs, which represent a Level 2 measurement within the fair value hierarchy. The Company has no other cash equivalents. The following tables present the Company’s fair value hierarchy for its cash equivalents, which are measured at fair value on a recurring basis (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements at December 31, 2020 Using: ​ Level 1 Level 2 Level 3 Total Cash equivalents: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Money market funds ​ $ — ​ $ 97,302 ​ $ — ​ $ 97,302 ​ ​ $ — ​ $ 97,302 ​ $ — ​ $ 97,302 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements at December 31, 2019 Using: ​ Level 1 Level 2 Level 3 Total Cash equivalents: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Money market funds ​ $ — ​ $ 197,879 ​ $ — ​ $ 197,879 ​ ​ $ — ​ $ 197,879 ​ $ — ​ $ 197,879 ​ During the years ended December 31, 2020 and 2019, there were no transfers between Level 1, Level 2 and Level 3. Leases As described below, the Company adopted Topic 842 as of January 1, 2019. The Company determines if an arrangement includes a lease at inception. Operating lease right-of-use assets represent the Company’s right to use an underlying asset for the lease term and operating lease liabilities represent its obligation to make lease payments arising from the lease. Operating lease right-of-use assets and liabilities are recognized at the lease commencement date based on the present value of lease payments over the lease term. In determining the net present value of lease payments, the Company uses its estimated incremental borrowing rate based on information available at the lease commencement date. Because most of the Company’s leases do not provide an implicit rate of return, an incremental borrowing rate is used based on the information available at the commencement date in determining the present value of lease payments on an individual lease basis. The Company’s incremental borrowing rate for a lease is the estimated rate of interest it would have to pay on a collateralized basis to borrow an amount equal to the lease payments under similar terms. The Company’s leases may include options to extend or terminate the lease which are included in the lease term when it is reasonably certain that it will exercise any such options. None of the Company’s leases contain any residual value guarantees. Lease expense is recognized on a straight-line basis over the expected lease term. Related variable lease costs incurred are not material to the Company. Topic 842 also provides practical expedients and certain exemptions for an entity’s ongoing accounting post implementation. The Company currently elected the short-term lease recognition exemption for all leases that qualify. This means, for those leases that qualify, we will not recognize right-of-use assets or liabilities, and this includes not recognizing right-of-use assets or liabilities for existing short-term leases of those assets in transition. We also elected the practical expedient to not separate lease and non-lease components for all of our leases. The Company has made an accounting policy election to account for each separate lease component of a contract and its associated non-lease components as a single lease component. See the Lease Agreements section in Note 6 for the related disclosures. Property and Equipment Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization expense is recognized using the straight-line method over the estimated useful life of each asset as follows: ​ ​ ​ ​ ​ Estimated Useful Life Furniture and fixtures 5 years Machinery and equipment ​ 5 years Leasehold improvements Shorter of life of lease or 15 years ​ Depreciation expense on property and equipment was $396,000 and $166,000 for the years ended December 31, 2020 and 2019, respectively. Upon retirement or sale, the cost of assets disposed of and the related accumulated depreciation and amortization are removed from the accounts and any resulting gain or loss is included in loss from operations. Expenditures for repairs and maintenance are charged to expense as incurred. Impairment of Long-Lived Assets ASC 360, Property, Plant and Equipment, addresses the financial accounting and reporting for impairment or disposal of long-lived assets. The Company reviews the recorded values of long-lived assets for impairment whenever events or changes in business circumstance indicate that the carrying amount of an asset or group of assets may not be fully recoverable. Income Taxes The Company accounts for income taxes under the asset and liability approach for the financial accounting and reporting of income taxes. Deferred tax assets and liabilities are recognized for the estimated future tax consequences attributable to net operating loss carry forwards and temporary differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases. These assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which the temporary differences are expected to reverse. We maintain a full valuation allowance on our deferred tax assets based on cumulative historical and expected losses. If we successfully commercialize our products and achieve profitability, we will consider the continued need for such valuation allowance. The Company prepares and files tax returns based on its interpretation of tax laws and regulations. In the normal course of business, the Company’s tax returns are subject to examination by various taxing authorities. Such examinations may result in future tax and interest assessments by these taxing authorities. In determining the Company’s tax provision for financial reporting purposes, the Company establishes a reserve for uncertain tax positions unless such positions are determined to be more likely than not of being sustained upon examination based on their technical merits. The Company considers many factors when evaluating and estimating its tax positions and tax benefits, which may require periodic adjustments and which may not accurately anticipate actual outcomes. Accordingly, the Company will report a liability for unrecognized tax benefits resulting from any uncertain tax positions taken or expected to be taken on a tax return. The Company’s policy is to recognize, when applicable, interest and penalties on uncertain tax positions as part of income tax expense. In accordance with guidance issued by Financial Accounting Standards Board (“FASB”), companies should make and disclose a policy election as to whether they will recognize deferred taxes for basis differences expected to reverse as Global Intangible Low-Taxed Income (“GILTI”) or whether they will account for GILTI as period costs if and when incurred. The Company has elected to recognize the resulting tax with respect to the GILTI provision as a period cost. No costs were incurred by the Company through December 31, 2020 as a result of GILTI. Revenue Recognition To determine revenue recognition for arrangements that an entity determines are within the scope of ASC 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 arrangements that meet the definition of a contract with a customer under ASC 606, including 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 ASC 606, the company assesses the goods or services promised within each contract, determines those that are performance obligations, and assess 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. Under the practical expedient permitted under Topic 606, the Company expenses incremental costs of obtaining a contract as and when incurred if the expected amortization period of the assets is one year or less. If there are multiple distinct performance obligations, we allocate the transaction price to each distinct performance obligation based on its relative standalone selling price. In assessing whether a promised good or service is distinct in the evaluation of a license arrangement subject to ASC 606, the Company considers factors such as the research, manufacturing and commercialization capabilities of the licensing partner and the availability of the associated expertise in the general marketplace. The Company also considers the intended benefit of the contract in assessing whether a promised good or service is separately identifiable from other promises in the contract. If a promised good or service is not distinct, the Company combines that good or service with other promised goods or services until it identifies a bundle of goods or services that is distinct. The standalone selling price is generally determined based on the prices charged to customers or using expected cost-plus margin. The Company received FDA approval of DANYELZA in November 2020 and entered into exclusive licensing and distribution agreements for DANYELZA and omburtamab in certain international territories resulting in total license revenue of $20.8 million for the year ended December 31, 2020. Of this amount, we have $0.8 million as an outstanding receivable included within other current assets. In December 2020, the Company entered into a development and commercialization arrangement with SciClone International Pharamceuticals Ltd. (SciClone) for certain indications of DANYELZA and omburtamab within China. As part of the agreement, we received a nonrefundable up-front fee of $20 million for the transfer of the license and know-how related to the product indications. The Company may receive regulatory-based milestone payments up to $40 million and sales-based milestone payments up to $60 million and is entitled to royalties based upon the net sales generated by SciClone related to the product indications in the territory. We considered the license to be distinct from other promises within the arrangement based on the rights and know-how transferred, late-stage development of the underlying indications and anticipated lack of significant involvement required from the joint steering committee associated with the indications. Accordingly, the full transaction price of $20 million was recognized upon transferring of the license and know-how to SciClone. The future potential regulatory milestone amounts were not included in the transaction price, as they were all determined to be fully constrained following the concepts of ASC 606. As part of our evaluation of the regulatory milestones constraint, we determined that the achievement of such milestones are contingent upon regulatory approvals which are not within our control and therefore not deemed probable. We expect that the sales-based milestone payments and royalty arrangements will be recognized when the related sales occur or the milestone is achieved. We will re-evaluate the transaction price each quarter and as uncertain events are resolved or other changes in circumstances occur, we assess whether this resolves the constraint and revenue will be recognized. We also considered that the manufacturing and supply terms, including within the arrangement, did not represent a material right to SciClone at inception as the terms reflected stand-alone selling price for similar goods or services. There was no license revenue generated in the period ended December 31, 2019. In addition, the Company recorded $2.2 million in royalty expense which remained in accrued liabilities as of December 31, 2020 which are due to MSK based on the terms of the original MSK License agreement. Research and Development Costs Research and development costs are charged to operations when incurred and are included in operating expenses. Research and development costs consist principally of compensation cost for our employees and consultants that perform our research activities, the fees paid to maintain our licenses, the payments to third parties for manufacturing and clinical research organizations and additional product development, and consumables and other materials used in research and development. The Company records accruals for estimated ongoing research and development costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies or clinical trials, including the phase or completion of events, invoices received and contracted costs. Actual results could differ from the Company’s estimates. The Company is obligated to make certain milestone and royalty payments in accordance with the contractual terms of its license agreement with MSK based upon the resolution of certain contingencies. The Company records the milestone and royalty payments when the achievement of the milestone or payment of the milestone or royalty is probable and the amount of the payment is reasonably estimable. Research and development costs were $93.7 million and $63.5 million for the years ended December 31, 2020 and 2019, respectively. Additionally, the Company is obligated to make certain royalty and clinical, regulatory and sales-based milestone payments in accordance with the contractual terms of the MSK License, CD33 License, MabVax Sublicense, and SADA License based upon the resolution of certain contingencies. Royalty payments and sales-based milestones are not due and deemed not probable as of December 31, 2020, as we have not recognized revenue for our only approved product, DANYELZA. We record the clinical and regulatory milestone payments when the achievement of the milestones or payment of the milestones is deemed probable, and the amount of the payment is reasonably estimable. As it relates to clinical and regulatory milestone payments under the SADA agreement, those may become due and payable with the passage of time whether or not the milestones have actually been met. When evaluating whether licensing milestones should be recognized under the SADA agreement, the Company uses its collective clinical experience across the company to determine the likelihood of achievement, as well as the current stage of the compounds under development, and estimates the progress of its preclinical studies and clinical trials, completion of milestone events per underlying agreements, the time expected to complete certain development activities, each party’s termination right under the license agreements, invoices received and contracted costs when evaluating whether the clinical milestones should be recognized in each reporting period. We review our assessment each period and make revisions to such estimates as necessary. We recognized research and development costs in connection with SADA licensing milestones of $0.6 million for the year ended December 31, 2020 as we deemed them probable of occuring. The Company determined that the achievement of the regulatory milestones under the SADA agreement were not probable given the current stage of the product development. The remaining total clinical milestones of $4.1 million were determined to not be probable as of December 31, 2020, as detailed in Note 6 – License Agreements And Commitments ​ ​ Patent Costs The Company expenses the costs of obtaining and maintaining patents as general and administrative expenses. Advertising and Promotion Costs Advertising and promotion costs are included in general and administrative expenses and were immaterial in the years ended December 31, 2020 and 2019. Advertising and product promotion costs are expensed as incurred. Stock-Based Compensation The Company measures stock options granted to employees, directors and consultants based on the fair value on the date of the grant and recognizes compensation expense of those awards, over the requisite service period, which for employees and directors is the vesting period of the respective award. Forfeitures are accounted for as they occur. The Company issues stock options with only service-based or immediate vesting conditions and records the expense for these awards using the straight-line method over the requisite service period. Following the Company’s adoption of ASU 2018-07, Compensation—Stock Compensation (Topic 718), Improvements to Nonemployee Share-Based Payment Accounting (“ASU 2018-07”), on January 1, 2019, for stock-based option awards issued to non-employees, the Company no longer revalues non-employee awards at each reporting date and instead calculates the fair value of the awards as of the grant date using the Black-Scholes option-pricing model. The overall fair value measurement for non-employees awards is consistent with the awards issued to employees. The fair value of each stock option grant is estimated on the date of grant using the Black-Scholes option pricing model. Prior to September 2018, the Company historically was a private company and lacks sufficient company-specific historical and implied volatility information for its shares. Therefore, it estimates its expected share price volatility based on the historical volatility of a group of publicly-traded peer companies and expects to continue to do so until such time as it has adequate historical data regarding the volatility of its own traded share price. The expected term of the Company’s stock options has been determined utilizing the “simplified” method for awards as the Company has limited historical data to support the expected term assumption. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. The expected dividend yield is based on the fact that the Company has never paid cash dividends on common shares and does not expect to pay any cash dividends in the foreseeable future. Stock-based compensation costs were $16.1 million and $4.7 million for the years ended December 31, 2020 and 2019, respectively. Segment Information The Company is engaged solely in the discovery and development of novel antibody-based therapeutic products for the treatment of cancer. Accordingly, the Company has determined that it operates in one operating segment. Comprehensive Loss Comprehensive loss includes net loss as well as other changes in stockholders’ equity that result from transactions and economic events other than those with shareholders. The difference between net loss and comprehensive loss for the period presented in the accompanying financial statements was due to foreign currency translation. Foreign Currency The financial statements of our Danish subsidiary with a functional currency other than the U.S. dollar are translated into U.S. dollars using period-end exchange rates for assets and liabilities, historical exchange rates for stockholders’ equity and weighted average exchange rates during the period for operating results. Translation gains and losses are included in accumulated other comprehensive income / (loss), net of tax, in stockholders’ equity. Foreign currency transaction gains and losses are included in the results of operations in other income and expense, and totaled $(576,000) and $43,000 for the years ended December 31, 2020 and 2019, respectively. Recently Issued Accounting Pronouncements - Adopted In October 2020, the FASB issued Accounting Standards Update No. 2020-10 (“ASU 2020-10”), Codification improvements. The amendments in this Update represent changes to clarify the Codification, correct unintended application of guidance, or make minor improvements to the Codification. The adoption of this standard on December 31, 2020 did not have a material impact on our consolidated financial statements and related disclosures. In August 2018, the FASB issued Accounting Standards Update No. 2018 -13 (“ASU2018-13”), Fair Value Measurement (Topic 820) Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement. ASU 2018-13 allows to remove the reasons for transfer between Level 1 and Level 2 assets, and adds the changes in unrealized gains and losses for recurring level 3 fair value measurements. ASU 2018-13 is effective for fiscal years beginning after December 15, 2019, and interim periods within those fiscal years with early adoption permitted. The adoption of this standard on January 1, 2020 did not have a material impact on our consolidated financial statements and related disclosures. In August 2018, the FASB issued Accounting Standards Update No. 2018-15 (“ASU 2018-15”), Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract. ASU 2018-15 clarifies certain aspects of ASU 2015-05, Customer’s Accounting for Fees Paid in a Cloud Computing Arrangement, which was issued in April 2015. Specifically, ASU 2018-15 aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal use software (and hosting arrangements that include an internal-use software license). ASU 2018-15 is effective for fiscal years beginning after December 15, 2019, and interim periods within those fiscal years with early adoption permitted. The adoption of this standard on January 1, 2020 did not have a material impact on our consolidated financial statements and related disclosures. In July 2018, the FASB issued Accounting Standards Update No. 2018-09 (“ASU 2018-09”), Codification Improvements, which clarify, correct errors in, or make minor improvements to a variety of ASC topics. The changes in ASU 2018-09 are not expected to have a significant effect on current accounting practices. Some of the amendments in this update do not require transition guidance and will be effective upon this update. However, many of the updates do have transition guidance with effective dates for periods beginning after December 15, 2018. The adoption of this standard on January 1, 2019 did not have a material impact on our consolidated financial statements and related disclosures. In June 2018, the FASB issued ASU No. 2018-07, Compensation—Stock Compensation (Topic 718), Improvements to Nonemployee Share-Based Payment Accounting (“ASU 2018-07”). ASU 2018-07 is intended to simplify aspects of share-based compensation issued to non-employees by making the guidance consistent with the accounting for employee share-based compensation. ASU 2018-07 is required to be adopted for annual periods beginning after December 15, 2018, including interim periods within those fiscal years. Early adoption is permitted. The adoption of this standard on January 1, 2019 did not have a material impact on our consolidated financial statements and related disclosures. In February 2018, the FASB issued Accounting Standards Update No. 2018-02, (“ASU 2018-02”), Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income. ASU 2018-02 allows a reclassification from accumulated other comprehensive income to retained earnings for stranded tax effects resulting from the Tax Cuts and Jobs Act. ASU 2018-07 is required to be adopted for annual periods beginning after December 15, 2018, including interim periods within those fiscal years. Early adoption is permitted. The adoption of this standard on January 1, 2019 did not have a material impact on our consolidated financial statements and related disclosures. In June 2016, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update No. 2016-13 (“ASU 2016-13”) Financial Instruments—Credit Losses (Topic 326): Measurement of credit losses on financial instruments. ASU 2016-13 introduces the current expected credit losses methodology (CECL) for estimating allowances for credit losses. The standard is effective in fiscal years and interim periods beginning after December 15, 2019. The adoption of this standard on January 1, 2020 did not have a material impact on our consolidated financial statements and related disclosures. In February 2016, the FASB issued Accounting Standards Update No. 2016-02 (“ASU 2016-02”), Leases, which is effective for fiscal years, and interim periods within those years, beginning after December 15, 2018 with early adoption permitted. Under ASU 2016-02, lessees will be required to recognize for most leases, at the commencement date of the lease, a lease liability, which is a lessee’s obligation to make lease payments arising from a lease measured on a discounted basis, and a right-to-use asset, which is an asset that represents the lessee’s right to use or control the use of a specified asset for the lease term. Topic 842 was subsequently amended by ASU 2017-13, Revenue and Leases: Amendments to SEC Paragraphs Pursuant to the Staff Announcement at the July 20, 2017 EITF Meeting and Rescission of Prior SEC Staff Announcements and Observer Comments; ASU 2018-01, Land Easement Practical Expedient for Transition to Topic 842; ASU No. 2018-10, Codification Improvements to Topic 842, Leases; ASU No. 2018-11, Targeted Improvements and ASU No. 2018-20, Narrow Scope Improvements for Lessors. The Company adopted the new leasing standards using the modified retrospective restatement of prior periods or cumulative adjustment to retained earnings. An entity may choose to use either (1) its effective date or (2) the beginning of the earliest comparative period presented in the financial statements as its date of initial application. The Company used the effective date as our date of initial application. Consequently, financial information was not updated and the disclosures required under the new standard are not provided for dates and periods before January 1, 2019. The new standard also provides a number of optional practical expedients in transition. The Company elected the Upon adoption of the new leasing standards, the Company recognized a lease liability of $1.8 million and a related right-of-use asset of $1.5 million with the difference being due to the elimination of previously reported deferred rent. Please refer to lease footnotes at Note 6 License Agreements And Commitments.

NET LOSS PER SHARE

NET LOSS PER SHARE	12 Months Ended
	Dec. 31, 2020
NET LOSS PER SHARE
NET LOSS PER SHARE	NOTE 4—NET LOSS PER SHARE Basic net loss per share (“EPS”) is calculated by dividing net income or loss attributable to common stockholders by the weighted average common stock outstanding. Diluted EPS is calculated by adjusting weighted average common shares outstanding for the dilutive effect of common stock options and restricted stock units. In periods in which a net loss is recorded, no effect is given to potentially dilutive securities, since the effect would be antidilutive. Securities that could potentially dilute basic EPS in the future were not included in the computation of diluted EPS because to do so would have been antidilutive. The calculations of basic and diluted net loss per share are as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year ended December 31, ​ ​ 2020 ​ 2019 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Net loss (numerator) ​ $ (119,337) ​ $ (81,028) ​ Weighted-average shares (denominator) ​ 40,119 ​ 35,183 ​ Basic and diluted net loss per share ​ $ (2.97) ​ $ (2.30) ​ ​ Potentially dilutive securities excluded from the computation of diluted earnings per share relate to stock options and unvested restricted share units (“RSUs”) outstanding totaled 5,704,246 shares as of December 31, 2020 and 4,005,873 shares as of December 31, 2019, and were excluded because including them would have an anti-dilutive impact.

ACCRUED LIABILITIES

ACCRUED LIABILITIES	12 Months Ended
	Dec. 31, 2020
ACCRUED LIABILITIES
ACCRUED LIABILITIES	NOTE 5—ACCRUED LIABILITIES Accrued short-term liabilities are as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, December 31, ​ 2020 ​ 2019 ​ ​ (in thousands) ​ ​ ​ ​ ​ ​ ​ Accrued licensing, milestone and royalty payments ​ $ 3,608 ​ $ 354 Accrued clinical costs ​ 678 ​ 1,584 Accrued compensation and board fees ​ 2,603 ​ 1,475 Accrued manufacturing costs ​ ​ 983 ​ ​ 760 Other ​ 325 ​ 377 Total ​ $ 8,197 ​ $ 4,550 ​

LICENSE AGREEMENTS AND COMMITME

LICENSE AGREEMENTS AND COMMITMENTS	12 Months Ended
	Dec. 31, 2020
LICENSE AGREEMENTS AND COMMITMENTS
LICENSE AGREEMENTS AND COMMITMENTS	NOTE 6—LICENSE AGREEMENTS AND COMMITMENTS As of December 31, 2020, the Company has entered into two license agreements and certain other agreements with Memorial Sloan Kettering Cancer Center (“MSK”). The license agreements are the MSK License and the CD33 License Agreement. Through a Settlement and Assumption and Assignment of the MSK License and Y-mAbs Sublicense Agreement (“SAAA”) with MabVax, Inc. (“MabVax”) and MSK, the Company has established a direct license with MSK relating to the GD2-GD3 Vaccine, which was originally sublicensed by the Company in 2018 from MabVax. In addition, the Company has entered into the SADA License Agreement with MSK and Massachusetts Institute of Technology (“MIT”) in 2020. The license agreement with MSK and MIT grants the Company certain patent rights and intellectual property rights, and in consideration thereof, the Company agreed to make certain payments and issue shares of the Company’s common stock to MSK and MIT. Certain of the payments are contingent milestone and royalty payments, as disclosed in the table below. Amounts disclosed in Note 5 for accrued milestone and royalty payments are inclusive of obligations under the MSK License, CD33 License Agreement and SADA License Agreement, collectively. We have the following significant license agreements and related commitments which include all obligations that have been paid or are accrued as of and for the period ending December 31, 2020: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Cash paid Cash paid Expense Expense Accrued liabilities Accrued liabilities Accrued liabilities Accrued liabilities ​ ​ Twelve months ​ Twelve months ​ Twelve months ​ Twelve months ​ Current as of ​ Non-current ​ Current as of ​ Non-current ​ ​ ended December ​ ended December ​ ended December ​ ended December ​ December ​ December ​ December ​ December Agreements ​ 2020 ​ 2019 ​ 2020 ​ 2019 ​ 2020 ​ 2020 ​ 2019 ​ 2019 MSK ​ ​ $ 80,000 ​ ​ $ 725,000 ​ ​ $ - ​ ​ $ 75,000 ​ ​ $ 305,000 ​ ​ $ 1,640,000 ​ ​ $ 254,000 ​ ​ $ 1,471,000 CD33 ​ ​ — ​ ​ — ​ ​ — ​ ​ — ​ ​ 100,000 ​ ​ 450,000 ​ ​ 100,000 ​ ​ 450,000 MabVax ​ ​ — ​ ​ 600,000 ​ ​ — ​ ​ — ​ ​ — ​ ​ — ​ ​ — ​ ​ — ​ The below table represents the maximum clinical, regulatory or sales-based miletones as reflected within the agreements, certain of which have been paid in prior periods or are accrued as presented in the table above: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Maximum Maximum Maximum Agreements ​ Clinical Milestones ​ Regulatory Milestones ​ Sales-based milestones ​ MSK ​ ​ $ 2,450,000 ​ ​ $ 9,000,000 ​ ​ $ 20,000,000 ​ CD33 ​ ​ 550,000 ​ ​ 500,000 ​ ​ 7,500,000 ​ MabVax ​ ​ 200,000 ​ ​ 1,200,000 ​ ​ — ​ ​ Minimum royalties and certain clinical milestones that become due based upon the passage of time under the CD33 License Agreement and the MabVax Agreement are not recorded as a liability as the Company does not consider such obligations to be probable as of December 31, 2020. ​ SADA License Agreement On April 15, 2020, we entered into a license agreement (the “SADA License Agreement”) with MSK and Massachusetts Institute of Technology (“MIT”) that grants us an exclusive worldwide, sublicensable license to MSK’s and MIT’s rights to certain patent and intellectual property to develop, make, and commercialize licensed products and to perform services for all therapeutic and diagnostic uses in the field of cancer diagnostics and cancer treatments using the SADA BiDE Pre-targeted Radioimmunotherapy Platform (“SADA technology”). We have assessed the licensing and other rights acquired and given the lack of outputs upon acquisition and that no employees were acquired, among other factors, we have concluded that the licensing rights represented an asset acquisition. The patents and patent applications covered by this agreement are directed, in part, to the SADA technology, as well as a number of SADA constructs developed by MSK. Upon entering into the SADA License Agreement and in exchange for the licenses granted thereunder, we concluded that the technology acquired under the licensing arrangement had no alternative future use. This conclusion was based on consideration of the rights conveyed under the agreement, extent of further development necessary and presence of uncertainty prior to obtaining regulatory approval for any product. Accordingly, we expensed and paid an upfront payment to MSK and MIT of $1,995,000. During the year ended December 31, 2020, we expensed $3,331,000 associated with stock grants available to MSK and MIT. This includes of future stock grants which will be paid on the anniversary date of the SADA License Agreement in 2021 and 2022. These awards survive the potential termination of the licensing arrangement, unless a breach by the licensors occurs, and can be settled in cash or stock at the determination of the Company. During the year ended December 31, 2020, we expensed The SADA License Agreement requires us to pay to MSK and MIT mid to high single-digit royalties based on annual net sales of licensed products or the performance of licensed services by us and our affiliates and sublicensees. We are obligated to pay annual minimum royalties of $40,000, increasing to $60,000 once a patent has been issued, over the royalty term, commencing on the tenth anniversary of the license agreement. These amounts are non-refundable but are creditable against royalty payments otherwise due under the SADA License Agreement. The Company is also obligated to pay MSK and MIT certain clinical, regulatory and sales-based milestone payments under the SADA License Agreement. Certain of the clinical and regulatory milestone payments become due at the earlier of completion of the related milestone activity or the date indicated in the SADA License Agreement. In addition, to the extent we enter into sublicense arrangements, we are obligated to pay to MSK and MIT a percentage of certain payments received from sublicensees of the rights licensed to us by MSK and MIT, which percentage will be based upon the achievement of certain clinical milestones. The Company has not entered into any sublicenses related to the SADA License Agreement. For each of the constructs previously generated by MSK using the SADA technology and sold for the Company by a sublicensee, the Company may pay sales milestones up to Failure by the Company to meet certain conditions under the arrangement could cause the related license to such licensed products to be canceled and could result in termination of the entire arrangement with MSK and MIT. In addition, the Company may terminate the SADA License Agreement with prior written notice. Research and development is inherently uncertain and as described above, should such research and development fail, the SADA License Agreement is cancelable at the Company’s option. The Company will also consider the development risk and each party’s termination rights under the agreement when considering whether any clinical or regulatory based milestone payments, certain of which also contain time-based payment requirements, are probable. The Company records milestones in the period in which the contingent liability is probable and the amount is reasonably estimable. During the twelve months ended December 31, 2020, we expensed $605,000 of milestones under the SADA License Agreement. This includes all time-based milestones coming due within We have the following SADA related balances and commitments which include all obligations that have been paid or accrued as of and for the priod ending December 31, 2020: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Cash paid Expense Expense Accrued liabilities Accrued liabilities Accrued liabilities Accrued liabilities ​ ​ Agreements ​ Twelve months ended December 2020 ​ Twelve months ended December 2020 ​ Twelve months ended December 2019 ​ Current as of December 2020 ​ Non-current as of December 2020 ​ Current as of December 2019 ​ Non-current as of December 2019 ​ ​ SADA ​ ​ $ 2,707,000 ​ ​ $ 14,019,000 ​ ​ $ - ​ ​ $ 1,000,000 ​ ​ $ 1,605,000 ​ ​ $ - ​ ​ $ - ​ ​ ​ ​ The below table represents the maximum clinical, regulatory or sales-based miletones as reflected within the agreements, certain of which have been paid in prior periods or are accrued as presented in the table above: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Maximum Maximum Maximum Agreements ​ Clinical Milestones ​ Regulatory Milestones ​ Sales-based milestones SADA ​ ​ $ 4,730,000 ​ ​ $ 18,125,000 ​ ​ $ 23,750,000 ​ Minimum royalties and certain clinical milestones that become due based upon the passage of time under the SADA Agreement are not recorded as a liability as the Company does not consider such obligations to be probable as of December 31, 2020. ​ Other agreements We have also entered into various other support agreements with MSK including a sponsored research agreement to provide research services related to the intellectual property licensed under the MSK License Agreement; a master data services agreement, for services provided by approximately five full time employees at MSK, who are engaged in transferring clinical data, databases, regulatory files and other know-how included in the MSK License Agreement to the Company; a master clinical trial agreement pursuant to which we committed to fund certain clinical trials at MSK; two separate core facility service agreements pursuant to which we committed to obtaining certain laboratory services from MSK; and a CD33 sponsored research agreement pursuant to which we agreed to pay MSK to provide research services over a period of two years related to the intellectual property licensed under the CD33 License Agreement; and in October 2020 we entered into a SADA sponsored research agreement pursuant to which we agreed to pay MSK to provide research services over a period of three years related to the intellectual property licensed under the SADA License Agreement. During 2020 and 2019, we incurred research and development expenses of $4.2 million and $6.8 million, respectively, under these agreements. Lease Agreements In July 2019, the Company entered a development, manufacturing and supply agreement with SpectronRx in South Bend, Indiana, to secure access to clinical and commercial scale radiolabeling capacity for omburtamab. Under the terms of the agreement, SpectronRx has agreed to establish a manufacturing unit designated for the Company within its existing facilities, at which both clinical and commercial supply of radiolabeled omburtamab can be produced. Since the Company possesses the right to substantially all the economic benefits and directs the use of the production area, the Company accounts for the payments related to the access to the manufacturing space under ASC 842 as an operating lease. The term of the lease is two years from the commencement date of August 31, 2020. Upon the lease commencement date, we recorded $3,617,000 as right-of-use asset and $2,680,000 as lease liability with the difference of $937,000 being due to prepayment of an initial fee of $500,000 to commence design and construction of the production area and access fees of $437,000 . The company pays equal monthly installments of approximately no In February 2019, the Company entered into a lease agreement in connection with its 4,548 square feet laboratory in New Jersey. In December 2019, we expanded the space with an additional 235 square feet. The term of the lease is three years from the date the Company occupied the premises, with an option to extend for an additional two years which the Company expects to exercise and has included in the determination of the related lease liability. Fixed rent payable under the lease is approximately $144,000 per annum and is payable in equal monthly installments of approximately $12,000. January 2018, the Company entered into a lease agreement in connection with its corporate headquarters in New York. The term of the lease is five years from the date the Company begins to occupy the premises. Fixed rent payable under the lease is approximately $384,000 per annum and is payable in equal monthly installments of approximately $32,000, which are recognized on a straight-line basis. Additionally, the Company entered a three-year lease agreement for the lease of certain office space in Denmark in February 2018, as amended in November 2018 and February 2019. The lease is payable in monthly installments of approximately $19,000, which are recognized on a straight-line basis. ​ As described above in Note 3, the Company adopted Topic 842 as of January 1, 2019. Total operating lease costs were $1,324,000 and $676,000 for the year ended December 31, 2020 and 2019, respectively. During the year ended December 31, 2020, the expenses were recorded as $1,114,000 in research and development expense and $210,000 in general and administrative expense. During year ended December 31, 2019, the expenses were recorded as $493,000 in research and development expense and $183,000 in general and administrative expense. Cash paid for amounts included in the measurement of lease liabilities was $1,248,000 for year ended December 31, 2020 and $606,000 for year ended December 31, 2019, and was included in net cash used in operating activities in the Company’s Consolidated Statements of Cash Flows. Maturities of operating lease liabilities at December 31, 2020 and 2019 were as follows (in thousands): ​ ​ ​ ​ ​ ​ Operating Leases ​ at December 31, 2020 2021 ​ $ 2,180 2022 ​ ​ 1,593 2023 ​ ​ 540 2024 ​ ​ 64 Total lease payments ​ ​ 4,377 Less: Imputed interest ​ ​ (398) Total operating lease liabilities at December 31, 2020 ​ $ 3,979 ​ Future minimum lease payments, including imputed interest, under non-cancelable operating leases at December 31, 2019 were as follows (in thousands): ​ ​ ​ ​ ​ ​ Operating Leases ​ at December 31, 2019 2020 ​ $ 749 2021 ​ ​ 753 2022 ​ ​ 646 2023 ​ ​ 539 2024 ​ ​ 77 Total lease payments ​ ​ 2,764 Less: Imputed interest ​ ​ (534) Total operating lease liabilities at December 31, 2019 ​ $ 2,230 ​ Operating lease liabilities are based on the net present value of the remaining lease payments over the remaining lease term. In determining the present value of lease payments, the Company uses its estimate of its incremental borrowing rate based on the information available at the lease commencement date. As of December 31, 2020, the weighted average remaining lease term is 2.18 years and the weighted average discount rate used to determine the operating lease liability was 7.6%. As of December 31, 2019, the weighted average remaining lease term is 3.83 years and the weighted average discount rate used to determine the operating lease liability was 11.0%.

STOCKHOLDERS' EQUITY

STOCKHOLDERS' EQUITY	12 Months Ended
	Dec. 31, 2020
STOCKHOLDERS' EQUITY
STOCKHOLDERS' EQUITY	NOTE 7—STOCKHOLDERS’ EQUITY Authorized Stock As of December 31, 2020 and 2019, the Company has authorized a total of 105,500,000 shares, 100,000,000 of which are to be common stock, par value $0.0001 per stock, and 5,500,000 of which are to preferred stock, par value $0.0001 per share. Common Stock Each share of common stock is entitled to one vote. Common stockholders are entitled to receive dividends, as may be declared by the board of directors, if any, subject to preferential dividend rights of the preferred stock, none of which have been issued. The Company has issued 40,688,447 shares of its common stock as of December 31, 2020 and 39,728,416 shares of its common stock as of December 31, 2019. Preferred Stock Preferred stock may be issued from time to time in one or more series with such designations, preferences and relative participating, optional or other special rights and qualifications, limitations or restrictions as approved by the Company’s Board of Directors. No preferred stock has been issued as of December 31, 2020 or December 31, 2019. Stock grant agreements with non-employees In August 2015, we entered into certain stock grant agreements with non-employees of the Company. We agreed to issue a total of 2,800,000 shares to two non-employee researchers who were involved in the development of technology licensed from MSK in consideration for their prior service. The shares are released according to a vesting schedule. A total of 560,000 shares were issued in 2015, and a total of 448,000 shares issued in each of 2016 and 2017. In 2018, a total of 544,000 shares were issued to the two researchers, whereby one of the two grants was fully issued. In 2019 a total of 400,000 shares were issued to one of the physicians, and the remaining 400,000 shares were issued in August 2020, subject to certain conditions. No future shares will be issued under this award. The total award was expensed at its estimated fair value in 2015, as no future service was required to continue to vest in and receive the shares. In April 2020, in connection with the SADA License Agreement, we entered into certain stock grant agreements pursuant to which we agreed to issue a total of 213,996 shares to two non-employee researchers who were involved in the development of the SADA technology licensed from MSK and MIT in consideration for their prior service. All 213,996 shares were issued in April 2020 into escrow with 40% of the shares immediately vesting at the time of issuance and the remaining 60% of the shares subject to vesting ratably over the next three years on the anniversary date of the agreement. The shares are subject to forfeiture to the extent the SADA License Agreement is terminated prior to the vesting of the shares. There is no cash settlement feature, and no future service is required for researchers to vest and receive the shares. While the shares vest over time, there is no performance condition for the shares. In April 2020, we recorded an expense within research and development totaling $7,376,000 related to the shares which represents the fair value of the shares on the grant date. In July 2020, pursuant to the stock grant agreements, we also loaned the two researchers a total of $2,610,000 related to their individual tax payments due in conjunction with the stock grants. Each of the loans are evidenced by a three year Secured Promissory Note. The outstanding principal amounts of the loans, together with all accrued interest thereon at the rate of 1% per annum, is due and payable on the maturity date of the loans. The loans are secured by Pledge and Security Agreements, pursuant to which the researchers have pledged the shares as security for repayment of the loans with interest rates that are at market. The loans are recorded at amortized cost, which approximates fair value due to the short-term nature and minimal changes in market interest rates. Issuance of common stock In November 2019, we completed a secondary public offering and issued 5,134,750 shares of Common Stock at a purchase price of $28.00 per share for an aggregate consideration of $134,704,000, net of issuance costs of $9,100,000. As dosclosed in Note 12, subsequent to December 31, 2020, on February 22, 2021, we completed a secondary public offering of our common stock pursuant to which we issued and sold 2,804,878 shares of our common stock at a price to the public of $41.00 per share which included the exercise in full of the underwriters’ option to purchase additional shares. We received aggregate gross proceeds from our secondary public offering of $115.0 million, or aggregate net proceeds of approximately $107.7 million. ​

SHARE-BASED COMPENSATION

SHARE-BASED COMPENSATION	12 Months Ended
	Dec. 31, 2020
SHARE-BASED COMPENSATION
SHARE-BASED COMPENSATION	NOTE 8—SHARE-BASED COMPENSATION 2015 Equity Incentive Plan Our board of directors and stockholders have approved and adopted the 2015 Plan, which provided for the grant of incentive stock options, within the meaning of Section 422 of the Code (the Internal Revenue Code), to our employees and any parent and subsidiary corporations’ employees, and for the grant of incentive stock options, nonqualified stock options, stock appreciation rights, restricted stock and restricted stock units to our employees, directors and consultants and our parent and subsidiary corporations’ employees and consultants. A total of 4,500,000 shares of our common stock were reserved for issuance pursuant to the 2015 Plan. Options granted under the 2015 Plan vest according to the schedule specified in the grant agreements, which is generally a four year period and generally become immediately exercisable upon the occurrence of a change in control, as defined. Upon the 2018 Equity Incentive Plan (the “2018 Plan’) becoming effective in September 2018, no further grants were allowed under the 2015 Plan. 2018 Equity Incentive Plan Our board of directors and stockholders approved and adopted the 2018 Plan, which became effective upon the Company’s initial public offering in September 2018 and which provides for the grant of incentive stock options, within the meaning of Section 422 of the Code (the Internal Revenue Code), to our employees and any parent and subsidiary corporations’ employees, and for the grant of incentive stock options, nonqualified stock options, stock appreciation rights, restricted stock and restricted stock units to our employees, directors and consultants and our parent and subsidiary corporations’ employees and consultants. A total of 5,500,000 shares of our common stock, inclusive of the awards previously granted under the 2015 Equity Incentive Plan, are reserved for issuance pursuant to the 2018 Plan. In addition, the number of shares available for issuance under the 2018 Plan will also include an annual increase on the first day of each fiscal year beginning in 2019, equal to 4% of the outstanding shares of common stock as of the last day of our immediately preceding fiscal year. The exercise price of options granted under the plans must at least be equal to the fair market value of our common stock on the date of grant. The term of an incentive stock option may not exceed 10 years , except that with respect to any participant who owns more than 10% of the voting power of all classes of our outstanding stock, the term must not exceed five years and the exercise price must equal at least 110% of the fair market value on the grant date. The administrator will determine the methods of payment of the exercise price of an option, which may include cash, shares or other property acceptable to the administrator, as well as other types of consideration permitted by applicable law. Options granted under the 2018 Plan vest according to the schedule specified in the grant agreements, which is generally a four-year period and generally become immediately exercisable upon the occurrence of a change in control, as defined. Stock Option Valuation and Restricted Stock Units During the years ended December 31, 2020 and 2019, stock-based compensation expenses for stock option grants were $15,949,000 and $4,581,000, respectively. During 2020 the expenses were recorded as $7,401,000 in research and development expense and $8,548,000 in general and administrative expense. During 2019 the expenses were recorded as $933,000 in research and development expense and $3,648,000 in general and administrative expense. Other than $15,949,000 stock option compensation in 2020, another $186,000 restricted stock units, $169,000 in research and development expense and $17,000 in general and administrative expense were recorded in 2020. Other than $4,581,000 stock option compensation in 2019, another $76,000 restricted stock units, $69,000 in research and development expense and $7,000 in general and administrative expense were recorded in 2019. The total stock based compensation was $16,135,000 in 2020 and $4,657,000 in 2019, respectively. Stock Option Valuation Activity The assumptions that the Company used to determine the fair value of the stock options granted to employees, directors and consultants were as follows, presented on a weighted average basis: ​ ​ ​ ​ ​ ​ Year Ended Year Ended ​ ​ December 31, 2020 ​ December 31, 2019 ​ ​ ​ ​ ​ ​ Risk-free interest rate 0.64 % 1.78 % Expected term (in years) 6.3 6.3 Expected volatility 65.0 % 60.6 % Expected dividend yield — % — % ​ During the year ended December 31, 2020, the Company recognized $4.8 million in stock-based compensation related to an option grant of 150,000 options to a non-employee consultant for past service and contributions to a development program. No further service is required by the non-employee consultant to obtain the options and therefore the Company recorded the full value of the grant on the date of the grant. The Company recognizes compensation expense for only the portion of awards that vest. The following table summarizes common stock options issued and outstanding: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted ​ ​ ​ ​ Weighted ​ Aggregate ​ average ​ ​ ​ ​ average ​ intrinsic ​ remaining ​ ​ ​ ​ exercise ​ value ​ contractual ​ ​ Options ​ price ​ (in thousands) ​ life (years) Outstanding and expected to vest at December 31, 2019 4,005,873 ​ $ 10.67 ​ $ 82,944 7.34 Granted 2,057,600 ​ ​ 43.47 ​ ​ ​ ​ ​ Exercised ​ (299,706) ​ ​ 6.69 ​ ​ ​ ​ ​ Forfeited ​ (89,667) ​ ​ 25.00 ​ ​ ​ ​ ​ Outstanding and expected to vest at December 31, 2020 5,674,100 ​ $ 22.55 ​ $ 156,726 ​ 7.51 Exercisable at December 31, 2020 2,907,404 ​ $ 8.42 ​ $ 119,454 ​ 5.93 ​ The weighted average grant-date fair value of stock options granted during the years ended December 31, 2020 and 2019 was $25.40 and $14.92 per share, respectively. The aggregate intrinsic value of stock options is calculated as the difference between the exercise price of the stock options and the fair value of the Company’s common stock for those stock options that had exercise prices lower than the fair value of the Company’s common stock. The fair value of each stock option grant is estimated on the date of grant using the Black-Scholes option pricing model. We estimate our expected share price volatility based on the historical volatility of a group of publicly-traded peer companies and we expect to continue to do so until such time as we have adequate historical data regarding the volatility of our own traded share price. The expected term of our stock options has been determined utilizing the “simplified” method for awards as we have limited historical data to support the expected term assumption. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. The expected dividend yield is based on the fact that we have never paid cash dividends on shares of our common stock and do not expect to pay any cash dividends in the foreseeable future. There were no significant changes to the inputs included in the Black-Scholes option pricing model during the period ended December 31, 2020. ​ As of December 31, 2020 and 2019, we had $52,239,000 and $15,942,000, respectively, of unrecognized compensation related to employee stock options that are expected to vest over a period of 3.09 years in 2020 and 2.72 years in 2019. ​ ​ Restricted Stock Unit Activity The following table summarizes restricted stock units issued and outstanding: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted ​ ​ ​ ​ Weighted ​ average ​ ​ ​ ​ ​ average ​ remaining ​ ​ ​ ​ ​ grant ​ vesting ​ ​ ​ Restricted Stock Units ​ price ​ life (years) ​ Outstanding and expected to vest at December 31, 2019 ​ 10,296 ​ $ 23.11 2.05 ​ Granted ​ 23,939 ​ ​ 26.02 ​ Vested ​ (3,429) ​ ​ 23.11 ​ ​ Forfeited ​ (660) ​ ​ 20.76 ​ ​ ​ Outstanding and expected to vest at December 31, 2020 ​ 30,146 ​ $ 25.45 2.18 ​ ​ As of December 31, 2020, we had $586,000 of unrecognized compensation related to employee restricted stock units that are expected to vest over a period of 2.18 years. As of December 31, 2019, we had $163,000 of unrecognized compensation related to employee restricted stock units that are expected to vest over a period of 2.05 years.

RELATED PARTY TRANSACTIONS

RELATED PARTY TRANSACTIONS	12 Months Ended
	Dec. 31, 2020
RELATED PARTY TRANSACTIONS
RELATED PARTY TRANSACTIONS	NOTE 9—RELATED PARTY TRANSACTIONS MSK is a shareholder of the Company. Under the MSK License Agreement, SADA License Agreement, the CD33 License Agreement, and various other supporting agreements with MSK, we have expensed costs in the total amount of $11,556,000 and $6,832,000 in the years ended December 31, 2020 and 2019, respectively, for milestones, minimum royalties, research and development costs, and patent activities. Please refer to Note 6 – License Agreements and Commitments for additional details on our agreements with MSK. As of December 31, 2020, we had a total of $833,000 recorded as accounts payable, $7,161,000 as accrued liabilities, thereby totaling $7,994,000 due to MSK. As of December 31, 2019, we had a total of $188,000 recorded as accounts payable, $3,983,000 as accrued liabilities, thereby totaling $4,171,000 due to MSK.

INCOME TAXES

INCOME TAXES	12 Months Ended
	Dec. 31, 2020
INCOME TAXES
INCOME TAXES	NOTE 10—INCOME TAXES Domestic and foreign loss before income taxes are as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ For The For The ​ ​ Year Ended ​ Year Ended ​ ​ December 31, ​ December 31, ​ ​ 2020 ​ 2019 ​ ​ (thousands) ​ (thousands) United States ​ $ (119,153) ​ $ (80,598) Foreign ​ (184) ​ (430) Total ​ $ (119,337) ​ $ (81,028) ​ The Company provided no current and deferred income tax benefits on net losses of $(119,337,000) and $(81,028,000) for years ended December 31, 2020 and 2019, respectively, and maintains a full valuation allowance against its net deferred tax assets. The difference between income taxes expected at the U.S. federal statutory income tax rate of 21% for tax years ended December 31, 2020 and December 31, 2019, respectively, and income taxes provided are set forth below: ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2020 December 31, 2019 ​ ​ (thousands) ​ (thousands) Taxes on income at U.S. federal statutory rate ​ $ (25,061) ​ $ (17,016) State and local taxes, net of federal tax effects ​ (3,059) ​ (11,022) Effect of rate change ​ 1,647 ​ (22) Foreign tax rate differential ​ (2) ​ (4) Valuation allowance ​ 30,986 ​ 33,984 Tax credits ​ (4,734) ​ (5,924) Other ​ 223 ​ 4 Total ​ — ​ — ​ Significant components of the Company’s net deferred tax assets/(liabilities) are as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2020 December 31, 2019 ​ ​ (thousands) ​ (thousands) Deferred tax assets/(liabilities): ​ ​ Acquired intangibles ​ $ 4,424 ​ $ 2,504 Accrued bonus ​ 34 ​ 153 Unrealized foreign exchange loss ​ (448) ​ (261) Accrued royalty ​ 284 ​ 415 Stock based compensation ​ 3,803 ​ 2,594 Net operating loss carryforwards ​ 75,711 ​ 51,649 Tax credit carryforwards ​ 13,146 ​ 8,412 ROU asset ​ ​ (1,068) ​ ​ (625) Lease liability ​ ​ 931 ​ ​ 711 Other ​ (357) ​ (63) Total deferred tax assets/(liabilities) ​ 96,460 ​ 65,489 Valuation allowance ​ (96,460) ​ (65,489) Net deferred tax assets/(liabilities) ​ — ​ — ​ The Company recognizes income tax benefits for tax positions determined more likely than not to be sustained upon examination, based on the technical merits of the positions. As of December 31, 2020, and 2019, the Company has determined that there were no uncertain tax positions. The Company’s tax returns for the years 2019, 2018, 2017 and 2016 are open for tax examination by U.S. federal and state, and the Danish tax authorities. The Company maintains a full valuation allowance on its U.S. and foreign deferred tax assets. The assessment regarding whether a valuation allowance is required considers both positive and negative evidence when determining whether it is more-likely-than-not that deferred tax assets are recoverable. In making this assessment, significant weight is given to evidence that can be objectively verified. In its evaluation, the Company considered its cumulative losses historically and in recent years and its forecasted losses in the near-term as significant negative evidence. Based upon review of available positive and negative evidence, the Company determined that the negative evidence outweighed the positive evidence and a full valuation allowance on its U.S. and foreign deferred tax assets will be maintained. The Company will continue to assess the realizability of its deferred tax assets and will adjust the valuation allowance as needed. As of December 31, 2020, the Company had U.S. federal and state net operating loss (“NOL”) carryforwards of approximately $248,942,000 and $168,792,000, respectively, which are available to reduce future taxable income. The Company also had approximately $168,301,000 of unused NOL carryforwards for New York City purposes. The Company also had U.S. federal tax credits of $13,146,000 as of December 31, 2020, which may be used to offset future tax liabilities. The federal NOL carryforwards of approximately $29,909,000 will expire through 2037. The federal NOL of approximately $219,033,000 can be carried forward indefinitely but limited to offset 80% of taxable income. The Arizona, Massachusetts, Montana, New York State, and New York City NOL and tax credit carryforwards will begin to expire in 2035. The NOL and tax credit carryforwards may become subject to an annual limitation in the event of certain cumulative changes in the ownership interest of significant stockholders over a three year period in excess of 50%, as defined under Sections 382 and 383 of the Internal Revenue Code of 1986 (“IRC”). The Company has performed an analysis of its Section 382 ownership changes through December 31, 2018. Due to the large annual limitation, the Company believes that it is more likely than not that none of the net operating loss carryforwards will expire as a result of the limitation from the ownership change under Section 382. The Company also has Danish NOL carryforwards of $1,850,000, which have an indefinite carryforward period.

OTHER BENEFITS

OTHER BENEFITS	12 Months Ended
	Dec. 31, 2020
OTHER BENEFITS
OTHER BENEFITS	NOTE 11—OTHER BENEFITS The Company has established a retirement program for employees of our Danish subsidiary pursuant to which all such employees can contribute an amount at their election from their base compensation and may receive contributions from our Danish subsidiary. Contributions from our Danish subsidiary were immaterial during the years ended December 31, 2020 and 2019. In addition, health insurance benefits for our Danish employees are fully paid for by such employees. Our Danish subsidiary does not incur any costs for these health insurance benefits. ​ On October 1, 2018, the Company adopted a defined contribution 401(k) savings plan (the “401(k) plan”) covering all U.S. employees of the Company. Participants may elect to defer a percentage of their pretax or after-tax compensation to the 401(k) plan, subject to defined limitations. The plan allows for a discretionary match by the Company. The Company made no matching contributions to the plan during the years ended December 31, 2020 and December 31, 2019.

SUBSEQUENT EVENTS

SUBSEQUENT EVENTS	12 Months Ended
	Dec. 31, 2020
SUBSEQUENT EVENTS
SUBSEQUENT EVENTS	NOTE 12 —SUBSEQUENT EVENTS On December 28, 2020, the Company announced that it entered into a definitive agreement to sell its DANYELZA Priority Review Voucher to United Therapeutics Corporation for $105 million. The PRV was granted in conjunction with the approval by the U.S. Food and Drug Administration (“FDA”) of DANYELZA®, for the treatment of refractory/relapsed high-risk neuroblastoma. Under the terms of the Company’s license agreement with MSK, Y-mAbs will retain 60% of the net proceeds received from the sale, and the remaining 40% will be paid to MSK. The transaction closed on January 21, 2021 once the substantive closing conditions including within the agreement were resolved. ​ On February 22, 2021, the Company announced the closing of its public offering of 2,804,878 shares of its common stock, at a public offering price of $41.00 per share, which includes the exercise in full of the underwriters' option to purchase 365,853 additional shares of common stock. The aggregate gross proceeds to Y-mAbs, before deducting underwriting discounts and commissions and estimated offering expenses payable by the Company, were approximately $115 million.

QUARTERLY CONSOLIDATED FINANCIA

QUARTERLY CONSOLIDATED FINANCIAL DATA (unaudited)	12 Months Ended
	Dec. 31, 2020
QUARTERLY CONSOLIDATED FINANCIAL DATA (unaudited)
QUARTERLY CONSOLIDATED FINANCIAL DATA (unaudited)	NOTE 13—QUARTERLY CONSOLIDATED FINANCIAL DATA (unaudited) ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ (In thousands, except per share amounts) ​ 2020 ​ March 31 June 30 September 30 December 31 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ License revenue ​ $ — ​ $ — ​ $ — ​ $ 20,750 Loss from operations ​ ​ (26,747) ​ ​ (40,452) ​ ​ (32,641) ​ ​ (20,095) Net loss ​ ​ (26,179) ​ ​ (40,393) ​ ​ (32,832) ​ ​ (19,933) Net loss per share - basic and diluted ​ $ (0.66) ​ $ (1.01) ​ $ (0.82) ​ $ (0.48) ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ 2019 ​ March 31 June 30 September 30 December 31 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ License revenue ​ $ — ​ $ — ​ $ — ​ $ — Loss from operations ​ ​ (16,253) ​ ​ (18,634) ​ ​ (24,359) ​ ​ (23,758) Net loss ​ ​ (15,934) ​ ​ (18,036) ​ ​ (23,922) ​ ​ (23,136) Net loss per share - basic and diluted ​ $ (0.47) ​ $ (0.53) ​ $ (0.70) ​ $ (0.60) ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

SUMMARY OF SIGNIFICANT ACCOUN_2

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)	12 Months Ended
	Dec. 31, 2020
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of expenses during the reporting period. Significant estimates and assumptions reflected in these financial statements include, but are not limited to, the accrual for research and development expenses, the accrual of milestone and royalty payments, and the valuation of stock options. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. Actual results could differ from those estimates. The full extent to which the COVID-19 pandemic will directly or indirectly impact our business, results of operations and financial condition, including expenses, manufacturing, clinical trials, research and development costs and employee-related amounts, will depend on future developments that are highly uncertain, including as a result of new information that may emerge concerning COVID-19 and the actions taken to contain it or treat COVID-19, as well as the economic impact on local, regional, national and international markets. We have made estimates of the impact of COVID-19 within our financial statements and there may be changes to those estimates in future periods. Actual results may differ from these estimates.
Cash and Cash Equivalents	Cash and Cash Equivalents The Company considers all highly liquid instruments with original maturities of three months or less from date of purchase to be cash equivalents. All cash and cash equivalents are held in highly rated securities including a Treasury money market fund which is unrestricted as to withdrawal or use. To date, the Company has not experienced any losses on its cash and cash equivalents. The carrying amount of cash and cash equivalents approximates its fair value due to its short-term and liquid nature. We maintain cash balances in excess of insured limits. We do not anticipate any losses with respect to such cash balances.
Fair Value Measurements	Fair Value Measurements Certain assets and liabilities are carried at fair value under GAAP. ​ • Level 1 — Unadjusted quoted prices for identical assets or liabilities in active markets; • Level 2 — Inputs other than quoted prices in active markets for identical assets and liabilities that are observable either directly or indirectly for substantially the full term of the asset or liability; and • Level 3 — Unobservable inputs for the asset or liability, which include management's own assumption about the assumptions market participants would use in pricing the asset or liability, including assumptions about risk. Cash equivalents held in money market funds are valued using other significant observable inputs, which represent a Level 2 measurement within the fair value hierarchy. The Company has no other cash equivalents. The following tables present the Company’s fair value hierarchy for its cash equivalents, which are measured at fair value on a recurring basis (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements at December 31, 2020 Using: ​ Level 1 Level 2 Level 3 Total Cash equivalents: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Money market funds ​ $ — ​ $ 97,302 ​ $ — ​ $ 97,302 ​ ​ $ — ​ $ 97,302 ​ $ — ​ $ 97,302 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements at December 31, 2019 Using: ​ Level 1 Level 2 Level 3 Total Cash equivalents: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Money market funds ​ $ — ​ $ 197,879 ​ $ — ​ $ 197,879 ​ ​ $ — ​ $ 197,879 ​ $ — ​ $ 197,879 ​ During the years ended December 31, 2020 and 2019, there were no transfers between Level 1, Level 2 and Level 3.
Leases	Leases As described below, the Company adopted Topic 842 as of January 1, 2019. The Company determines if an arrangement includes a lease at inception. Operating lease right-of-use assets represent the Company’s right to use an underlying asset for the lease term and operating lease liabilities represent its obligation to make lease payments arising from the lease. Operating lease right-of-use assets and liabilities are recognized at the lease commencement date based on the present value of lease payments over the lease term. In determining the net present value of lease payments, the Company uses its estimated incremental borrowing rate based on information available at the lease commencement date. Because most of the Company’s leases do not provide an implicit rate of return, an incremental borrowing rate is used based on the information available at the commencement date in determining the present value of lease payments on an individual lease basis. The Company’s incremental borrowing rate for a lease is the estimated rate of interest it would have to pay on a collateralized basis to borrow an amount equal to the lease payments under similar terms. The Company’s leases may include options to extend or terminate the lease which are included in the lease term when it is reasonably certain that it will exercise any such options. None of the Company’s leases contain any residual value guarantees. Lease expense is recognized on a straight-line basis over the expected lease term. Related variable lease costs incurred are not material to the Company. Topic 842 also provides practical expedients and certain exemptions for an entity’s ongoing accounting post implementation. The Company currently elected the short-term lease recognition exemption for all leases that qualify. This means, for those leases that qualify, we will not recognize right-of-use assets or liabilities, and this includes not recognizing right-of-use assets or liabilities for existing short-term leases of those assets in transition. We also elected the practical expedient to not separate lease and non-lease components for all of our leases. The Company has made an accounting policy election to account for each separate lease component of a contract and its associated non-lease components as a single lease component. See the Lease Agreements section in Note 6 for the related disclosures.
Property and Equipment	Property and Equipment Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization expense is recognized using the straight-line method over the estimated useful life of each asset as follows: ​ ​ ​ ​ ​ Estimated Useful Life Furniture and fixtures 5 years Machinery and equipment ​ 5 years Leasehold improvements Shorter of life of lease or 15 years ​ Depreciation expense on property and equipment was $396,000 and $166,000 for the years ended December 31, 2020 and 2019, respectively. Upon retirement or sale, the cost of assets disposed of and the related accumulated depreciation and amortization are removed from the accounts and any resulting gain or loss is included in loss from operations. Expenditures for repairs and maintenance are charged to expense as incurred.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets ASC 360, Property, Plant and Equipment, addresses the financial accounting and reporting for impairment or disposal of long-lived assets. The Company reviews the recorded values of long-lived assets for impairment whenever events or changes in business circumstance indicate that the carrying amount of an asset or group of assets may not be fully recoverable.
Income Taxes	Income Taxes The Company accounts for income taxes under the asset and liability approach for the financial accounting and reporting of income taxes. Deferred tax assets and liabilities are recognized for the estimated future tax consequences attributable to net operating loss carry forwards and temporary differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases. These assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which the temporary differences are expected to reverse. We maintain a full valuation allowance on our deferred tax assets based on cumulative historical and expected losses. If we successfully commercialize our products and achieve profitability, we will consider the continued need for such valuation allowance. The Company prepares and files tax returns based on its interpretation of tax laws and regulations. In the normal course of business, the Company’s tax returns are subject to examination by various taxing authorities. Such examinations may result in future tax and interest assessments by these taxing authorities. In determining the Company’s tax provision for financial reporting purposes, the Company establishes a reserve for uncertain tax positions unless such positions are determined to be more likely than not of being sustained upon examination based on their technical merits. The Company considers many factors when evaluating and estimating its tax positions and tax benefits, which may require periodic adjustments and which may not accurately anticipate actual outcomes. Accordingly, the Company will report a liability for unrecognized tax benefits resulting from any uncertain tax positions taken or expected to be taken on a tax return. The Company’s policy is to recognize, when applicable, interest and penalties on uncertain tax positions as part of income tax expense. In accordance with guidance issued by Financial Accounting Standards Board (“FASB”), companies should make and disclose a policy election as to whether they will recognize deferred taxes for basis differences expected to reverse as Global Intangible Low-Taxed Income (“GILTI”) or whether they will account for GILTI as period costs if and when incurred. The Company has elected to recognize the resulting tax with respect to the GILTI provision as a period cost. No costs were incurred by the Company through December 31, 2020 as a result of GILTI.
Revenue Recognition	Revenue Recognition To determine revenue recognition for arrangements that an entity determines are within the scope of ASC 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 arrangements that meet the definition of a contract with a customer under ASC 606, including 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 ASC 606, the company assesses the goods or services promised within each contract, determines those that are performance obligations, and assess 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. Under the practical expedient permitted under Topic 606, the Company expenses incremental costs of obtaining a contract as and when incurred if the expected amortization period of the assets is one year or less. If there are multiple distinct performance obligations, we allocate the transaction price to each distinct performance obligation based on its relative standalone selling price. In assessing whether a promised good or service is distinct in the evaluation of a license arrangement subject to ASC 606, the Company considers factors such as the research, manufacturing and commercialization capabilities of the licensing partner and the availability of the associated expertise in the general marketplace. The Company also considers the intended benefit of the contract in assessing whether a promised good or service is separately identifiable from other promises in the contract. If a promised good or service is not distinct, the Company combines that good or service with other promised goods or services until it identifies a bundle of goods or services that is distinct. The standalone selling price is generally determined based on the prices charged to customers or using expected cost-plus margin. The Company received FDA approval of DANYELZA in November 2020 and entered into exclusive licensing and distribution agreements for DANYELZA and omburtamab in certain international territories resulting in total license revenue of $20.8 million for the year ended December 31, 2020. Of this amount, we have $0.8 million as an outstanding receivable included within other current assets. In December 2020, the Company entered into a development and commercialization arrangement with SciClone International Pharamceuticals Ltd. (SciClone) for certain indications of DANYELZA and omburtamab within China. As part of the agreement, we received a nonrefundable up-front fee of $20 million for the transfer of the license and know-how related to the product indications. The Company may receive regulatory-based milestone payments up to $40 million and sales-based milestone payments up to $60 million and is entitled to royalties based upon the net sales generated by SciClone related to the product indications in the territory. We considered the license to be distinct from other promises within the arrangement based on the rights and know-how transferred, late-stage development of the underlying indications and anticipated lack of significant involvement required from the joint steering committee associated with the indications. Accordingly, the full transaction price of $20 million was recognized upon transferring of the license and know-how to SciClone. The future potential regulatory milestone amounts were not included in the transaction price, as they were all determined to be fully constrained following the concepts of ASC 606. As part of our evaluation of the regulatory milestones constraint, we determined that the achievement of such milestones are contingent upon regulatory approvals which are not within our control and therefore not deemed probable. We expect that the sales-based milestone payments and royalty arrangements will be recognized when the related sales occur or the milestone is achieved. We will re-evaluate the transaction price each quarter and as uncertain events are resolved or other changes in circumstances occur, we assess whether this resolves the constraint and revenue will be recognized. We also considered that the manufacturing and supply terms, including within the arrangement, did not represent a material right to SciClone at inception as the terms reflected stand-alone selling price for similar goods or services. There was no license revenue generated in the period ended December 31, 2019. In addition, the Company recorded $2.2 million in royalty expense which remained in accrued liabilities as of December 31, 2020 which are due to MSK based on the terms of the original MSK License agreement.
Research and Development Costs	Research and Development Costs Research and development costs are charged to operations when incurred and are included in operating expenses. Research and development costs consist principally of compensation cost for our employees and consultants that perform our research activities, the fees paid to maintain our licenses, the payments to third parties for manufacturing and clinical research organizations and additional product development, and consumables and other materials used in research and development. The Company records accruals for estimated ongoing research and development costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies or clinical trials, including the phase or completion of events, invoices received and contracted costs. Actual results could differ from the Company’s estimates. The Company is obligated to make certain milestone and royalty payments in accordance with the contractual terms of its license agreement with MSK based upon the resolution of certain contingencies. The Company records the milestone and royalty payments when the achievement of the milestone or payment of the milestone or royalty is probable and the amount of the payment is reasonably estimable. Research and development costs were $93.7 million and $63.5 million for the years ended December 31, 2020 and 2019, respectively. Additionally, the Company is obligated to make certain royalty and clinical, regulatory and sales-based milestone payments in accordance with the contractual terms of the MSK License, CD33 License, MabVax Sublicense, and SADA License based upon the resolution of certain contingencies. Royalty payments and sales-based milestones are not due and deemed not probable as of December 31, 2020, as we have not recognized revenue for our only approved product, DANYELZA. We record the clinical and regulatory milestone payments when the achievement of the milestones or payment of the milestones is deemed probable, and the amount of the payment is reasonably estimable. As it relates to clinical and regulatory milestone payments under the SADA agreement, those may become due and payable with the passage of time whether or not the milestones have actually been met. When evaluating whether licensing milestones should be recognized under the SADA agreement, the Company uses its collective clinical experience across the company to determine the likelihood of achievement, as well as the current stage of the compounds under development, and estimates the progress of its preclinical studies and clinical trials, completion of milestone events per underlying agreements, the time expected to complete certain development activities, each party’s termination right under the license agreements, invoices received and contracted costs when evaluating whether the clinical milestones should be recognized in each reporting period. We review our assessment each period and make revisions to such estimates as necessary. We recognized research and development costs in connection with SADA licensing milestones of $0.6 million for the year ended December 31, 2020 as we deemed them probable of occuring. The Company determined that the achievement of the regulatory milestones under the SADA agreement were not probable given the current stage of the product development. The remaining total clinical milestones of $4.1 million were determined to not be probable as of December 31, 2020, as detailed in Note 6 – License Agreements And Commitments ​
Patent Costs	Patent Costs The Company expenses the costs of obtaining and maintaining patents as general and administrative expenses.
Advertising and Promotion Costs	Advertising and Promotion Costs Advertising and promotion costs are included in general and administrative expenses and were immaterial in the years ended December 31, 2020 and 2019. Advertising and product promotion costs are expensed as incurred.
Stock-Based Compensation	Stock-Based Compensation The Company measures stock options granted to employees, directors and consultants based on the fair value on the date of the grant and recognizes compensation expense of those awards, over the requisite service period, which for employees and directors is the vesting period of the respective award. Forfeitures are accounted for as they occur. The Company issues stock options with only service-based or immediate vesting conditions and records the expense for these awards using the straight-line method over the requisite service period. Following the Company’s adoption of ASU 2018-07, Compensation—Stock Compensation (Topic 718), Improvements to Nonemployee Share-Based Payment Accounting (“ASU 2018-07”), on January 1, 2019, for stock-based option awards issued to non-employees, the Company no longer revalues non-employee awards at each reporting date and instead calculates the fair value of the awards as of the grant date using the Black-Scholes option-pricing model. The overall fair value measurement for non-employees awards is consistent with the awards issued to employees. The fair value of each stock option grant is estimated on the date of grant using the Black-Scholes option pricing model. Prior to September 2018, the Company historically was a private company and lacks sufficient company-specific historical and implied volatility information for its shares. Therefore, it estimates its expected share price volatility based on the historical volatility of a group of publicly-traded peer companies and expects to continue to do so until such time as it has adequate historical data regarding the volatility of its own traded share price. The expected term of the Company’s stock options has been determined utilizing the “simplified” method for awards as the Company has limited historical data to support the expected term assumption. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. The expected dividend yield is based on the fact that the Company has never paid cash dividends on common shares and does not expect to pay any cash dividends in the foreseeable future. Stock-based compensation costs were $16.1 million and $4.7 million for the years ended December 31, 2020 and 2019, respectively.
Segment Information	Segment Information The Company is engaged solely in the discovery and development of novel antibody-based therapeutic products for the treatment of cancer. Accordingly, the Company has determined that it operates in one operating segment.
Comprehensive Loss	Comprehensive Loss Comprehensive loss includes net loss as well as other changes in stockholders’ equity that result from transactions and economic events other than those with shareholders. The difference between net loss and comprehensive loss for the period presented in the accompanying financial statements was due to foreign currency translation.
Foreign Currency	Foreign Currency The financial statements of our Danish subsidiary with a functional currency other than the U.S. dollar are translated into U.S. dollars using period-end exchange rates for assets and liabilities, historical exchange rates for stockholders’ equity and weighted average exchange rates during the period for operating results. Translation gains and losses are included in accumulated other comprehensive income / (loss), net of tax, in stockholders’ equity. Foreign currency transaction gains and losses are included in the results of operations in other income and expense, and totaled $(576,000) and $43,000 for the years ended December 31, 2020 and 2019, respectively.
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements - Adopted In October 2020, the FASB issued Accounting Standards Update No. 2020-10 (“ASU 2020-10”), Codification improvements. The amendments in this Update represent changes to clarify the Codification, correct unintended application of guidance, or make minor improvements to the Codification. The adoption of this standard on December 31, 2020 did not have a material impact on our consolidated financial statements and related disclosures. In August 2018, the FASB issued Accounting Standards Update No. 2018 -13 (“ASU2018-13”), Fair Value Measurement (Topic 820) Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement. ASU 2018-13 allows to remove the reasons for transfer between Level 1 and Level 2 assets, and adds the changes in unrealized gains and losses for recurring level 3 fair value measurements. ASU 2018-13 is effective for fiscal years beginning after December 15, 2019, and interim periods within those fiscal years with early adoption permitted. The adoption of this standard on January 1, 2020 did not have a material impact on our consolidated financial statements and related disclosures. In August 2018, the FASB issued Accounting Standards Update No. 2018-15 (“ASU 2018-15”), Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract. ASU 2018-15 clarifies certain aspects of ASU 2015-05, Customer’s Accounting for Fees Paid in a Cloud Computing Arrangement, which was issued in April 2015. Specifically, ASU 2018-15 aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal use software (and hosting arrangements that include an internal-use software license). ASU 2018-15 is effective for fiscal years beginning after December 15, 2019, and interim periods within those fiscal years with early adoption permitted. The adoption of this standard on January 1, 2020 did not have a material impact on our consolidated financial statements and related disclosures. In July 2018, the FASB issued Accounting Standards Update No. 2018-09 (“ASU 2018-09”), Codification Improvements, which clarify, correct errors in, or make minor improvements to a variety of ASC topics. The changes in ASU 2018-09 are not expected to have a significant effect on current accounting practices. Some of the amendments in this update do not require transition guidance and will be effective upon this update. However, many of the updates do have transition guidance with effective dates for periods beginning after December 15, 2018. The adoption of this standard on January 1, 2019 did not have a material impact on our consolidated financial statements and related disclosures. In June 2018, the FASB issued ASU No. 2018-07, Compensation—Stock Compensation (Topic 718), Improvements to Nonemployee Share-Based Payment Accounting (“ASU 2018-07”). ASU 2018-07 is intended to simplify aspects of share-based compensation issued to non-employees by making the guidance consistent with the accounting for employee share-based compensation. ASU 2018-07 is required to be adopted for annual periods beginning after December 15, 2018, including interim periods within those fiscal years. Early adoption is permitted. The adoption of this standard on January 1, 2019 did not have a material impact on our consolidated financial statements and related disclosures. In February 2018, the FASB issued Accounting Standards Update No. 2018-02, (“ASU 2018-02”), Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income. ASU 2018-02 allows a reclassification from accumulated other comprehensive income to retained earnings for stranded tax effects resulting from the Tax Cuts and Jobs Act. ASU 2018-07 is required to be adopted for annual periods beginning after December 15, 2018, including interim periods within those fiscal years. Early adoption is permitted. The adoption of this standard on January 1, 2019 did not have a material impact on our consolidated financial statements and related disclosures. In June 2016, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update No. 2016-13 (“ASU 2016-13”) Financial Instruments—Credit Losses (Topic 326): Measurement of credit losses on financial instruments. ASU 2016-13 introduces the current expected credit losses methodology (CECL) for estimating allowances for credit losses. The standard is effective in fiscal years and interim periods beginning after December 15, 2019. The adoption of this standard on January 1, 2020 did not have a material impact on our consolidated financial statements and related disclosures. In February 2016, the FASB issued Accounting Standards Update No. 2016-02 (“ASU 2016-02”), Leases, which is effective for fiscal years, and interim periods within those years, beginning after December 15, 2018 with early adoption permitted. Under ASU 2016-02, lessees will be required to recognize for most leases, at the commencement date of the lease, a lease liability, which is a lessee’s obligation to make lease payments arising from a lease measured on a discounted basis, and a right-to-use asset, which is an asset that represents the lessee’s right to use or control the use of a specified asset for the lease term. Topic 842 was subsequently amended by ASU 2017-13, Revenue and Leases: Amendments to SEC Paragraphs Pursuant to the Staff Announcement at the July 20, 2017 EITF Meeting and Rescission of Prior SEC Staff Announcements and Observer Comments; ASU 2018-01, Land Easement Practical Expedient for Transition to Topic 842; ASU No. 2018-10, Codification Improvements to Topic 842, Leases; ASU No. 2018-11, Targeted Improvements and ASU No. 2018-20, Narrow Scope Improvements for Lessors. The Company adopted the new leasing standards using the modified retrospective restatement of prior periods or cumulative adjustment to retained earnings. An entity may choose to use either (1) its effective date or (2) the beginning of the earliest comparative period presented in the financial statements as its date of initial application. The Company used the effective date as our date of initial application. Consequently, financial information was not updated and the disclosures required under the new standard are not provided for dates and periods before January 1, 2019. The new standard also provides a number of optional practical expedients in transition. The Company elected the Upon adoption of the new leasing standards, the Company recognized a lease liability of $1.8 million and a related right-of-use asset of $1.5 million with the difference being due to the elimination of previously reported deferred rent. Please refer to lease footnotes at Note 6 License Agreements And Commitments.

SUMMARY OF SIGNIFICANT ACCOUN_3

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Tables)	12 Months Ended
	Dec. 31, 2020
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Schedule of cash equivalents measured at fair value on recurring basis	The following tables present the Company’s fair value hierarchy for its cash equivalents, which are measured at fair value on a recurring basis (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements at December 31, 2020 Using: ​ Level 1 Level 2 Level 3 Total Cash equivalents: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Money market funds ​ $ — ​ $ 97,302 ​ $ — ​ $ 97,302 ​ ​ $ — ​ $ 97,302 ​ $ — ​ $ 97,302 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements at December 31, 2019 Using: ​ Level 1 Level 2 Level 3 Total Cash equivalents: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Money market funds ​ $ — ​ $ 197,879 ​ $ — ​ $ 197,879 ​ ​ $ — ​ $ 197,879 ​ $ — ​ $ 197,879
Schedule of property and equipment	​ ​ ​ ​ ​ Estimated Useful Life Furniture and fixtures 5 years Machinery and equipment ​ 5 years Leasehold improvements Shorter of life of lease or 15 years

NET LOSS PER SHARE (Tables)

NET LOSS PER SHARE (Tables)	12 Months Ended
	Dec. 31, 2020
NET LOSS PER SHARE
Schedule of basic and diluted net loss per share	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year ended December 31, ​ ​ 2020 ​ 2019 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Net loss (numerator) ​ $ (119,337) ​ $ (81,028) ​ Weighted-average shares (denominator) ​ 40,119 ​ 35,183 ​ Basic and diluted net loss per share ​ $ (2.97) ​ $ (2.30) ​

ACCRUED LIABILITIES (Tables)

ACCRUED LIABILITIES (Tables)	12 Months Ended
	Dec. 31, 2020
ACCRUED LIABILITIES
Summary of accrued short-term liabilities	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, December 31, ​ 2020 ​ 2019 ​ ​ (in thousands) ​ ​ ​ ​ ​ ​ ​ Accrued licensing, milestone and royalty payments ​ $ 3,608 ​ $ 354 Accrued clinical costs ​ 678 ​ 1,584 Accrued compensation and board fees ​ 2,603 ​ 1,475 Accrued manufacturing costs ​ ​ 983 ​ ​ 760 Other ​ 325 ​ 377 Total ​ $ 8,197 ​ $ 4,550

LICENSE AGREEMENTS AND COMMIT_2

LICENSE AGREEMENTS AND COMMITMENTS (Tables)	12 Months Ended
	Dec. 31, 2020
Agreements
Maturities of operating lease liabilities	Maturities of operating lease liabilities at December 31, 2020 and 2019 were as follows (in thousands): ​ ​ ​ ​ ​ ​ Operating Leases ​ at December 31, 2020 2021 ​ $ 2,180 2022 ​ ​ 1,593 2023 ​ ​ 540 2024 ​ ​ 64 Total lease payments ​ ​ 4,377 Less: Imputed interest ​ ​ (398) Total operating lease liabilities at December 31, 2020 ​ $ 3,979 ​ Future minimum lease payments, including imputed interest, under non-cancelable operating leases at December 31, 2019 were as follows (in thousands): ​ ​ ​ ​ ​ ​ Operating Leases ​ at December 31, 2019 2020 ​ $ 749 2021 ​ ​ 753 2022 ​ ​ 646 2023 ​ ​ 539 2024 ​ ​ 77 Total lease payments ​ ​ 2,764 Less: Imputed interest ​ ​ (534) Total operating lease liabilities at December 31, 2019 ​ $ 2,230 ​
MSK, CD33 and MabVax License Agreements
Agreements
Summary of significant license agreements, related commitments and milestone information	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Cash paid Cash paid Expense Expense Accrued liabilities Accrued liabilities Accrued liabilities Accrued liabilities ​ ​ Twelve months ​ Twelve months ​ Twelve months ​ Twelve months ​ Current as of ​ Non-current ​ Current as of ​ Non-current ​ ​ ended December ​ ended December ​ ended December ​ ended December ​ December ​ December ​ December ​ December Agreements ​ 2020 ​ 2019 ​ 2020 ​ 2019 ​ 2020 ​ 2020 ​ 2019 ​ 2019 MSK ​ ​ $ 80,000 ​ ​ $ 725,000 ​ ​ $ - ​ ​ $ 75,000 ​ ​ $ 305,000 ​ ​ $ 1,640,000 ​ ​ $ 254,000 ​ ​ $ 1,471,000 CD33 ​ ​ — ​ ​ — ​ ​ — ​ ​ — ​ ​ 100,000 ​ ​ 450,000 ​ ​ 100,000 ​ ​ 450,000 MabVax ​ ​ — ​ ​ 600,000 ​ ​ — ​ ​ — ​ ​ — ​ ​ — ​ ​ — ​ ​ — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Maximum Maximum Maximum Agreements ​ Clinical Milestones ​ Regulatory Milestones ​ Sales-based milestones ​ MSK ​ ​ $ 2,450,000 ​ ​ $ 9,000,000 ​ ​ $ 20,000,000 ​ CD33 ​ ​ 550,000 ​ ​ 500,000 ​ ​ 7,500,000 ​ MabVax ​ ​ 200,000 ​ ​ 1,200,000 ​ ​ — ​
SADA License Agreement
Agreements
Summary of significant license agreements, related commitments and milestone information	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Cash paid Expense Expense Accrued liabilities Accrued liabilities Accrued liabilities Accrued liabilities ​ ​ Agreements ​ Twelve months ended December 2020 ​ Twelve months ended December 2020 ​ Twelve months ended December 2019 ​ Current as of December 2020 ​ Non-current as of December 2020 ​ Current as of December 2019 ​ Non-current as of December 2019 ​ ​ SADA ​ ​ $ 2,707,000 ​ ​ $ 14,019,000 ​ ​ $ - ​ ​ $ 1,000,000 ​ ​ $ 1,605,000 ​ ​ $ - ​ ​ $ - ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Maximum Maximum Maximum Agreements ​ Clinical Milestones ​ Regulatory Milestones ​ Sales-based milestones SADA ​ ​ $ 4,730,000 ​ ​ $ 18,125,000 ​ ​ $ 23,750,000

SHARE-BASED COMPENSATION (Table

SHARE-BASED COMPENSATION (Tables)	12 Months Ended
	Dec. 31, 2020
Share-Based Compensation Awards
Schedule of common stock options issued and outstanding	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted ​ ​ ​ ​ Weighted ​ Aggregate ​ average ​ ​ ​ ​ average ​ intrinsic ​ remaining ​ ​ ​ ​ exercise ​ value ​ contractual ​ ​ Options ​ price ​ (in thousands) ​ life (years) Outstanding and expected to vest at December 31, 2019 4,005,873 ​ $ 10.67 ​ $ 82,944 7.34 Granted 2,057,600 ​ ​ 43.47 ​ ​ ​ ​ ​ Exercised ​ (299,706) ​ ​ 6.69 ​ ​ ​ ​ ​ Forfeited ​ (89,667) ​ ​ 25.00 ​ ​ ​ ​ ​ Outstanding and expected to vest at December 31, 2020 5,674,100 ​ $ 22.55 ​ $ 156,726 ​ 7.51 Exercisable at December 31, 2020 2,907,404 ​ $ 8.42 ​ $ 119,454 ​ 5.93
Schedule of restricted stock units issued and outstanding	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted ​ ​ ​ ​ Weighted ​ average ​ ​ ​ ​ ​ average ​ remaining ​ ​ ​ ​ ​ grant ​ vesting ​ ​ ​ Restricted Stock Units ​ price ​ life (years) ​ Outstanding and expected to vest at December 31, 2019 ​ 10,296 ​ $ 23.11 2.05 ​ Granted ​ 23,939 ​ ​ 26.02 ​ Vested ​ (3,429) ​ ​ 23.11 ​ ​ Forfeited ​ (660) ​ ​ 20.76 ​ ​ ​ Outstanding and expected to vest at December 31, 2020 ​ 30,146 ​ $ 25.45 2.18 ​
Employees
Share-Based Compensation Awards
Schedule of assumptions that Company used to determine the fair value of the stock options granted	​ ​ ​ ​ ​ ​ Year Ended Year Ended ​ ​ December 31, 2020 ​ December 31, 2019 ​ ​ ​ ​ ​ ​ Risk-free interest rate 0.64 % 1.78 % Expected term (in years) 6.3 6.3 Expected volatility 65.0 % 60.6 % Expected dividend yield — % — %

INCOME TAXES (Tables)

INCOME TAXES (Tables)	12 Months Ended
	Dec. 31, 2020
INCOME TAXES
Schedule of domestic and foreign loss before income taxes	​ ​ ​ ​ ​ ​ ​ ​ ​ For The For The ​ ​ Year Ended ​ Year Ended ​ ​ December 31, ​ December 31, ​ ​ 2020 ​ 2019 ​ ​ (thousands) ​ (thousands) United States ​ $ (119,153) ​ $ (80,598) Foreign ​ (184) ​ (430) Total ​ $ (119,337) ​ $ (81,028)
Schedule of difference between incomes taxes expected at U.S. federal statutory income tax rate and income taxes	​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2020 December 31, 2019 ​ ​ (thousands) ​ (thousands) Taxes on income at U.S. federal statutory rate ​ $ (25,061) ​ $ (17,016) State and local taxes, net of federal tax effects ​ (3,059) ​ (11,022) Effect of rate change ​ 1,647 ​ (22) Foreign tax rate differential ​ (2) ​ (4) Valuation allowance ​ 30,986 ​ 33,984 Tax credits ​ (4,734) ​ (5,924) Other ​ 223 ​ 4 Total ​ — ​ —
Schedule of significant components of net deferred tax assets/(liabilities)	​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2020 December 31, 2019 ​ ​ (thousands) ​ (thousands) Deferred tax assets/(liabilities): ​ ​ Acquired intangibles ​ $ 4,424 ​ $ 2,504 Accrued bonus ​ 34 ​ 153 Unrealized foreign exchange loss ​ (448) ​ (261) Accrued royalty ​ 284 ​ 415 Stock based compensation ​ 3,803 ​ 2,594 Net operating loss carryforwards ​ 75,711 ​ 51,649 Tax credit carryforwards ​ 13,146 ​ 8,412 ROU asset ​ ​ (1,068) ​ ​ (625) Lease liability ​ ​ 931 ​ ​ 711 Other ​ (357) ​ (63) Total deferred tax assets/(liabilities) ​ 96,460 ​ 65,489 Valuation allowance ​ (96,460) ​ (65,489) Net deferred tax assets/(liabilities) ​ — ​ —

QUARTERLY CONSOLIDATED FINANC_2

QUARTERLY CONSOLIDATED FINANCIAL DATA (unaudited) (Tables)	12 Months Ended
	Dec. 31, 2020
QUARTERLY CONSOLIDATED FINANCIAL DATA (unaudited)
Schedule of quarterly consolidated financial data	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ (In thousands, except per share amounts) ​ 2020 ​ March 31 June 30 September 30 December 31 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ License revenue ​ $ — ​ $ — ​ $ — ​ $ 20,750 Loss from operations ​ ​ (26,747) ​ ​ (40,452) ​ ​ (32,641) ​ ​ (20,095) Net loss ​ ​ (26,179) ​ ​ (40,393) ​ ​ (32,832) ​ ​ (19,933) Net loss per share - basic and diluted ​ $ (0.66) ​ $ (1.01) ​ $ (0.82) ​ $ (0.48) ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ 2019 ​ March 31 June 30 September 30 December 31 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ License revenue ​ $ — ​ $ — ​ $ — ​ $ — Loss from operations ​ ​ (16,253) ​ ​ (18,634) ​ ​ (24,359) ​ ​ (23,758) Net loss ​ ​ (15,934) ​ ​ (18,036) ​ ​ (23,922) ​ ​ (23,136) Net loss per share - basic and diluted ​ $ (0.47) ​ $ (0.53) ​ $ (0.70) ​ $ (0.60) ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

BASIS OF PRESENTATION - Accumul

BASIS OF PRESENTATION - Accumulated deficit and cash (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
BASIS OF PRESENTATION
Accumulated deficit	$ 285,200	$ 165,863
Cash and cash equivalents	$ 114,634	$ 207,136

BASIS OF PRESENTATION - Seconda

BASIS OF PRESENTATION - Secondary public offering (Details) - USD ($) $ / shares in Units, $ in Millions	Feb. 22, 2021	Dec. 31, 2019
Common Stock
Issuance of stock
Issuance of common stock, Stock offering (in shares)		5,134,750
Secondary public offering, February 22, 2021 | Subsequent Event
Issuance of stock
Aggregate gross proceeds	$ 115
Secondary public offering, February 22, 2021 | Common Stock | Subsequent Event
Issuance of stock
Issuance of common stock, Stock offering (in shares)	2,804,878
Share price (in dollars per share)	$ 41
Secondary public offering, February 22, 2021, Underwriters' option | Common Stock | Subsequent Event
Issuance of stock
Issuance of common stock, Stock offering (in shares)	365,853

SUMMARY OF SIGNIFICANT ACCOUN_4

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Fair Value Measurement (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Fair value measurements
Level 1 to Level 2 Transfers	$ 0	$ 0
Level 2 to Level 1 Transfers	0	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Recurring
Fair value measurements
Cash equivalents	97,302	197,879
Recurring | Money market funds
Fair value measurements
Cash equivalents	97,302	197,879
Recurring | Level 2
Fair value measurements
Cash equivalents	97,302	197,879
Recurring | Level 2 | Money market funds
Fair value measurements
Cash equivalents	$ 97,302	$ 197,879

SUMMARY OF SIGNIFICANT ACCOUN_5

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Operating Leases (Details)	Dec. 31, 2020USD ($)
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Residual value guarantees	$ 0

SUMMARY OF SIGNIFICANT ACCOUN_6

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Property and Equipment (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Property and Equipment
Depreciation expense	$ 396,000	$ 166,000
Furniture and fixtures
Property and Equipment
Estimated Useful Life	5 years
Machinery and equipment
Property and Equipment
Estimated Useful Life	5 years
Leasehold improvements | Maximum
Property and Equipment
Estimated Useful Life	15 years

SUMMARY OF SIGNIFICANT ACCOUN_7

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Income Taxes (Details)	12 Months Ended
	Dec. 31, 2020USD ($)
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Cost incurred as a result of GILTI	$ 0

SUMMARY OF SIGNIFICANT ACCOUN_8

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Revenue Recognition (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Agreements
Revenue		$ 20,750	$ 20,750	$ 0
Revenue from Contract with Customer, Product and Service [Extensible List]		us-gaap:LicenseMember	Us-gaap:LicenseMember	Us-gaap:LicenseMember
Royalties			$ 2,203
Accrued liabilities	$ 8,197	$ 8,197	8,197	$ 4,550
General and administrative
Agreements
Royalties			2,200
Other current assets
Agreements
Accounts receivable	800	800	800
Development and commercialization arrangement | SciClone
Agreements
Revenue	$ 20,000
Revenue from Contract with Customer, Product and Service [Extensible List]	Us-gaap:LicenseMember
Upfront payment	$ 20,000
Maximum regulatory milestones	40,000	40,000	40,000
Maximums sales-based milestones	$ 60,000	$ 60,000	$ 60,000

SUMMARY OF SIGNIFICANT ACCOUN_9

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Research and development (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Agreements
Research and development	$ 93,697	$ 63,492
SADA License Agreement
Agreements
Milestone expenses	605
Clinical milestone payments considered not probable	$ 4,100

SUMMARY OF SIGNIFICANT ACCOU_10

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Other policies (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)segment	Dec. 31, 2019USD ($)
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Stock-based compensation	$ 16,135	$ 4,657
Number of operating segments | segment	1
Foreign currency transaction (gain) loss	$ (576)	$ 43

SUMMARY OF SIGNIFICANT ACCOU_11

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Recently issued accounting pronouncements (Details) - USD ($) $ in Thousands	Jan. 01, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
New accounting pronouncements
Lease liabilities		$ 3,979	$ 2,230
Operating lease right-of-use assets		$ 4,569	$ 1,989
ASU 2016-02
New accounting pronouncements
Change in Accounting Principle, Accounting Standards Update, Adopted	true
Change in Accounting Principle, Accounting Standards Update, Transition Option Elected	us-gaap:AccountingStandardsUpdate201602CumulativeEffectPeriodOfAdoptionMember
New Accounting Pronouncement or Change in Accounting Principle, Prior Period Not Restated	true
Lease, Practical Expedients, Package	true
ASU 2016-02 | Cumulative Effect, Period of Adoption, Adjustment
New accounting pronouncements
Lease liabilities				$ 1,800
Operating lease right-of-use assets				$ 1,500

NET LOSS PER SHARE - Basic and

NET LOSS PER SHARE - Basic and diluted (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019
NET LOSS PER SHARE
Net loss (numerator)	$ (19,933)	$ (32,832)	$ (40,393)	$ (26,179)	$ (23,136)	$ (23,922)	$ (18,036)	$ (15,934)	$ (119,337)	$ (81,028)
Weighted-average shares (denominator)									40,118,537	35,183,488
Basic and diluted net loss per share (in dollars per share)	$ (0.48)	$ (0.82)	$ (1.01)	$ (0.66)	$ (0.60)	$ (0.70)	$ (0.53)	$ (0.47)	$ (2.97)	$ (2.30)

NET LOSS PER SHARE - Anti-dilut

NET LOSS PER SHARE - Anti-dilutive securities (Details) - shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
NET LOSS PER SHARE
Potentially dilutive securities outstanding	5,704,246	4,005,873

ACCRUED LIABILITIES (Details)

ACCRUED LIABILITIES (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Accrued short-term liabilities
Accrued licensing, milestone and royalty payments	$ 3,608	$ 354
Accrued clinical costs	678	1,584
Accrued compensation and board fees	2,603	1,475
Accrued manufacturing costs	983	760
Other	325	377
Total	$ 8,197	$ 4,550

LICENSE AGREEMENTS AND COMMIT_3

LICENSE AGREEMENTS AND COMMITMENTS - MSK, CD33 and MabVax License Agreement (Details)	12 Months Ended
	Dec. 31, 2020USD ($)agreement	Dec. 31, 2019USD ($)
Financial statement information
Expense	$ 140,685,000	$ 83,004,000
Accrued liabilities, current	8,197,000	4,550,000
Accrued liabilities, noncurrent	$ 2,695,000	1,921,000
MSK
Agreements
Number of license agreements | agreement	2
Msk License Agreement
Financial statement information
Cash paid	$ 80,000	725,000
Expense		75,000
Accrued liabilities, current	305,000	254,000
Accrued liabilities, noncurrent	1,640,000	1,471,000
Milestones
Maximum Clinical Milestones	2,450,000
Maximum Regulatory Milestones	9,000,000
Maximums Sales-based Milestones	20,000,000
CD33 License Agreement
Financial statement information
Accrued liabilities, current	100,000	100,000
Accrued liabilities, noncurrent	450,000	450,000
Milestones
Maximum Clinical Milestones	550,000
Maximum Regulatory Milestones	500,000
Maximums Sales-based Milestones	7,500,000
MabVax Agreement
Financial statement information
Cash paid		$ 600,000
Milestones
Maximum Clinical Milestones	200,000
Maximum Regulatory Milestones	$ 1,200,000

LICENSE AGREEMENTS AND COMMIT_4

LICENSE AGREEMENTS AND COMMITMENTS - SADA License Agreement (Details) $ in Thousands	Apr. 15, 2020USD ($)individual	Aug. 31, 2020shares	Jul. 31, 2020individual	Apr. 30, 2020USD ($)individualshares	Aug. 31, 2015individual	Dec. 31, 2020USD ($)individualshares	Dec. 31, 2019USD ($)individualshares	Dec. 31, 2018individualshares	Dec. 31, 2017shares	Dec. 31, 2016shares	Dec. 31, 2015shares
Agreements
Non-cash expense in connection with equity issuance to inventors						$ 7,376
Financial statement information
Expense						140,685	$ 83,004
Accrued liabilities, current						8,197	4,550
Accrued liabilities, noncurrent						$ 2,695	$ 1,921
Common Stock
Agreements
Issuance of common stock to non-employees (in shares) | shares						656,896	400,000
Nonemployees | Researchers, Employees of MSK
Agreements
Number of individuals | individual					2		1	2
Nonemployees | Researchers, Employees of MSK | Common Stock
Agreements
Issuance of common stock to non-employees (in shares) | shares		400,000					400,000	544,000	448,000	448,000	560,000
SADA License Agreement
Agreements
Number of employees acquired | individual	0
Upfront payment	$ 1,995
Stock grant expense						$ 3,331
Issuance of common stock to strategic partner						$ 1,331
Issuance of common stock to strategic partner (in shares) | shares						42,900
Accrued stock issuance costs						$ 2,000
Annual minimum royalties payable (non-refundable)	40
Annual minimum royalties payable from tenth anniversary (non-refundable)	60
Maximum sales based milestone payments due for each construct previously generated	$ 60,000
Milestone expenses						605
Milestone period	36 months
Financial statement information
Cash paid						2,707
Expense						14,019
Accrued liabilities, current						1,000
Accrued liabilities, noncurrent						1,605
Milestones
Maximum Clinical Milestones						4,730
Maximum Regulatory Milestones						18,125
Maximums Sales-based Milestones						23,750
SADA License Agreement | Researchers, Employees of MSK
Agreements
Number of individuals | individual			2
SADA License Agreement | Nonemployees | Researchers, Employees of MSK
Agreements
Non-cash expense in connection with equity issuance to inventors				$ 7,376		$ 7,376
Number of individuals | individual				2		2
SADA License Agreement | Nonemployees | Researchers, Employees of MSK | Common Stock
Agreements
Issuance of common stock to non-employees (in shares) | shares				213,996		213,996

LICENSE AGREEMENTS AND COMMIT_5

LICENSE AGREEMENTS AND COMMITMENTS - Other agreements (Details) $ in Thousands	1 Months Ended	12 Months Ended
	Oct. 31, 2020	Dec. 31, 2020USD ($)agreementindividual	Dec. 31, 2019USD ($)	Dec. 31, 2018
Agreements
Research and development		$ 93,697	$ 63,492
Other Agreements with MSK
Agreements
Research and development		$ 4,200	$ 6,800
CFSAs
Agreements
Number of service agreements | agreement		2
CD33-SRA
Agreements
Research service period				2 years
SRA
Agreements
Research service period	3 years
Researchers, Employees of MSK | MDSA
Agreements
Number of individuals | individual		5

LICENSE AGREEMENTS AND COMMIT_6

LICENSE AGREEMENTS AND COMMITMENTS - Lease agreements (Details) $ in Thousands	1 Months Ended
	Aug. 31, 2020USD ($)	Feb. 28, 2019USD ($)ft²	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)ft²	Jan. 31, 2018USD ($)
Leases
Operating lease right-of-use assets			$ 4,569	$ 1,989
Lease liabilities			$ 3,979	$ 2,230
Manufacturing facility, Indiana
Leases
Lease term	2 years
Operating lease right-of-use assets	$ 3,617
Lease liabilities	2,680
Difference between right of use asset and lease liability recognized	937
Initial fee	500
Access fee paid	437
Access fee monthly installment	117
Amount of additional access fees to be paid in monthly installments	$ 2,330
Option to extend	false
Laboratory, New Jersey
Leases
Leased area (in square feet) | ft²		4,548
Additional leased area (in square feet) | ft²				235
Lease term		3 years
Option to extend		true
Option to extend, period		2 years
Fixed rent payable per annum		$ 144
Lease payable per month		$ 12
Corporate headquarters, New York
Leases
Lease term					5 years
Fixed rent payable per annum					$ 384
Lease payable per month					$ 32
Office space, Denmark
Leases
Lease term		3 years
Lease payable per month		$ 19

LICENSE AGREEMENTS AND COMMIT_7

LICENSE AGREEMENTS AND COMMITMENTS - Lease costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Leases
Total operating lease costs	$ 1,324	$ 676
Cash paid for amounts included in the measurement of lease liabilities	1,248	606
Research and development expense
Leases
Total operating lease costs	1,114	493
General and administrative
Leases
Total operating lease costs	$ 210	$ 183

LICENSE AGREEMENTS AND COMMIT_8

LICENSE AGREEMENTS AND COMMITMENTS - Lease maturities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Future minimum commitments under all non-cancelable operating leases
First fiscal year	$ 2,180	$ 749
Second fiscal year	1,593	753
Third fiscal year	540	646
Fourth fiscal year	64	539
Fifth fiscal year		77
Total lease payments	4,377	2,764
Less: Imputed interest	(398)	(534)
Total operating lease liabilities at end of period	$ 3,979	$ 2,230

LICENSE AGREEMENTS AND COMMIT_9

LICENSE AGREEMENTS AND COMMITMENTS - Lease term and discount rate (Details)	Dec. 31, 2020	Dec. 31, 2019
LICENSE AGREEMENTS AND COMMITMENTS
Weighted average remaining lease term	2 years 2 months 4 days	3 years 9 months 29 days
Weighted average discount rate	7.60%	11.00%

STOCKHOLDERS' EQUITY - Authoriz

STOCKHOLDERS' EQUITY - Authorized, Common and Preferred Stock (Details) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)Vote$ / sharesshares	Dec. 31, 2019USD ($)$ / sharesshares
STOCKHOLDERS' EQUITY
Total shares authorized	105,500,000	105,500,000
Common stock, authorized shares	100,000,000	100,000,000
Common stock, par value (in dollars per share) | $ / shares	$ 0.0001	$ 0.0001
Preferred stock, authorized shares	5,500,000	5,500,000
Preferred stock, par value (in dollars per share) | $ / shares	$ 0.0001	$ 0.0001
Number of votes per common share | Vote	1
Common stock dividends issued | $	$ 0	$ 0
Common stock, shares issued	40,688,447	39,728,416
Preferred stock, shares issued	0	0

STOCKHOLDERS' EQUITY - Stock gr

STOCKHOLDERS' EQUITY - Stock grant agreements with non-employees, Stock (Details) $ in Thousands	1 Months Ended				12 Months Ended
	Aug. 31, 2020shares	Jul. 31, 2020USD ($)individual	Apr. 30, 2020USD ($)individualshares	Aug. 31, 2015individualshares	Dec. 31, 2020USD ($)individualshares	Dec. 31, 2019individualshares	Dec. 31, 2018itemindividualshares	Dec. 31, 2017shares	Dec. 31, 2016shares	Dec. 31, 2015shares
Share-Based Compensation Awards
Non-cash expense in connection with equity issuance to inventors | $					$ 7,376
Nonemployees | Researchers, Employees of MSK
Share-Based Compensation Awards
Number of individuals | individual				2		1	2
Number of stock grants fully issued | item							1
SADA License Agreement | Researchers, Employees of MSK
Share-Based Compensation Awards
Number of individuals | individual		2
Notes receivable, noncurrent | $		$ 2,610
Notes receivable term		3 years
Notes receivable interest rate (as a percent)		1.00%
SADA License Agreement | Nonemployees | Researchers, Employees of MSK
Share-Based Compensation Awards
Number of individuals | individual			2		2
Non-cash expense in connection with equity issuance to inventors | $			$ 7,376		$ 7,376
SADA License Agreement | Nonemployees | Researchers, Employees of MSK | Tranche one
Share-Based Compensation Awards
Vesting (as a percent)			40.00%
SADA License Agreement | Nonemployees | Researchers, Employees of MSK | Tranche two
Share-Based Compensation Awards
Vesting (as a percent)			60.00%
Vesting period			3 years
Common Stock
Share-Based Compensation Awards
Issuance of common stock to non-employees (in shares)					656,896	400,000
Common Stock | Nonemployees | Researchers, Employees of MSK
Share-Based Compensation Awards
Shares agreed to be issued				2,800,000
Issuance of common stock to non-employees (in shares)	400,000					400,000	544,000	448,000	448,000	560,000
Common Stock | SADA License Agreement | Nonemployees | Researchers, Employees of MSK
Share-Based Compensation Awards
Shares agreed to be issued			213,996
Issuance of common stock to non-employees (in shares)			213,996		213,996

STOCKHOLDERS' EQUITY - Issuance

STOCKHOLDERS' EQUITY - Issuance of common stock (Details) - USD ($) $ / shares in Units, $ in Thousands	Feb. 22, 2021	Nov. 30, 2019	Dec. 31, 2019
Issuance of stock
Issuance of common stock to investors, net of issuance costs			$ 134,704
Secondary public offering, November 2019
Issuance of stock
Payment of offering costs		$ 9,100
Secondary public offering, February 22, 2021 | Subsequent Event
Issuance of stock
Aggregate gross proceeds	$ 115,000
Common Stock
Issuance of stock
Issuance of common stock, Stock offering (in shares)			5,134,750
Issuance of common stock to investors, net of issuance costs			$ 1
Common Stock | Secondary public offering, November 2019
Issuance of stock
Issuance of common stock, Stock offering (in shares)		5,134,750
Share price (in dollars per share)		$ 28
Issuance of common stock to investors, net of issuance costs		$ 134,704
Common Stock | Secondary public offering, February 22, 2021 | Subsequent Event
Issuance of stock
Issuance of common stock, Stock offering (in shares)	2,804,878
Share price (in dollars per share)	$ 41
Issuance of common stock to investors, net of issuance costs	$ 107,700

SHARE-BASED COMPENSATION - 2015

SHARE-BASED COMPENSATION - 2015 Plan (Details) - 2015 Plan - shares	12 Months Ended
	Dec. 31, 2015	Sep. 30, 2018
Share-Based Compensation Awards
Shares reserved for issuance pursuant to the plan	4,500,000
Shares available for grant		0
Stock options
Share-Based Compensation Awards
Vesting period	4 years

SHARE-BASED COMPENSATION - 2018

SHARE-BASED COMPENSATION - 2018 Plan (Details) - 2018 Plan	12 Months Ended
	Dec. 31, 2020shares
Share-Based Compensation Awards
Shares reserved for issuance pursuant to the plan	5,500,000
Annual increase on share reserve (as a percent)	4.00%
Stock options | Employees and nonemployees owning less than 10% of voting power
Share-Based Compensation Awards
Term of award	10 years
Vesting period	4 years
Stock options | Participants owning more than 10% of voting power
Share-Based Compensation Awards
Term of award	5 years
Stock options | Participants owning more than 10% of voting power | Minimum
Share-Based Compensation Awards
Ownership (as a percent)	10.00%
Option price as percentage of fair market value of common stock on the date of grant	110.00%

SHARE-BASED COMPENSATION - Stoc

SHARE-BASED COMPENSATION - Stock option expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Share-Based Compensation Awards
Stock-based compensation expenses	$ 16,135	$ 4,657
Stock options | Employees
Share-Based Compensation Awards
Stock-based compensation expenses	15,949	4,581
Stock options | Employees | Research and development expense
Share-Based Compensation Awards
Stock-based compensation expenses	7,401	933
Stock options | Employees | General and administrative
Share-Based Compensation Awards
Stock-based compensation expenses	8,548	3,648
Stock options | Nonemployees
Share-Based Compensation Awards
Stock-based compensation expenses	4,800
RSUs
Share-Based Compensation Awards
Stock-based compensation expenses	186	76
RSUs | Research and development expense
Share-Based Compensation Awards
Stock-based compensation expenses	169	69
RSUs | General and administrative
Share-Based Compensation Awards
Stock-based compensation expenses	$ 17	$ 7

SHARE-BASED COMPENSATION - St_2

SHARE-BASED COMPENSATION - Stock Option Valuation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Stock Option Valuation
Stock-based compensation expenses	$ 16,135	$ 4,657
Granted (in shares)	2,057,600
Stock options
Stock Option Valuation
Risk-free interest rate	0.64%	1.78%
Expected term (in years)	6 years 3 months 18 days	6 years 3 months 18 days
Expected volatility	65.00%	60.60%
Stock options | Employees
Stock Option Valuation
Stock-based compensation expenses	$ 15,949	$ 4,581
Stock options | Nonemployees
Stock Option Valuation
Stock-based compensation expenses	$ 4,800
Granted (in shares)	150,000

SHARE-BASED COMPENSATION - St_3

SHARE-BASED COMPENSATION - Stock option activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Options
Outstanding at beginning of period (in shares)	4,005,873
Granted (in shares)	2,057,600
Exercised (in shares)	(299,706)
Forfeited, shares (in shares)	(89,667)
Outstanding at end of period (in shares)	5,674,100	4,005,873
Exercisable at end of period (in shares)	2,907,404
Weighted average exercise price
Outstanding at beginning of period (in dollars per share)	$ 10.67
Granted (in dollars per share)	43.47
Exercised (in dollars per share)	6.69
Forfeited (in dollars per share)	25
Outstanding at end of period (in dollars per share)	22.55	$ 10.67
Exercisable at end of period (in dollars per share)	$ 8.42
Aggregate intrinsic value and Weighted average remaining contractual life (years)
Outstanding (in dollars)	$ 156,726	$ 82,944
Exercisable (in dollars)	$ 119,454
Outstanding (in years)	7 years 6 months 3 days	7 years 4 months 2 days
Exercisable (in years)	5 years 11 months 4 days
Weighted average grant-date fair value of stock options granted (in dollars per share)	$ 25.40	$ 14.92

SHARE-BASED COMPENSATION - St_4

SHARE-BASED COMPENSATION - Stock option unrecognized compensation (Details) - Stock options - Employees - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Unrecognized compensation related to employee stock options
Unrecognized compensation	$ 52,239	$ 15,942
Expected vesting period	3 years 1 month 2 days	2 years 8 months 19 days

SHARE-BASED COMPENSATION - Rest

SHARE-BASED COMPENSATION - Restricted Stock Unit Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Share-Based Compensation Awards
Stock-based compensation expenses	$ 16,135	$ 4,657
RSUs
Share-Based Compensation Awards
Stock-based compensation expenses	$ 186	$ 76
Restricted stock units issued and outstanding
Outstanding and expected to vest at beginning of period (in shares)	10,296
Granted (in shares)	23,939
Vested (in shares)	(3,429)
Forfeited (in shares)	(660)
Outstanding and expected to vest at end of period (in shares)	30,146	10,296
Weighted average grant price
Weighted average grant price, Outstanding and expected to vest, Beginning of period (in dollars per share)	$ 23.11
Weighted average grant price, Grants (in dollars per share)	26.02
Weighted average grant price, Vested (in dollars per share)	23.11
Weighted average grant price, Forfeited (in dollars per share)	20.76
Weighted average grant price, Outstanding and expected to vest, End of period (in dollars per share)	$ 25.45	$ 23.11
Unrecognized compensation	$ 586	$ 163
Weighted average remaining vesting life	2 years 2 months 4 days	2 years 18 days

RELATED PARTY TRANSACTIONS (Det

RELATED PARTY TRANSACTIONS (Details) - MSK - MSK License Agreement, CD33 License Agreement, CTA, CFAs, SRA and MDSA - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Related party transactions
Expensed costs	$ 11,556,000	$ 6,832,000
Due to related parties, Accounts payable	833,000	188,000
Due to related parties, Accrued liabilities	7,161,000	3,983,000
Due to related parties	$ 7,994,000	$ 4,171,000

INCOME TAXES - Expense (Details

INCOME TAXES - Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Domestic and foreign loss before income taxes:
United States	$ (119,153)	$ (80,598)
Foreign	(184)	(430)
Total losses before taxes	(119,337)	(81,028)
Income tax expense
Current income taxes	0	0
Deferred income taxes	$ 0	$ 0

INCOME TAXES - Reconciliation o

INCOME TAXES - Reconciliation of Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
INCOME TAXES
U.S. federal corporate tax rate (as a percent)	21.00%	21.00%
Difference between income taxes expected at U.S. federal statutory income tax rate and income taxes provided:
Taxes on income at U.S. federal statutory rate	$ (25,061)	$ (17,016)
State and local taxes, net of federal tax effects	(3,059)	(11,022)
Effect of rate change	1,647	(22)
Foreign tax rate differential	(2)	(4)
Valuation allowance	30,986	33,984
Tax credits	(4,734)	(5,924)
Other	223	4
Total

INCOME TAXES - Deferred tax ass

INCOME TAXES - Deferred tax assets/(liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets/(liabilities)::
Acquired intangibles	$ 4,424	$ 2,504
Accrued bonus	34	153
Unrealized foreign exchange loss	(448)	(261)
Accrued royalty	284	415
Stock based compensation	3,803	2,594
Net operating loss carryforwards	75,711	51,649
Tax credit carryforwards	13,146	8,412
ROU asset	(1,068)	(625)
Lease liability	931	711
Other liabilities	(357)	(63)
Total deferred tax assets/(liabilities)	96,460	65,489
Valuation allowance	(96,460)	(65,489)
Net deferred tax assets/(liabilities)

INCOME TAXES - Uncertain tax po

INCOME TAXES - Uncertain tax positions (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
INCOME TAXES
Uncertain tax position	$ 0	$ 0

INCOME TAXES - Net Operating Lo

INCOME TAXES - Net Operating Loss Carryforwards (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)
U.S. federal
Operating Loss Carryforwards
Net operating loss carryforwards	$ 248,942
Tax credit	13,146
Operating loss carryforward, expiring through 2037	29,909
Operating loss carryforward, indefinite	219,033
State
Operating Loss Carryforwards
Net operating loss carryforwards	168,792
New York City
Operating Loss Carryforwards
Net operating loss carryforwards	168,301
Danish
Operating Loss Carryforwards
Net operating loss carryforwards	$ 1,850

OTHER BENEFITS (Details)

OTHER BENEFITS (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
OTHER BENEFITS
Employer matching contribution to defined contribution plan (as a percent)	0.00%	0.00%

SUBSEQUENT EVENTS (Details)

SUBSEQUENT EVENTS (Details) - USD ($) $ / shares in Units, $ in Millions	Feb. 22, 2021	Jan. 21, 2021	Dec. 31, 2019
Common Stock
Subsequent Events
Issuance of common stock, Stock offering (in shares)			5,134,750
Subsequent Event | Secondary public offering, February 22, 2021
Subsequent Events
Aggregate gross proceeds	$ 115
Subsequent Event | Secondary public offering, February 22, 2021 | Common Stock
Subsequent Events
Issuance of common stock, Stock offering (in shares)	2,804,878
Share price (in dollars per share)	$ 41
Subsequent Event | Secondary public offering, February 22, 2021, Underwriters' option | Common Stock
Subsequent Events
Issuance of common stock, Stock offering (in shares)	365,853
Subsequent Event | Priority Review Voucher | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Subsequent Events
Consideration		$ 105
Subsequent Event | Priority Review Voucher | Disposal Group, Disposed of by Sale, Not Discontinued Operations | Msk License Agreement
Subsequent Events
Percentage of net proceeds from monetization entitled to be retained		60.00%
Subsequent Event | Priority Review Voucher | Disposal Group, Disposed of by Sale, Not Discontinued Operations | Msk License Agreement | MSK
Subsequent Events
Percentage of net proceeds from monetization to be paid to MSK		40.00%

QUARTERLY CONSOLIDATED FINANC_3

QUARTERLY CONSOLIDATED FINANCIAL DATA (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019
QUARTERLY CONSOLIDATED FINANCIAL DATA (unaudited)
Revenue	$ 20,750								$ 20,750	$ 0
Revenue from Contract with Customer, Product and Service [Extensible List]	us-gaap:LicenseMember								Us-gaap:LicenseMember	Us-gaap:LicenseMember
Loss from operations	$ (20,095)	$ (32,641)	$ (40,452)	$ (26,747)	$ (23,758)	$ (24,359)	$ (18,634)	$ (16,253)	$ (119,935)	$ (83,004)
Net loss	$ (19,933)	$ (32,832)	$ (40,393)	$ (26,179)	$ (23,136)	$ (23,922)	$ (18,036)	$ (15,934)	$ (119,337)	$ (81,028)
Basic and diluted net loss per share (in dollars per share)	$ (0.48)	$ (0.82)	$ (1.01)	$ (0.66)	$ (0.60)	$ (0.70)	$ (0.53)	$ (0.47)	$ (2.97)	$ (2.30)