Y-mAbs Therapeutics (YMAB) 10-Q 6 May 21 2021 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2021	May 04, 2021
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2021
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 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
Entity Common Stock, Shares Outstanding		43,571,419
Entity Central Index Key	0001722964
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q1
Amendment Flag	false

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
CURRENT ASSETS
Cash and cash equivalents	$ 252,271	$ 114,634
Accounts receivable, net	5,997
Inventories	1,005
Other current assets	4,736	7,729
Total current assets	264,009	122,363
Property and equipment, net	2,095	1,825
Operating lease right-of-use assets	3,979	4,569
Other assets	4,833	3,290
TOTAL ASSETS	274,916	132,047
LIABILITIES
Accounts payable	6,543	9,372
Accrued liabilities	8,486	8,197
Operating lease liabilities, current portion	1,971	1,966
Total current liabilities	17,000	19,535
Accrued milestone and royalty payments	2,250	2,695
Operating lease liabilities, long-term portion	1,515	2,013
Other liabilities	1,934	1,968
TOTAL LIABILITIES	22,699	26,211
Commitments and contingencies (Note 6)
STOCKHOLDERS' EQUITY
Preferred stock, $0.0001 par value, 5,500,000 shares authorized at March 31, 2021 and December 31, 2020; none issued at March 31, 2021 and December 31, 2020
Common stock, $0.0001 par value, 100,000,000 shares authorized at March 31, 2021 and December 31, 2020; 43,548,419 and 40,688,447 shares issued at March 31, 2021 and December 31, 2020, respectively	4	4
Additional paid in capital	504,091	391,558
Accumulated other comprehensive loss	(91)	(526)
Accumulated deficit	(251,787)	(285,200)
TOTAL STOCKHOLDERS' EQUITY	252,217	105,836
TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY	$ 274,916	$ 132,047

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2021	Dec. 31, 2020
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	43,548,419	40,688,447

Consolidated Statements of Net

Consolidated Statements of Net Income/(Loss) and Comprehensive Income/(Loss) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
OPERATING COSTS AND EXPENSES
Cost of goods sold	$ 93
Research and development	21,579	$ 18,622
Selling, general and administrative	11,970	8,125
Total operating cost and expenses	33,642	26,747
Loss from operations	(28,259)	(26,747)
OTHER INCOME, NET
Gain from sale of priority review voucher, net	62,010
Interest and other income / (loss)	(338)	568
NET INCOME / (LOSS)	33,413	(26,179)
Other comprehensive income
Foreign currency translation	435	25
COMPREHENSIVE INCOME / (LOSS)	$ 33,848	$ (26,154)
Net income/(loss) per share attributable to common stockholders, basic (in dollars per share)	$ 0.80	$ (0.66)
Weighted average common shares outstanding, basic (in shares)	41,870,759	39,753,583
Net income/(loss) per share attributable to common stockholders, diluted (in dollars per share)	$ 0.75	$ (0.66)
Weighted average common shares outstanding, diluted (in shares)	44,383,791	39,753,583
Product
Product revenue, net	$ 5,383
License
Product revenue, net	$ 0	$ 0

Consolidated Statements of Chan

Consolidated Statements of Changes in Stockholders' Equity $ in Thousands	Common StockUSD ($)shares	Common StockUSS ($)shares	Additional Paid-in CapitalUSD ($)	Accumulated Other Comprehensive Income/(Loss)USD ($)	Accumulated DeficitUSD ($)	USD ($)shares
Balance at the beginning of period at Dec. 31, 2019	$ 3		$ 364,712	$ 50	$ (165,863)	$ 198,903
Balance at the beginning of period (in shares) at Dec. 31, 2019 | shares	39,728,416	39,728,416
Increase (Decrease) in Stockholders' Equity
Exercise of stock options			370			370
Exercise of stock options (in shares) | shares	25,778	25,778
Stock-based compensation expense		$ 3,429	2,211			2,211
Foreign currency translation				25		25
Net income (loss)					(26,179)	(26,179)
Balance at the end of period at Mar. 31, 2020	$ 3		367,293	75	(192,042)	175,330
Balance at the end of period (in shares) at Mar. 31, 2020 | shares	39,757,623	39,757,623
Balance at the beginning of period at Dec. 31, 2020	$ 4		391,558	(526)	(285,200)	105,836
Balance at the beginning of period (in shares) at Dec. 31, 2020 | shares	40,688,447	40,688,447
Increase (Decrease) in Stockholders' Equity
Issuance of common stock to investors, net of issuance costs			107,725			107,725
Issuance of common stock to investors, net of issuance costs (in shares) | shares	2,804,878	2,804,878
Exercise of stock options			110			$ 110
Exercise of stock options (in shares) | shares	46,000	46,000				46,000
Stock-based compensation expense			4,698			$ 4,698
Stock-based compensation expense (in shares) | shares	9,094	9,094
Foreign currency translation				435		435
Net income (loss)					33,413	33,413
Balance at the end of period at Mar. 31, 2021	$ 4		$ 504,091	$ (91)	$ (251,787)	$ 252,217
Balance at the end of period (in shares) at Mar. 31, 2021 | shares	43,548,419	43,548,419

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
CASH FLOWS FROM OPERATING ACTIVITIES
Net income / (loss)	$ 33,413	$ (26,179)
Adjustments to reconcile net loss to net cash used in operating activities:
Gain from sale of priority review voucher, net	(62,010)
Depreciation and amortization	130	86
Stock-based compensation	4,698	2,211
Foreign currency transactions	435	25
Changes in assets and liabilities:
Accounts receivable, net	(5,997)
Inventories	(1,005)
Other current assets	2,993	759
Other assets	(1,543)	(11)
Accounts payable	(2,829)	1,740
Accrued liabilities and other	(146)	(387)
NET CASH USED IN OPERATING ACTIVITIES	(31,861)	(21,756)
CASH FLOWS FROM INVESTING ACTIVITIES
Purchase of property and equipment	(400)	(26)
Net proceeds from sale of priority review voucher	62,010
NET CASH USED IN INVESTING ACTIVITIES	61,610	(26)
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from issuance of common stock, net of issuance costs	107,725
Proceeds from exercised stock options	110	370
NET CASH PROVIDED BY FINANCING ACTIVITIES	107,835	370
Effect of exchange rates on cash and cash equivalents	53	50
NET INCREASE / (DECREASE) IN CASH AND CASH EQUIVALENTS	137,637	(21,362)
Cash and cash equivalents at the beginning of period	114,634	207,136
Cash and cash equivalents at the end of period	252,271	185,774
SUPPLEMENTAL DISCLOSURE OF NON-CASH ACTIVITIES
Property and equipment purchases in accounts payable	$ 60
Right-of-use assets obtained in exchange for lease obligations in 2020		$ 2,679

ORGANIZATION AND DESCRIPTION OF

ORGANIZATION AND DESCRIPTION OF BUSINESS	3 Months Ended
	Mar. 31, 2021
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	3 Months Ended
	Mar. 31, 2021
BASIS OF PRESENTATION
BASIS OF PRESENTATION	NOTE 2—BASIS OF PRESENTATION Except for the quarter ended March 31, 2021, 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 pre-clinical 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 $251.8 million as of March 31, 2021 and $285.2 million as of December 31, 2020. Through March 31, 2021, the Company has funded its operations primarily through proceeds from sales of shares of its common stock, including its initial public offering in September 2018 and its subsequent public offerings in November 2019 and February 2021. 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 the Company, before deducting underwriting discounts and commissions and offering expenses payable by the Company, were approximately $115 million. As of March 31, 2021, the Company had cash and cash equivalents of $252.3 million, and as of December 31, 2020 the Company had cash and cash equivalents of $114.6 million. As of the issuance date of the financial statements for the first quarter ended March 31, 2021, the Company expects that its cash and cash equivalents at March 31, 2021 will be sufficient to fund its operating expenses and capital expenditure requirements through at least the next 12 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 unaudited 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”) for interim financial information, Accounting Standards Codification (“ASC”) Topic 270-10 and with the instructions to Form 10-Q. Accordingly, these financial statements do not include all of the information and notes required by GAAP for complete financial statements. The unaudited interim financial statements include all adjustments (consisting only of normal recurring nature) necessary in the judgment of management for a fair statement of the results for the periods presented. All intercompany balances and transactions have been eliminated. The Company has evaluated subsequent events through the date of this filing. Operating results for the three-month period ended March 31, 2021 are not necessarily indicative of the results that may be expected for the year ended December 31, 2021, any other interim periods, or any future year or period. The December 31, 2020 consolidated balance sheet data was derived from audited financial statements, but does not include all disclosures required by GAAP. You should read these unaudited interim condensed consolidated financial statements in conjunction with the consolidated financial statements and notes included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2020.

SUMMARY OF SIGNIFICANT ACCOUNTI

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	3 Months Ended
	Mar. 31, 2021
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	NOTE 3—SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Our signifcant accounting policies are detailed in our Annual Report on Form 10-K for the year ended December 31, 2020. Operating Leases 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. The Company currently elects 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 elect 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 8 for the related disclosures. 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. The carrying amount of cash and cash equivalents approximates its fair value due to its short-term and liquid nature. To date, the Company has not experienced any losses on its cash and cash equivalents, and we do not anticipate any losses with respect to such cash balances. While we maintain cash balances in excess of insured limits within a limited number of financial institutions, we mitigate our risk by maintaining the majority of our cash and equivalents with high quality financial institutions. Trade Accounts Receivables The Company’s trade accounts receivable balance consists of amounts due from sales of our approved product, DANYELZA. Receivables from product sales are recorded net of allowances which generally include chargebacks, doubtful accounts, returns, and discounts. The company accrues allowances based on the estimation of each individual sales transaction. The Company has not experienced any write-offs related to our customers and has not recognized any allowance for doubtful accounts. Concentration of Credit Risk The Company product sales are made through arrangements primarily with two national speciality distributors in the United States of America. As of March 31, 2021, the receivables balances from such distributors totaled 96% of our outstanding accounts receivable. The Company has contractual payment terms with each of its customers and the Company monitors their financial performance, historical payment terms and credit worthiness to timely assess and respond to any changes in their credit profile. Inventory The Company values its inventories at the lower of cost or net realizable value on a first-in, first-out basis. The Company’s inventory cost includes amounts related to materials, third-party contract manufacturing, third-party packaging services, freight, labor costs for personnel involved in the manufacturing process, and indirect overhead costs. Raw and intermediate materials that may be utilized for both commercial and clinical programs are identical and given the alternative future use such amounts are initially classified as inventory. Amounts in inventory associated with clinical development programs are charged to research and development expense when the product enters the research and development process and can no longer be used for commercial purposes and, therefore, does not have an alternative future use. The Company performs an assessment of the recoverability of capitalized inventory during each reporting period, and writes down any excess and obsolete inventories to their estimated realizable value in the period in which the impairment is first identified. Such impairment charges, should they occur, are recorded within cost of sales. The determination of whether inventory costs will be realizable requires estimates by management. No inventory write-downs occurred in the three months ended March 31, 2021. The Company capitalizes inventory costs related to products to be sold in the ordinary course of business. The Company makes a determination of capitalizing inventory costs for a product based on, among other factors, status of regulatory approval, information regarding safety, efficacy and expectations relating to commercial sales and recoverability of costs. For DANYELZA, the Company commenced capitalization of inventory at the receipt of FDA approval. 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 March 31, 2021 Using: ​ Level 1 Level 2 Level 3 Total Cash equivalents: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Money market funds ​ $ — ​ $ 235,686 ​ $ — ​ $ 235,686 ​ ​ $ — ​ $ 235,686 ​ $ — ​ $ 235,686 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ 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 ​ During the quarter ended March 31, 2021, there were no transfers Level 3 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, net product revenues, 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. Revenue Recognition - Product revenue We recognize revenue from sales of DANYELZA at a point in time when our customer is deemed to have obtained control of the product, which generally occurs upon receipt at the end-user hospital. ​ The amount of revenue we recognize from sales of DANYELZA varies due to rebates, chargebacks and discounts provided under governmental and other programs, distribution related fees and other sales-related deductions. In order to determine those deductions, we estimate, utilizing the expected value method, the amount of revenue that we will ultimately be entitled to. This estimate is based upon contracts with customers and government agencies, statutorily-defined discounts applicable to government-funded programs, estimated payor mix, and other relevant factors. Calculating these amounts involves estimates and judgments. ​ ​ Revenue Recognition - License revenue 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. 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 $20,000,000 for the transfer of the license and know-how related to the product indications. The Company may receive regulatory-based milestone payments up to $40,000,000 and sales-based milestone payments up to $60,000,000 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 $20,000,000 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 for three months ended March 31, 2021 and March 31, 2020. ​ 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. Recently Issued Accounting Pronouncements - Adopted In March 2020, the FASB issued Accounting Standards Update No. 2020-04 (“ASU 2020-04”), Reference rate reform (Topic 848)—Facilitation of the effects of reference rate reform on financial reporting. The amendments in this Update provide optional guidance for a limited time to ease the potential burden in accounting for (or recognizing the effects) of reference rate reform on financial reporting. The amendments in this Update provide optional expedients and exceptions for applying generally accepted accounting principles (GAAP) to contracts, hedging relationships, and other transactions affected by reference rate reform if certain criteria are met. The amendments in this Update are effective for all entities as of March 12, 2020 through December 31, 2022. The adoption of this standard on January 1, 2021 did not have a material impact on our consolidated financial statements and related disclosures. In December 2019, the FASB issued Accounting Standards Update No. 2019-12 (“ASU 2019-12”), Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes. The amendments in this Update affect entities within the scope of Topic 740, Income Taxes, and are effective for fiscal years beginning after December 15, 2020, and interim periods within those fiscal years. The adoption of this standard on January 1, 2021 did not have a material impact on our consolidated financial statements and related disclosures.

PRODUCT REVENUE

PRODUCT REVENUE	3 Months Ended
	Mar. 31, 2021
REVENUE RECOGNITION
REVENUE RECOGNITION	NOTE 4—PRODUCT REVENUE The Company’s product revenues were generated from sales of DANYELZA and totaled $5.4 million for the three months ended March 31, 2021. There were no product sales during the three months ended March 31, 2020. ​ Revenue from product sales is recorded net of applicable provisions for rebates, chargebacks, discounts, distribution-related fees and other sales-related deductions. Accruals for chargebacks and discounts are recorded as a direct reduction to accounts receivable. Accruals for rebates, distribution-related fees and other sales-related deductions are recorded within accrued liabilities. As of March 31, 2021, the company had recorded accounts receivable allowances of approximately $184,000 and accrued liabilities of $614,000 related to product sales during the three months ended March 31, 2021. ​ Substantially all of the Company’s product sales were in the United States. The Company had product sales to certain customers that accounted for more than 10% of total gross product revenue for the three months ended March 31, 2021. Mckesson and Cardinal Health accounted for 80% and 16%, respectively, of our gross product revenue for the three months ended March 31, 2021. ​ ​

NET LOSS PER SHARE

NET LOSS PER SHARE	3 Months Ended
	Mar. 31, 2021
NET LOSS PER SHARE
NET LOSS PER SHARE	NOTE 5—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 (in thousands, except per share amounts): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three months ended March 31, ​ ​ 2021 ​ 2020 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Net income / (loss) (numerator) ​ $ 33,413 ​ $ (26,179) ​ Weighted-average shares (denominator), basic ​ 41,871 ​ 39,754 ​ Basic net income / (loss) per share ​ $ 0.80 ​ $ (0.66) ​ Weighted-average shares (denominator), diluted ​ ​ 44,384 ​ ​ 39,754 ​ Diluted net income / (loss) per share ​ $ 0.75 ​ $ (0.66) ​ ​ Potentially dilutive securities excluded from the computation of diluted earnings per share relate to stock options outstanding totaled 1,177,600 shares as of March 31, 2021 and 4,721,824 shares as of March 31, 2020.

INVENTORY

INVENTORY	3 Months Ended
	Mar. 31, 2021
INVENTORY
INVENTORY	NOTE 6—INVENTORY Inventories consist of the following (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of March 31, 2021 ​ Raw Material ​ Work In Progress ​ Finished Goods ​ Total ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Inventories ​ $ — ​ $ 697 ​ $ 308 ​ $ 1,005 ​

ACCRUED LIABILITIES

ACCRUED LIABILITIES	3 Months Ended
	Mar. 31, 2021
ACCRUED LIABILITIES
ACCRUED LIABILITIES	NOTE 7—ACCRUED LIABILITIES Accrued short-term liabilities at March 31, 2021 and December 31, 2020 are as follows (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ March 31, December 31, ​ 2021 ​ 2020 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Accrued licensing, milestone and royalty payments ​ $ 2,800 ​ $ 3,608 Accrued clinical costs ​ 1,077 ​ 678 Accrued compensation and board fees ​ 3,000 ​ 2,603 Accrued manufacturing costs ​ ​ — ​ ​ 983 Sales reserves accruals ​ ​ 614 ​ ​ — Other ​ 995 ​ 325 Total ​ $ 8,486 ​ $ 8,197 ​

LICENSE AGREEMENTS AND COMMITME

LICENSE AGREEMENTS AND COMMITMENTS	3 Months Ended
	Mar. 31, 2021
LICENSE AGREEMENTS AND COMMITMENTS
LICENSE AGREEMENTS AND COMMITMENTS	NOTE 8—LICENSE AGREEMENTS AND COMMITMENTS As of March 31, 2021, the Company has entered into two license agreements and certain other agreements with Memorial Sloan Kettering Cancer Center (“MSK”). The license agreements, as previously disclosed in our annual report on Form 10-K, are the MSK License and the CD33 License Agreement. In addition, the Company entered into the SADA License Agreement with MSK and Massachusetts Institute of Technology (“MIT”) in 2020. Through a 2019 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. These license agreements with MSK and MIT grant 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 7 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 accrued as of and for the period ending March 31, 2021 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Cash paid Cash paid Expense Expense Accrued liabilities Accrued liabilities Accrued liabilities Accrued liabilities ​ ​ Three months ​ Three months ​ Three months ​ Three months ​ Current as of ​ Non-current ​ Current as of ​ Non-current ​ ​ ended March ​ ended March ​ ended March ​ ended March ​ March ​ March ​ December ​ December Agreements ​ 2021 ​ 2020 ​ 2021 ​ 2020 ​ 2021 ​ 2021 ​ 2020 ​ 2020 MSK ​ ​ $ 450 ​ ​ $ - ​ ​ $ - ​ ​ $ - ​ ​ $ 1,195 ​ ​ $ 1,800 ​ ​ $ 305 ​ ​ $ 1,640 CD33 ​ ​ 100 ​ ​ — ​ ​ — ​ ​ — ​ ​ — ​ ​ 450 ​ ​ 100 ​ ​ 450 MabVax ​ ​ — ​ ​ — ​ ​ — ​ ​ — ​ ​ — ​ ​ — ​ ​ — ​ ​ — SADA ​ ​ — ​ ​ — ​ ​ — ​ ​ — ​ ​ 1,605 ​ ​ 1,000 ​ ​ 1,000 ​ ​ 1,605 ​ As of March 31, 2021, the Company has $1.8 million of intangible assets related to DANYELZA that were recorded under the MSK license and are included in other assets on the consolidated balance sheet. The below table represents the maximum clinical, regulatory or sales-based milestones as reflected within the agreements, certain of which have been paid in prior periods or are accrued as presented in the table above (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Maximum Maximum Maximum Agreements ​ Clinical Milestones ​ Regulatory Milestones ​ Sales-based milestones ​ MSK ​ ​ $ 2,450 ​ ​ $ 9,000 ​ ​ $ 20,000 ​ CD33 ​ ​ 550 ​ ​ 500 ​ ​ 7,500 ​ MabVax ​ ​ 200 ​ ​ 1,200 ​ ​ — ​ SADA ​ ​ 4,730 ​ ​ 18,125 ​ ​ 23,750 ​ ​ Minimum royalties and certain clinical and regulatory milestones that become due based upon the passage of time under the CD33 License Agreement, the SADA Agreement and the MabVax Agreement are not recorded as a liability as the Company does not consider such obligations to be probable as of March 31, 2021. 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 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. For three months ended March 31, 2021 and 2020, we incurred research and development expenses of $0.9 million and $1.0 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 $3.6 million as right-of-use asset and $2.7 million as lease liability with the difference of $0.9 million resulting from certain prepayments and other costs incurred. The company pays equal monthly installments of approximately $0.1 million per month in additional access fees through September 2022 resulting in total payments of $2.0 million remaining under the agreement. There are no renewal options in this agreement. 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. Total operating lease costs were $646,000 and $174,000 for the three months ended March 31, 2021 and 2020, respectively. During the three months ended March 31, 2021, the expenses were recorded as $587,000 in research and development expense and $59,000 in general and administrative expense. During the three months ended March 31, 2020, the expenses were recorded as $123,000 in research and development expense and $51,000 in general and administrative expense. Cash paid for amounts included in the measurement of lease liabilities for the three months ended March 31, 2021 and 2020 was $544,000 and $188,000, respectively, 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 March 31, 2021 were as follows (in thousands): ​ ​ ​ ​ ​ ​ Operating Leases ​ at March 31, 2021 Remainder of 2021 ​ $ 1,627 Years ending December 31, ​ ​ ​ 2022 ​ ​ 1,587 2023 ​ ​ 539 2024 ​ ​ 64 Total lease payments ​ ​ 3,817 Less: Imputed interest ​ ​ (331) Total operating lease liabilities at March 31, 2021 ​ $ 3,486 ​ Maturities of operating lease liabilities at December 31, 2020 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 ​ 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 March 31, 2021, the weighted average remaining lease term is 1.97 years and the weighted average discount rate used to determine the operating lease liability was 7.6%. As of March 31, 2020, the weighted average remaining lease term is 3.51 years and the weighted average discount rate used to determine the operating lease liability was 11.0%.

STOCKHOLDERS' EQUITY

STOCKHOLDERS' EQUITY	3 Months Ended
	Mar. 31, 2021
STOCKHOLDERS' EQUITY
STOCKHOLDERS' EQUITY	NOTE 9—STOCKHOLDERS’ EQUITY Authorized Stock As of March 31, 2021 and December 31, 2020, the Company has authorized a total of 105,500,000 shares, 100,000,000 of which are common stock, par value $0.0001 per share, and 5,500,000 of which are 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 had issued 43,548,419 shares of its common stock as of March 31, 2021 and 40,688,447 shares of its common stock as of December 31, 2020. 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 March 31, 2021 or December 31, 2020. 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 On February 22, 2021, we completed a third 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	3 Months Ended
	Mar. 31, 2021
SHARE-BASED COMPENSATION
SHARE-BASED COMPENSATION	NOTE 10—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 are 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 During the three-month periods ended March 31, 2021, there were no new stock options granted. During the three month periods ended in March 31, 2021 and 2020, stock-based compensation for stock option grants were $4,629,000 and $2,188,000, respectively, for options granted to employees and directors. During the three months ended March 31, 2021, the expenses were recorded as $1,692,000 in research and development expense and $2,937,000 in general and administrative expense. During the three months ended March 31, 2020, the expenses were recorded as $489,000 in research and development expense and $1,699,000 in general and administrative expense. 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, 2020 5,674,100 ​ $ 22.55 ​ $ 156,726 7.51 Exercised ​ (46,000) ​ ​ 2.40 ​ ​ ​ ​ ​ Forfeited ​ (26,000) ​ ​ 39.08 ​ ​ ​ ​ ​ Outstanding and expected to vest at March 31, 2021 5,602,100 ​ $ 22.64 ​ $ 70,905 ​ 7.28 Exercisable at March 31, 2021 3,189,667 ​ $ 10.48 ​ $ 64,628 ​ 5.99 ​ The weighted average fair value of stock options granted during the three months ended March 31, 2020 was $16.94. No stock options were granted during the three months ended March 31, 2021. 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. As of March 31, 2021, we had $47.6 million of unrecognized compensation related to employee stock options that are expected to vest over a period of 2.92 years. As of March 31, 2020, we had $25.9 million of unrecognized compensation related to employee stock options that are expected to vest over a period of 3.03 years. Restricted Stock Unit Activity During the three months ended March 31, 2021 and March 31, 2020, stock-based compensation for restricted stock unit grants was $69,000 and $23,000, respectively. During the three months ended March 31, 2021, the expenses were recorded as $63,000 in research and development expense and $6,000 in general and administrative expense. During the three months ended March 31, 2020, the expenses were recorded as $21,000 in research and development expense and $2,000 in general and administrative expense. 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, 2020 ​ 30,146 ​ $ 25.45 2.18 ​ Granted ​ 372 ​ ​ 35.51 ​ Vested ​ (9,094) ​ ​ 20.73 ​ ​ Outstanding and expected to vest at March 31, 2021 ​ 21,424 ​ $ 27.63 1.96 ​ ​ As of March 31, 2021, we had $523,000 of unrecognized compensation related to employee restricted stock units that are expected to vest over a period of 1.96 years. As of March 31, 2020, we had $486,000 of unrecognized compensation related to employee restricted stock units that are expected to vest over a period of 2.54 years.

RELATED PARTY TRANSACTIONS

RELATED PARTY TRANSACTIONS	3 Months Ended
	Mar. 31, 2021
RELATED PARTY TRANSACTIONS
RELATED PARTY TRANSACTIONS	NOTE 11—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 $948,000 and $1,049,000 in the three months ended March 31, 2021 and 2020, respectively, for milestones, and research and development costs, under these agreements with MSK. Please refer to Note 8 – License Agreements and Commitments for additional details on our agreements with MSK. As of March 31, 2021, we had a total of $487,000 recorded as accounts payable, $5,903,000 as accrued liabilities, thereby totaling $6,390,000 due to 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.

INCOME TAXES

INCOME TAXES	3 Months Ended
	Mar. 31, 2021
INCOME TAXES
INCOME TAXES	NOTE 12—INCOME TAXES The Company provided no current and deferred income taxes on net income of $33,413,000 and net loss of $26,179,000 for the three-month periods ended March 31, 2021 and 2020, respectively. 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 March 31, 2021 and December 31, 2020, the Company has determined that there were no uncertain tax positions. The Company’s tax returns for years 2020, 2019, 2018, 2017, 2016, and 2015 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 valuation allowance related primarily to net U.S. deferred tax assets from operating losses, and research and development tax credit carryforwards. 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.

OTHER BENEFITS

OTHER BENEFITS	3 Months Ended
	Mar. 31, 2021
OTHER BENEFITS
OTHER BENEFITS	NOTE 13—OTHER BENEFITS The Company has 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 three months ended March 31, 2021 and 2020. The Company has established a retirement program for employees of the Company’s 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. No contributions from our Danish subsidiary were made during the three months ended March 31, 2021 and 2020. 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.

GAIN FROM SALE OF PRIORITY REVI

GAIN FROM SALE OF PRIORITY REVIEW VOUCHER	3 Months Ended
	Mar. 31, 2021
GAIN FROM SALE OF PRIORITY REVIEW VOUCHER
GAIN FROM SALE OF PRIORITY REVIEW VOUCHER	NOTE 14-GAIN FROM SALE OF PRIORITY REVIEW VOUCHER 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 $105,000,000. 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 retained 60% of the net proceeds received from the sale, and the remaining 40% was paid to MSK. The transaction closed on January 21, 2021 once the substantive closing conditions included within the agreement were resolved. The Company recognized a net gain of $62,010,000 during the quarter ended March 31, 2021 related to the sale. ​

SUMMARY OF SIGNIFICANT ACCOUN_2

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)	3 Months Ended
	Mar. 31, 2021
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Operating Leases	Operating Leases 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. The Company currently elects 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 elect 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 8 for the related disclosures.
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. The carrying amount of cash and cash equivalents approximates its fair value due to its short-term and liquid nature. To date, the Company has not experienced any losses on its cash and cash equivalents, and we do not anticipate any losses with respect to such cash balances. While we maintain cash balances in excess of insured limits within a limited number of financial institutions, we mitigate our risk by maintaining the majority of our cash and equivalents with high quality financial institutions.
Trade Accounts Receivables	Trade Accounts Receivables The Company’s trade accounts receivable balance consists of amounts due from sales of our approved product, DANYELZA. Receivables from product sales are recorded net of allowances which generally include chargebacks, doubtful accounts, returns, and discounts. The company accrues allowances based on the estimation of each individual sales transaction. The Company has not experienced any write-offs related to our customers and has not recognized any allowance for doubtful accounts.
Concentration of Credit Risk	Concentration of Credit Risk The Company product sales are made through arrangements primarily with two national speciality distributors in the United States of America. As of March 31, 2021, the receivables balances from such distributors totaled 96% of our outstanding accounts receivable. The Company has contractual payment terms with each of its customers and the Company monitors their financial performance, historical payment terms and credit worthiness to timely assess and respond to any changes in their credit profile.
Inventory	Inventory The Company values its inventories at the lower of cost or net realizable value on a first-in, first-out basis. The Company’s inventory cost includes amounts related to materials, third-party contract manufacturing, third-party packaging services, freight, labor costs for personnel involved in the manufacturing process, and indirect overhead costs. Raw and intermediate materials that may be utilized for both commercial and clinical programs are identical and given the alternative future use such amounts are initially classified as inventory. Amounts in inventory associated with clinical development programs are charged to research and development expense when the product enters the research and development process and can no longer be used for commercial purposes and, therefore, does not have an alternative future use. The Company performs an assessment of the recoverability of capitalized inventory during each reporting period, and writes down any excess and obsolete inventories to their estimated realizable value in the period in which the impairment is first identified. Such impairment charges, should they occur, are recorded within cost of sales. The determination of whether inventory costs will be realizable requires estimates by management. No inventory write-downs occurred in the three months ended March 31, 2021. The Company capitalizes inventory costs related to products to be sold in the ordinary course of business. The Company makes a determination of capitalizing inventory costs for a product based on, among other factors, status of regulatory approval, information regarding safety, efficacy and expectations relating to commercial sales and recoverability of costs. For DANYELZA, the Company commenced capitalization of inventory at the receipt of FDA approval.
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 March 31, 2021 Using: ​ Level 1 Level 2 Level 3 Total Cash equivalents: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Money market funds ​ $ — ​ $ 235,686 ​ $ — ​ $ 235,686 ​ ​ $ — ​ $ 235,686 ​ $ — ​ $ 235,686 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ 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 ​ During the quarter ended March 31, 2021, there were no transfers Level 3
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, net product revenues, 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.
Revenue Recognition	Revenue Recognition - Product revenue We recognize revenue from sales of DANYELZA at a point in time when our customer is deemed to have obtained control of the product, which generally occurs upon receipt at the end-user hospital. ​ The amount of revenue we recognize from sales of DANYELZA varies due to rebates, chargebacks and discounts provided under governmental and other programs, distribution related fees and other sales-related deductions. In order to determine those deductions, we estimate, utilizing the expected value method, the amount of revenue that we will ultimately be entitled to. This estimate is based upon contracts with customers and government agencies, statutorily-defined discounts applicable to government-funded programs, estimated payor mix, and other relevant factors. Calculating these amounts involves estimates and judgments. ​ ​ Revenue Recognition - License revenue 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. 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 $20,000,000 for the transfer of the license and know-how related to the product indications. The Company may receive regulatory-based milestone payments up to $40,000,000 and sales-based milestone payments up to $60,000,000 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 $20,000,000 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 for three months ended March 31, 2021 and March 31, 2020.
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.

SUMMARY OF SIGNIFICANT ACCOUN_3

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Tables)	3 Months Ended
	Mar. 31, 2021
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 March 31, 2021 Using: ​ Level 1 Level 2 Level 3 Total Cash equivalents: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Money market funds ​ $ — ​ $ 235,686 ​ $ — ​ $ 235,686 ​ ​ $ — ​ $ 235,686 ​ $ — ​ $ 235,686 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ 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

NET LOSS PER SHARE (Tables)

NET LOSS PER SHARE (Tables)	3 Months Ended
	Mar. 31, 2021
NET LOSS PER SHARE
Schedule of basic and diluted net loss per share	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three months ended March 31, ​ ​ 2021 ​ 2020 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Net income / (loss) (numerator) ​ $ 33,413 ​ $ (26,179) ​ Weighted-average shares (denominator), basic ​ 41,871 ​ 39,754 ​ Basic net income / (loss) per share ​ $ 0.80 ​ $ (0.66) ​ Weighted-average shares (denominator), diluted ​ ​ 44,384 ​ ​ 39,754 ​ Diluted net income / (loss) per share ​ $ 0.75 ​ $ (0.66) ​

INVENTORY (Tables)

INVENTORY (Tables)	3 Months Ended
	Mar. 31, 2021
INVENTORY
Schedule of inventory	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of March 31, 2021 ​ Raw Material ​ Work In Progress ​ Finished Goods ​ Total ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Inventories ​ $ — ​ $ 697 ​ $ 308 ​ $ 1,005

ACCRUED LIABILITIES (Tables)

ACCRUED LIABILITIES (Tables)	3 Months Ended
	Mar. 31, 2021
ACCRUED LIABILITIES
Summary of accrued short-term liabilities	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ March 31, December 31, ​ 2021 ​ 2020 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Accrued licensing, milestone and royalty payments ​ $ 2,800 ​ $ 3,608 Accrued clinical costs ​ 1,077 ​ 678 Accrued compensation and board fees ​ 3,000 ​ 2,603 Accrued manufacturing costs ​ ​ — ​ ​ 983 Sales reserves accruals ​ ​ 614 ​ ​ — Other ​ 995 ​ 325 Total ​ $ 8,486 ​ $ 8,197

LICENSE AGREEMENTS AND COMMIT_2

LICENSE AGREEMENTS AND COMMITMENTS (Tables)	3 Months Ended
	Mar. 31, 2021
LICENSE AGREEMENTS AND COMMITMENTS
Summary of significant license agreements, related commitments and milestone information	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Cash paid Cash paid Expense Expense Accrued liabilities Accrued liabilities Accrued liabilities Accrued liabilities ​ ​ Three months ​ Three months ​ Three months ​ Three months ​ Current as of ​ Non-current ​ Current as of ​ Non-current ​ ​ ended March ​ ended March ​ ended March ​ ended March ​ March ​ March ​ December ​ December Agreements ​ 2021 ​ 2020 ​ 2021 ​ 2020 ​ 2021 ​ 2021 ​ 2020 ​ 2020 MSK ​ ​ $ 450 ​ ​ $ - ​ ​ $ - ​ ​ $ - ​ ​ $ 1,195 ​ ​ $ 1,800 ​ ​ $ 305 ​ ​ $ 1,640 CD33 ​ ​ 100 ​ ​ — ​ ​ — ​ ​ — ​ ​ — ​ ​ 450 ​ ​ 100 ​ ​ 450 MabVax ​ ​ — ​ ​ — ​ ​ — ​ ​ — ​ ​ — ​ ​ — ​ ​ — ​ ​ — SADA ​ ​ — ​ ​ — ​ ​ — ​ ​ — ​ ​ 1,605 ​ ​ 1,000 ​ ​ 1,000 ​ ​ 1,605 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Maximum Maximum Maximum Agreements ​ Clinical Milestones ​ Regulatory Milestones ​ Sales-based milestones ​ MSK ​ ​ $ 2,450 ​ ​ $ 9,000 ​ ​ $ 20,000 ​ CD33 ​ ​ 550 ​ ​ 500 ​ ​ 7,500 ​ MabVax ​ ​ 200 ​ ​ 1,200 ​ ​ — ​ SADA ​ ​ 4,730 ​ ​ 18,125 ​ ​ 23,750 ​
Maturities of operating lease liabilities	Maturities of operating lease liabilities at March 31, 2021 were as follows (in thousands): ​ ​ ​ ​ ​ ​ Operating Leases ​ at March 31, 2021 Remainder of 2021 ​ $ 1,627 Years ending December 31, ​ ​ ​ 2022 ​ ​ 1,587 2023 ​ ​ 539 2024 ​ ​ 64 Total lease payments ​ ​ 3,817 Less: Imputed interest ​ ​ (331) Total operating lease liabilities at March 31, 2021 ​ $ 3,486 ​ Maturities of operating lease liabilities at December 31, 2020 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

SHARE-BASED COMPENSATION (Table

SHARE-BASED COMPENSATION (Tables)	3 Months Ended
	Mar. 31, 2021
SHARE-BASED COMPENSATION
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, 2020 5,674,100 ​ $ 22.55 ​ $ 156,726 7.51 Exercised ​ (46,000) ​ ​ 2.40 ​ ​ ​ ​ ​ Forfeited ​ (26,000) ​ ​ 39.08 ​ ​ ​ ​ ​ Outstanding and expected to vest at March 31, 2021 5,602,100 ​ $ 22.64 ​ $ 70,905 ​ 7.28 Exercisable at March 31, 2021 3,189,667 ​ $ 10.48 ​ $ 64,628 ​ 5.99
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, 2020 ​ 30,146 ​ $ 25.45 2.18 ​ Granted ​ 372 ​ ​ 35.51 ​ Vested ​ (9,094) ​ ​ 20.73 ​ ​ Outstanding and expected to vest at March 31, 2021 ​ 21,424 ​ $ 27.63 1.96 ​

BASIS OF PRESENTATION - Accumul

BASIS OF PRESENTATION - Accumulated deficit and cash (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
BASIS OF PRESENTATION
Accumulated deficit	$ 251,787	$ 285,200
Cash and cash equivalents	$ 252,271	$ 114,634

BASIS OF PRESENTATION - Seconda

BASIS OF PRESENTATION - Secondary public offering (Details) - USD ($) $ / shares in Units, $ in Millions	Feb. 22, 2021	Mar. 31, 2021
Common Stock
Issuance of stock
Issuance of common stock, Stock offering (in shares)		2,804,878
Secondary public offering, February 22, 2021
Issuance of stock
Aggregate gross proceeds	$ 115
Secondary public offering, February 22, 2021 | Common Stock
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
Issuance of stock
Issuance of common stock, Stock offering (in shares)	365,853

SUMMARY OF SIGNIFICANT ACCOUN_4

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Operating Leases (Details) $ in Millions	Mar. 31, 2021USD ($)
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Residual value guarantees	$ 0

SUMMARY OF SIGNIFICANT ACCOUN_5

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Concentration of Credit Risk (Details) - Credit Concentration Risk - Accounts Receivable - Two Customers	3 Months Ended
	Mar. 31, 2021customer
Concentration risk
Number of customers	2
Concentration risk percentage (as a percent)	96.00%

SUMMARY OF SIGNIFICANT ACCOUN_6

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Inventory (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2021USD ($)
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Inventory write-downs	$ 0

SUMMARY OF SIGNIFICANT ACCOUN_7

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Fair Value Measurement (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Fair value measurements
Level 1 to Level 2 Transfers	$ 0
Level 2 to Level 1 Transfers	0
Transfers into Level 3	0
Transfers out of Level 3	0
Recurring
Fair value measurements
Cash equivalents	235,686	$ 97,302
Recurring | Money market funds
Fair value measurements
Cash equivalents	235,686	97,302
Recurring | Level 2
Fair value measurements
Cash equivalents	235,686	97,302
Recurring | Level 2 | Money market funds
Fair value measurements
Cash equivalents	$ 235,686	$ 97,302

SUMMARY OF SIGNIFICANT ACCOUN_8

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Revenue Recognition - License Revenue (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		12 Months Ended
	Dec. 31, 2020	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Agreements
Accounts receivable		$ 5,997
License
Agreements
Revenue	$ 20,000	$ 0	$ 0
License | Development and commercialization arrangement | SciClone
Agreements
Revenue				$ 20,800
Upfront payment	20,000
Maximum regulatory milestones	40,000			40,000
Maximum sales-based milestones	$ 60,000			$ 60,000

SUMMARY OF SIGNIFICANT ACCOUN_9

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Segment (Details)	3 Months Ended
	Mar. 31, 2021segment
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Number of operating segments	1

PRODUCT REVENUE (Details)

PRODUCT REVENUE (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Revenue
Accrued liabilities, Sales reserves accruals	$ 614
Product
Revenue
Revenue	5,383
Accounts receivable allowances	184
Accrued liabilities, Sales reserves accruals	$ 614
Product | Revenue | Customer Concentration Risk | McKesson
Revenue
Concentration risk percentage (as a percent)	80.00%
Product | Revenue | Customer Concentration Risk | Cardinal Health
Revenue
Concentration risk percentage (as a percent)	16.00%
Product, DANYELZA
Revenue
Revenue	$ 5,400	$ 0

NET LOSS PER SHARE - Basic and

NET LOSS PER SHARE - Basic and diluted (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
NET LOSS PER SHARE
Net income / (loss) (numerator), Basic (in dollars)	$ 33,413	$ (26,179)
Net income / (loss) (numerator), Diluted (in dollars)	$ 33,413	$ (26,179)
Weighted-average shares (denominator), basic (in dollars per share)	41,870,759	39,753,583
Basic net income / (loss) per share (in dollars per share)	$ 0.80	$ (0.66)
Weighted-average shares (denominator), diluted (in dollars per share)	44,383,791	39,753,583
Diluted net income / (loss) per share (in dollars per share)	$ 0.75	$ (0.66)

NET LOSS PER SHARE - Anti-dilut

NET LOSS PER SHARE - Anti-dilutive securities (Details) - shares	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
NET LOSS PER SHARE
Potentially dilutive securities outstanding	1,177,600	4,721,824

INVENTORY (Details)

INVENTORY (Details) $ in Thousands	Mar. 31, 2021USD ($)
INVENTORY
Work in Process	$ 697
Finished Goods	308
Inventory Total	$ 1,005

ACCRUED LIABILITIES (Details)

ACCRUED LIABILITIES (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Accrued short-term liabilities
Accrued licensing, milestone and royalty payments	$ 2,800	$ 3,608
Accrued clinical costs	1,077	678
Accrued compensation and board fees	3,000	2,603
Accrued manufacturing costs		983
Sales reserves accruals	614
Other	995	325
Total	$ 8,486	$ 8,197

LICENSE AGREEMENTS AND COMMIT_3

LICENSE AGREEMENTS AND COMMITMENTS - MSK, CD33 and MabVax License Agreement (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2021USD ($)agreement	Dec. 31, 2020USD ($)
Financial statement information
Accrued liabilities, current	$ 8,486	$ 8,197
Accrued liabilities, noncurrent	$ 2,250	2,695
MSK
Agreements
Number of license agreements | agreement	2
MSK License Agreement
Financial statement information
Cash paid	$ 450
Accrued liabilities, current	1,195	305
Accrued liabilities, noncurrent	1,800	1,640
Milestones
Maximum Clinical Milestones	2,450
Maximum Regulatory Milestones	9,000
Maximums Sales-based Milestones	20,000
MSK License Agreement | Other long-term assets
Financial statement information
Intangible assets related to Danyelza	1,800
CD33 License Agreement
Financial statement information
Cash paid	100
Accrued liabilities, current		100
Accrued liabilities, noncurrent	450	450
Milestones
Maximum Clinical Milestones	550
Maximum Regulatory Milestones	500
Maximums Sales-based Milestones	7,500
MabVax Agreement
Milestones
Maximum Clinical Milestones	200
Maximum Regulatory Milestones	1,200
SADA License Agreement
Financial statement information
Accrued liabilities, current	1,605	1,000
Accrued liabilities, noncurrent	1,000	$ 1,605
Milestones
Maximum Clinical Milestones	4,730
Maximum Regulatory Milestones	18,125
Maximums Sales-based Milestones	$ 23,750

LICENSE AGREEMENTS AND COMMIT_4

LICENSE AGREEMENTS AND COMMITMENTS - Other agreements (Details) $ in Thousands	1 Months Ended	3 Months Ended
	Oct. 31, 2020	Mar. 31, 2021USD ($)agreementemployee	Mar. 31, 2020USD ($)agreementemployee
Agreements
Research and development		$ 21,579	$ 18,622
Other Agreements with MSK
Agreements
Research and development		$ 900	$ 1,000
CFSAs
Agreements
Number of service agreements | agreement		2	2
SRA
Agreements
Research service period	3 years
Researchers, Employees of MSK | MDSA
Agreements
Number of individuals | employee		5	5

LICENSE AGREEMENTS AND COMMIT_5

LICENSE AGREEMENTS AND COMMITMENTS - Lease agreements (Details) $ in Thousands, $ in Thousands	1 Months Ended
	Aug. 31, 2020USS ($)	Feb. 28, 2019USS ($)ft²	Mar. 31, 2021USD ($)	Dec. 31, 2020USD ($)	Dec. 31, 2019ft²	Jan. 31, 2018USS ($)
Leases
Operating lease right-of-use assets			$ 3,979	$ 4,569
Lease liabilities			$ 3,486	$ 3,979
Manufacturing facility, Indiana
Leases
Lease term	2 years
Operating lease right-of-use assets	$ 3,600
Lease liabilities	2,700
Difference between right of use asset and lease liability recognized	900
Access fee monthly installment	100
Amount of additional access fees to be paid in monthly installments	$ 2,000
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_6

LICENSE AGREEMENTS AND COMMITMENTS - Lease costs (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Leases
Total operating lease costs	$ 646	$ 174
Cash paid for amounts included in the measurement of lease liabilities	544	188
Research and development expense
Leases
Total operating lease costs	587	123
General and administrative
Leases
Total operating lease costs	$ 59	$ 51

LICENSE AGREEMENTS AND COMMIT_7

LICENSE AGREEMENTS AND COMMITMENTS - Lease maturities (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Future minimum commitments under all non-cancelable operating leases
Remainder of year	$ 1,627
First fiscal year	1,587	$ 2,180
Second fiscal year	539	1,593
Third fiscal year	64	540
Fourth fiscal year		64
Total lease payments	3,817	4,377
Less: Imputed interest	(331)	(398)
Total operating lease liabilities at end of period	$ 3,486	$ 3,979

LICENSE AGREEMENTS AND COMMIT_8

LICENSE AGREEMENTS AND COMMITMENTS - Lease term and discount rate (Details)	Mar. 31, 2021	Mar. 31, 2020
LICENSE AGREEMENTS AND COMMITMENTS
Weighted average remaining lease term	1 year 11 months 19 days	3 years 6 months 3 days
Weighted average discount rate	7.60%	11.00%

STOCKHOLDERS' EQUITY - Authoriz

STOCKHOLDERS' EQUITY - Authorized, Common and Preferred Stock (Details)	3 Months Ended	12 Months Ended
	Mar. 31, 2021USD ($)Vote$ / sharesshares	Dec. 31, 2020USD ($)Vote$ / 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	1
Common stock dividends issued | $	$ 0	$ 0
Common stock, shares issued	43,548,419	40,688,447
Preferred stock, shares issued	0	0

STOCKHOLDERS' EQUITY - Stock gr

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

STOCKHOLDERS' EQUITY - Issuance

STOCKHOLDERS' EQUITY - Issuance of common stock (Details) - USD ($) $ / shares in Units, $ in Thousands	Feb. 22, 2021	Mar. 31, 2021
Issuance of stock
Issuance of common stock to investors, net of issuance costs		$ 107,725
Secondary public offering, February 22, 2021
Issuance of stock
Aggregate gross proceeds	$ 115,000
Common Stock
Issuance of stock
Issuance of common stock, Stock offering (in shares)		2,804,878
Common Stock | Secondary public offering, February 22, 2021
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 | Employees and nonemployees owning less than 10% of voting power
Share-Based Compensation Awards
Vesting period	4 years

SHARE-BASED COMPENSATION - 2018

SHARE-BASED COMPENSATION - 2018 Plan (Details) - 2018 Plan	3 Months Ended
	Mar. 31, 2021shares
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	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Share-Based Compensation Awards
Options granted (in shares)	0
Stock options | Employees and directors
Share-Based Compensation Awards
Stock-based compensation expenses	$ 4,629	$ 2,188
Stock options | Employees and directors | Research and development expense
Share-Based Compensation Awards
Stock-based compensation expenses	1,692	489
Stock options | Employees and directors | General and administrative
Share-Based Compensation Awards
Stock-based compensation expenses	$ 2,937	$ 1,699

SHARE-BASED COMPENSATION - St_2

SHARE-BASED COMPENSATION - Stock option activity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Options
Outstanding at beginning of period (in shares)	5,674,100
Exercised (in shares)	(46,000)
Forfeited, shares (in shares)	(26,000)
Outstanding at end of period (in shares)	5,602,100		5,674,100
Exercisable at end of period (in shares)	3,189,667
Weighted average exercise price
Outstanding at beginning of period (in dollars per share)	$ 22.55
Exercised (in dollars per share)	2.40
Forfeited (in dollars per share)	39.08
Outstanding at end of period (in dollars per share)	22.64		$ 22.55
Exercisable at end of period (in dollars per share)	$ 10.48
Aggregate intrinsic value and Weighted average remaining contractual life (years)
Outstanding (in dollars)	$ 70,905		$ 156,726
Exercisable (in dollars)	$ 64,628
Outstanding (in years)	7 years 3 months 10 days		7 years 6 months 3 days
Exercisable (in years)	5 years 11 months 26 days
Weighted average grant-date fair value of stock options granted (in dollars per share)		$ 16.94
Options granted (in shares)	0

SHARE-BASED COMPENSATION - St_3

SHARE-BASED COMPENSATION - Stock option unrecognized compensation (Details) - Stock options - Employees - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Unrecognized compensation related to employee stock options
Unrecognized compensation	$ 47.6	$ 25.9
Expected vesting period	2 years 11 months 1 day	3 years 10 days

SHARE-BASED COMPENSATION - Rest

SHARE-BASED COMPENSATION - Restricted Stock Unit Activity (Details) - RSUs - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Share-Based Compensation Awards
Stock-based compensation expenses	$ 69	$ 23
Restricted stock units issued and outstanding
Outstanding and expected to vest at beginning of period (in shares)	30,146
Granted (in shares)	372
Vested (in shares)	(9,094)
Outstanding and expected to vest at end of period (in shares)	21,424		30,146
Weighted average grant price
Weighted average grant price, Outstanding and expected to vest, Beginning of period (in dollars per share)	$ 25.45
Weighted average grant price, Grants (in dollars per share)	35.51
Weighted average grant price, Vested (in dollars per share)	20.73
Weighted average grant price, Outstanding and expected to vest, End of period (in dollars per share)	$ 27.63		$ 25.45
Unrecognized compensation	$ 523	$ 486
Weighted average remaining vesting life	1 year 11 months 15 days	2 years 6 months 14 days	2 years 2 months 4 days
Research and development expense
Share-Based Compensation Awards
Stock-based compensation expenses	$ 63	$ 21
General and administrative
Share-Based Compensation Awards
Stock-based compensation expenses	$ 6	$ 2

RELATED PARTY TRANSACTIONS (Det

RELATED PARTY TRANSACTIONS (Details) - MSK - MSK License Agreement, CD33 License Agreement, CTA, CFAs, SRA and MDSA - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Related party transactions
Expensed costs	$ 948	$ 1,049
Due to related parties, Accounts payable	487		$ 833
Due to related parties, Accrued liabilities	5,903		7,161
Due to related parties	$ 6,390		$ 7,994

INCOME TAXES - Expense (Details

INCOME TAXES - Expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Income tax expense
Current income taxes	$ 0	$ 0
Deferred income taxes	0	0
Domestic and foreign loss before income taxes:
Income (loss) before income tax benefit	$ 33,413	$ (26,179)

INCOME TAXES - Uncertain tax po

INCOME TAXES - Uncertain tax positions (Details) - USS ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
INCOME TAXES
Uncertain tax position	$ 0	$ 0

OTHER BENEFITS (Details)

OTHER BENEFITS (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
UNITED STATES
Benefit plan
Employer matching contribution to defined contribution plan (as a percent)	0.00%	0.00%
DENMARK
Benefit plan
Employer matching contribution to defined contribution plan (as a percent)	0.00%
Costs for employee health insurance benefits	$ 0	$ 0

GAIN FROM SALE OF PRIORITY RE_2

GAIN FROM SALE OF PRIORITY REVIEW VOUCHER (Details) - USD ($) $ in Thousands	Jan. 21, 2021	Mar. 31, 2021
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net gain recognized		$ 62,010
Priority Review Voucher | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Consideration	$ 105,000
Percentage of net proceeds from monetization entitled to be retained	60.00%
Percentage of net proceeds from monetization to be paid to MSK	40.00%
Net gain recognized		$ 62,010