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Table of Contents ( Mark One ) UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington , D.C. 20549 FORM 10 - K ANNUAL REPORT PURSUANT TO SECTION 13 OR 15 ( d ) OF THE SECURITIES EXCHANGE ACT OF 1934 For the fiscal year ended December 31 , 2020 . OR TRANSITION REPORT PURSUANT TO SECTION 13 OR 15 ( d ) OF THE SECURITIES EXCHANGE ACT OF 1934 For the transition period from Commission file number 001-38650 Y - mAbs Therapeutics , Inc. ( Exact name of registrant as specified in its charter ) Delaware ( State or Other Jurisdiction of Incorporation or Organization ) 230 Park Avenue , Suite 3350 New York , NY ( Address of Principal Executive Offices ) Title of each class : Common Stock , $ 0.0001 par value to Trading Symbol YMAB ( I.R.S. Employer Identification No. ) Registrant's telephone number , including area code ( 646 ) -885-8505 Securities registered pursuant to Section 12 ( b ) of the Act : 47-4619612 Securities registered pursuant to Section 12 ( g ) of the Act : 10169 ( Zip Code ) Name of each exchange on which registered : NASDAQ Global Select Market NONE ( Title of class ) Indicate by check mark if the registrant is a well - known seasoned issuer , as defined in Rule 405 of the Securities Act . Yes No | Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or Section 15 ( d ) of the Act . No X Yes Indicate by check mark whether the registrant ( 1 ) has filed all reports required to be filed by Section 13 or 15 ( d ) of the Securities Exchange Act of 1934 during the preceding 12 months ( or for such shorter period that registrant was required to file such reports ) , and ( 2 ) has been subject to such filing requirements for the past 90 days . Yes > No Indicate by check mark whether the registrant is a shell company ( as defined by Rule 12b - 2 of the Exchange Act ) . Yes No X Indicate by check mark whether the registrant has submitted electronically every Interactive Data File required to be submitted pursuant to Rule 405 of Regulation S - T ( §232.405 of this chapter ) during the preceding 12 months ( or for such shorter period that the registrant was required to submit such files ) . Yes X No Indicate by check mark if disclosure of delinquent filers pursuant to Item 405 of Regulation S - K ( §299.405 of this chapter ) is not contained herein , and will not be contained , to the best of the registrant's knowledge , in definitive proxy or information statements incorporated by reference in Part III of this Form 10 - K or any amendment to this Form 10 - K . > Indicate by check mark whether the registrant is a large accelerated filer , an accelerated filer , a non - accelerated filer , a smaller reporting company or an emerging growth company . See the definitions of " large accelerated filer " , " accelerated filer " , " smaller reporting company " and " emerging growth company " in Rule 12b - 2 of the Exchange Act . Large accelerated filer X Accelerated filer Non - accelerated filer Smaller reporting company Emerging growth company If an emerging growth company , indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13 ( a ) of the Exchange Act . Indicate by check mark whether the registrant has filed a report on and attestation to its management's assessment of the effectiveness of its internal control over financial reporting under Section 404 ( b ) of the Sarbanes - Oxley Act ( 15 USC . 7262 ( b ) ) by the registered public accounting firm that prepared or issued its audit report . Yes X No As of June 30 , 2020 the aggregate market value of the common stock held by non - affiliates of the registrant , based on the closing price of a share of common stock as reported by the NASDAQ Global Select Market on such date , was approximately $ 990.1 million . Shares of the registrant's common stock held by each executive officer , director and holder of 5 % or more of the outstanding common stock have been excluded in that such persons may be deemed to be affiliates . This calculation does not reflect a determination that certain persons are affiliates of the registrant for any other purpose . The number of outstanding shares of the registrant's common stock as of February 22 , 2021 was 43,526,254 . Documents Incorporated by Reference : Portions of the Registrant's Definitive Proxy Statement relating to the 2021 Annual Meeting of Stockholders , which will be filed with the Securities and Exchange Commission within 120 days after the end of the Registrant's fiscal year ended December 31 , 2020 are incorporated by reference into Part III of this Report .