Sec Form 4 Filing - Rossi Michael J @ Y-mAbs Therapeutics, Inc. - 2024-02-12

FORM 4 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person Rossi Michael J 2. Issuer Name and Ticker or Trading Symbol Y-mAbs Therapeutics, Inc. [ YMAB] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) X __ Director _____ 10% Owner X __ Officer (give title below) _____ Other (specify below) PRESIDENT & CEO (Last) (First) (Middle) C/O Y-MABS THERAPEUTICS, INC., 230 PARK AVENUE, SUITE 3350 3. Date of Earliest Transaction (MM/DD/YY) 02/12/2024 (Street) NEW YORK, NY10169 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) __ X __ Form filed by One Reporting Person _____ Form filed by More than One Reporting Person (City) (State) (Zip)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned 1.Title of Security (Instr. 3) 2. Transaction Date (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price Common Stock 02/12/2024 A 35,700 ( 1 ) A $ 0 35,700 D

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities) 1. Title of Derivative Security (Instr. 3) 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 4. Transaction Code (Instr. 8) 5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5) 6. Date Exercisable and Expiration Date (MM/DD/YY) 7. Title and Amount of Underlying Securities (Instr. 3 and 4) 8. Price of Derivative Security (Instr. 5) 9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4) 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4) 11. Nature of Indirect Beneficial Ownership (Instr. 4) Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares Employee Stock Option (right to buy) ( 2 ) $ 14.36 02/12/2024 A 214,200 ( 3 ) 02/12/2034 Com mon Stock 214,200 $ 0 214,200 D Restricted Stock Units ( 4 ) ( 5 ) 02/12/2024 A 54,000 ( 5 ) ( 5 ) Common Stock 54,000 $ 0 54,000 D

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Rossi Michael J C/O Y-MABS THERAPEUTICS, INC. 230 PARK AVENUE, SUITE 3350 NEW YORK, NY10169 X PRESIDENT & CEO Signatures /s/ Michael J. Rossi 02/14/2024 Signature of Reporting Person Date

Explanation of Responses:

( 1 )Represents 35,750 restricted stock units ("RSUs") granted to the Reporting Person pursuant to the 2018 Equity Incentive Plan of Y-mAbs Therapeutics, Inc. Each RSU represents a contingent right to receive one share of Common Stock on the vesting dates of the RSU. The RSUs vest by 1/3 on each of February 12, 2025, February 12, 2026 and February 12, 2027 subject to the continuing employment of the Reporting Person through each vesting date. ( 2 )Stock options granted to the Reporting Person pursuant to the 2018 Equity Incentive Plan of Y-mAbs Therapeutics, Inc. ( 3 )The stock options shall vest and become exercisable as to 25% of the shares subject to the option on February 12, 2025, and thereafter vest and become exercisable as to 1/48th of the shares in equal monthly installments, until such time as the option is 100% vested, subject to the continuing employment of the Reporting Person through each vesting date. ( 4 )Performance-Based RSUs ("PRSUs") granted to the Reporting Person pursuant to the 2018 Equity Incentive Plan of Y-mAbs Therapeutics, Inc. Each PRSU represents a contingent right to receive one share of Common Stock on the vesting dates of the PRSU. ( 5 )1/3 of the PRSUs vest on each of February 12, 2025, February 12, 2026 and February 12, 2027, in each case, if the average closing price per share of the Issuer's common stock over the preceding 30 trading day period is greater or equal to $10.00 per share, subject to the continuing employment of the Reporting Person through each vesting date.

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