Sec Form 4 Filing - Wilms Joris @ Y-mAbs Therapeutics, Inc. - 2023-01-17

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 Wilms Joris 2. Issuer Name and Ticker or Trading Symbol Y-mAbs Therapeutics, Inc. [ YMAB] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director _____ 10% Owner X __ Officer (give title below) _____ Other (specify below) SVP & CHIEF OPERATING OFFICER (Last) (First) (Middle) C/O Y-MABS THERAPEUTICS, INC., 230 PARK AVENUE SUITE 3350 3. Date of Earliest Transaction (MM/DD/YY) 01/17/2023 (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 01/17/2023 A 17,800( 1 ) A $ 0 17,800 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 ) $ 4.7 01/17/2023 A 35,700 ( 3 ) 01/17/2033 Common stock 35,700 $ 0 35,700 D

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Wilms Joris C/O Y-MABS THERAPEUTICS, INC. 230 PARK AVENUE SUITE 3350 NEW YORK, NY10169 SVP & CHIEF OPERATING OFFICER Signatures /s/ Sune Reinholth Nyland, Attorney-in-Fact for Joris Wilms 01/19/2023 Signature of Reporting Person Date

Explanation of Responses:

( 1 )Represents 17,800 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 January 17, 2024, January 17, 2025 and January 17, 2026 subject to the continuing employment of the Reporting Person on 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 option shall vest and become exercisable as to 25% of the shares subject to the option on January 17, 2024, and thereafter vests 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 on each vesting.

Remarks: Exhibit Index: Exhibit 24 - Power of Attorney for Joris Wilms dated December 5, 2022.

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.

* If the form is filed by more than one reporting person, see Instruction 4(b)(v).

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