Sec Form 4 Filing - Gad Thomas @ Y-mAbs Therapeutics, Inc. - 2025-09-16

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 Gad Thomas 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) CHIEF BUSINESS OFFICER (Last) (First) (Middle) C/O Y-MABS THERAPEUTICS, INC., 202 CARNEGIE CENTER, SUITE 301 3. Date of Earliest Transaction (MM/DD/YY) 09/16/2025 (Street) PRINCETON, NJ08540 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 09/16/2025( 1 ) U 245,486 D $ 8.6 ( 2 ) 0 D Common Stock 09/16/2025( 1 ) D 60,235 ( 3 ) D 0 D Common Stock 09/16/2025( 1 ) U 67,681 D $ 8.6 ( 2 ) 0 I By GAD Enterprises LLC ( 5 ) Common Stock 09/16/2025( 1 ) U 60,000 D $ 8.6 ( 2 ) 0 I By Children ( 6 )

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) $ 4.38 09/16/2025( 1 ) D 143,169 ( 7 ) 10/21/2026 Common Stock 143,169 ( 7 ) 0 D Employee Stock Option (right to buy) $ 8.5 09/16/2025( 1 ) D 16,000 ( 7 ) 12/14/2026 Common Stock 16,000 ( 7 ) 0 D Employee Stock Option (right to buy) $ 11.16 09/16/2025( 1 ) D 100,000 ( 7 ) 04/24/2028 Common Stock 100,000 ( 7 ) 0 D Employee Stock Option (right to buy) $ 21.97 09/16/2025( 1 ) D 107,500 ( 7 ) 12/11/2028 Common Stock 107,500 ( 7 ) 0 D Employee Stock Option (right to buy) $ 30.34 09/16/2025( 1 ) D 130,000 ( 7 ) 01/07/2030 Common Stock 130,000 ( 7 ) 0 D Employee Stock Option (right to buy) $ 53.88 09/16/2025( 1 ) D 91,450 ( 7 ) 12/15/2030 Common Stock 91,450 ( 7 ) 0 D Employee Stock Option (right to buy) $ 15.73 09/16/2025( 1 ) D 110,000 ( 7 ) 12/14/2031 Common Stock 110,000 ( 7 ) 0 D Employee Stock Option (right to buy) $ 9.52 09/16/2025( 1 ) D 60,000 ( 7 ) 05/18/2032 Common Stock 60,000 ( 7 ) 0 D Employee Stock Option (right to buy) $ 4.7 09/16/2025( 1 ) D 242,200 ( 7 ) 01/17/2033 Common Stock 242,200 ( 7 ) 0 D Employee Stock Option (right to buy) $ 5.75 09/16/2025( 1 ) D 75,000 ( 7 ) 11/06/2033 Common Stock 75,000 ( 7 ) 0 D Employee Stock Option (right to buy) $ 9.83 09/16/2025( 1 ) D 132,500 ( 7 ) 01/18/2034 Common Stock 132,500 ( 7 ) 0 D Employee Stock Option (right to buy) $ 6.16 09/16/2025( 1 ) D 64,000 ( 7 ) 01/17/2035 Common Stock 64,000 ( 7 ) 0 D Restricted Stock Units ( 8 ) ( 9 ) 09/16/2025( 1 ) D 16,000 ( 9 ) ( 9 ) Common Stock 16,000 ( 9 ) 0 D Restricted Stock Units ( 8 ) ( 9 ) 09/16/2025( 1 ) D 16,000 ( 9 ) ( 9 ) Common Stock 16,000 ( 9 ) 0 D

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Gad Thomas C/O Y-MABS THERAPEUTICS, INC. 202 CARNEGIE CENTER, SUITE 301 PRINCETON, NJ08540 X CHIEF BUSINESS OFFICER Signatures /s/ John LaRocca, Attorney-in-Fact 09/18/2025 Signature of Reporting Person Date

Explanation of Responses:

( 1 )Pursuant to the Agreement and Plan of Merger (the "Merger Agreement"), dated as of August 4, 2025, by and among Y-mAbs Therapeutics, Inc. (the "Issuer"), Yosemite Merger Sub, Inc. ("Purchaser"), Perseus BidCo US, Inc., a Delaware corporation ("Parent"), and Stark International Lux, a Luxembourg private limited liability company ("Ultimate Parent"), on September 16, 2025, Purchaser completed a tender offer for shares of common stock of the Issuer (each, a "Share") and thereafter merged with and into the Issuer, with the Issuer continuing as the surviving corporation and an indirect wholly owned subsidiary of Parent (the "Merger"). ( 2 )Pursuant to the terms of the Merger Agreement, the Shares were tendered at the Offer Acceptance Time (as defined in the Merger Agreement) in exchange for the right to receive $8.60 per Share, in cash, without interest, subject to any applicable withholding of taxes (the "Merger Consideration"). At the effective time of the Merger (being such date and at such time as the certificate of merger in respect of the Merger was duly filed with the Secretary of State of the State of Delaware in accordance with the DGCL) (the "Effective Time"), each issued and outstanding Share was cancelled in exchange for the Merger Consideration. ( 3 )Represents restricted stock units ("RSUs") granted to the Reporting Person pursuant to the Issuer's 2018 Equity Incentive Plan. Each RSU represents a contingent right to receive one share of Common Stock on the vesting dates of the RSU. ( 4 )At the Effective Time, pursuant to the Merger Agreement, each RSU award outstanding as of immediately prior to the Effective Time, whether vested or unvested, was cancelled and converted into the right to receive an amount in cash, without interest, equal to the product of (a) the total number of Shares issuable in settlement of such RSU award immediately prior to the Effective Time, multiplied by (b) the Merger Consideration. ( 5 )Reporting Person is sole member of GAD Enterprises, LLC and the sole member and manager of GAD Management, LLC, the manager of GAD Enterprises, LLC, and as such Reporting Person has sole voting and dispositive power with respect to such shares. ( 6 )Represents shares owned by Reporting Person's children who are deemed to share Reporting Person's household. ( 7 )At the Effective Time, pursuant to the Merger Agreement, each stock option ("Option") outstanding as of immediately prior to the Effective Time, whether vested or unvested, was cancelled and converted into the right to receive cash, without interest, equal to the product of (a) the total number of Shares subject to such Option immediately prior to the Effective Time, multiplied by (b) the excess of the Merger Consideration over the exercise price payable per Share under such Option. Any Options with an exercise price equal to or in excess of the Merger Consideration were cancelled for no consideration. ( 8 )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. ( 9 )At the Effective Time, pursuant to the Merger Agreement, each PRSU award outstanding as of immediately prior to the Effective Time, whether vested or unvested, was cancelled and converted into the right to receive an amount in cash, without interest, equal to the product of (a) the maximum number of Shares issuable in settlement of such PRSU award immediately prior to the Effective Time, multiplied by (b) the Merger Consideration.

Remarks: The foregoing descriptions in the footnotes to this Form 4 are qualified in their entirety by reference to the terms of the Merger Agreement. In the event of any conflict between the descriptions above and the terms set forth in the Merger Agreement, the terms set forth in the Merger Agreement shall control.

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

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