Sec Form 3 Filing - Merendino Lauren @ Day One Biopharmaceuticals, Inc. - 2025-02-26

FORM 3 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 Merendino Lauren 2. Issuer Name and Ticker or Trading Symbol Day One Biopharmaceuticals, Inc. [ DAWN] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director _____ 10% Owner X __ Officer (give title below) _____ Other (specify below) Chief Commercial Officer (Last) (First) (Middle) 1800 SIERRA POINT PARKWAY, SUITE 200 3. Date of Earliest Transaction (MM/DD/YY) 02/26/2025 (Street) BRISBANE, CA94005 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 28,552 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 Stock Option (right to buy Common Stock) $ 12.54 ( 1 ) 06/11/2033 Common Stock 330,200 D Stock Option (right to buy Common Stock) $ 14.44 ( 2 ) 01/04/2034 Common Stock 90,000 D Stock Option (right to buy Common Stock) $ 11.87 ( 3 ) 01/14/2035 Common Stock 90,000 D Restricted Stock Units ( 6 ) ( 4 ) ( 5 ) Common Stock 31,626 D Restricted Stock Units ( 6 ) ( 7 ) ( 5 ) Common Stock 40,565 D Restricted Stock Units ( 6 ) ( 8 ) ( 5 ) Common Stock 55,313 D

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Merendino Lauren 1800 SIERRA POINT PARKWAY, SUITE 200 BRISBANE, CA94005 Chief Commercial Officer Signatures /s/ Charles N. York II, as Attorney-in-Fact 02/28/2025 Signature of Reporting Person Date

Explanation of Responses:

( 1 )The option vests as to 25% of the total shares on June 12, 2024 and then 2.0833% of the total shares vest monthly thereafter, with 100% of the total shares vested on June 12, 2027, subject to the Reporting Person's provision of service to the Issuer on each vesting date. ( 2 )The option vests as to 1/48th of the total shares monthly, commencing February 5, 2024, subject to the Reporting Person's provision of service to the Issuer on each vesting date. ( 3 )The option vests as to 1/48th of the total shares monthly, commencing February 15, 2025, subject to the Reporting Person's provision of service to the Issuer on each vesting date. ( 4 )The original award of 50,600 restricted stock units ("RSUs") was granted on June 12, 2023, of which 18,974 RSUs have previously vested and settled. The RSUs vested as to 1/4th of the total award on August 15, 2024, and the remaining 3/4 of the award vests in 12 substantially equal quarterly installments thereafter on November 15, February 15, May 15 and August 15, subject to the Reporting Person's provision of service to the Issuer on each vesting date. ( 5 )RSUs do not expire; they either vest or are canceled prior to the vesting date. ( 6 )Each RSU represents a contingent right to receive one share of the Issuer's Common Stock upon settlement. ( 7 )The original award of 59,000 RSUs was granted on January 5, 2024, of which 18,435 RSUs have previously vested and settled. The RSUs vest as to 1/16th of the total award in quarterly installments on February 15, May 15, August 15 and November 15, subject to the Reporting Person's provision of service to the Issuer on each vesting date. ( 8 )The original award of 59,000 RSUs was granted on January 15, 2025, of which 3,687 RSUs have previously vested and settled. The RSUs vest as to 1/16th of the total award in quarterly installments on February 15, May 15, August 15 and November 15, subject to the Reporting Person's provision of service to the Issuer on each vesting date.

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).

** Intentional misstatements or omissions of facts constitute Federal Criminal Violations. See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).

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