Sec Form 4 Filing - Dubow Adam @ Day One Biopharmaceuticals, Inc. - 2025-11-06

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 Dubow Adam 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) Gen Counsel & Secretary (Last) (First) (Middle) 1800 SIERRA POINT PARKWAY, SUITE 200 3. Date of Earliest Transaction (MM/DD/YY) 11/06/2025 (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

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) $ 8.99 11/06/2025 A 309,000 ( 1 ) 10/30/2032 Common Stock 309,000 $ 0 309,000 D Stock Option (right to buy Common Stock) $ 23.41 11/06/2025 D( 2 )( 3 ) 90,000 ( 4 ) 01/16/2033 Common Stock 90,000 $ 0 0 D Stock Option (right to buy Common Stock) $ 8.99 11/06/2025 A( 2 )( 3 ) 90,000 ( 4 ) 01/16/2033 Common Stock 90,000 $ 0 90,000 D Stock Option (right to buy Common Stock) $ 14.44 11/06/2025 D( 2 )( 3 ) 90,000 ( 5 ) 01/04/2034 Common Stock 90,000 $ 0 0 D Stock Option (right to buy Common Stock) $ 8.99 11/06/2025 A( 2 )( 3 ) 90,000 ( 5 ) 01/04/2034 Common Stock 90,000 $ 0 90,000 D Stock Option (right to buy Common Stock) $ 11.87 11/06/2025 D( 2 )( 3 ) 90,000 ( 6 ) 01/14/2035 Common Stock 90,000 $ 0 0 D Stock Option (right to buy Common Stock) $ 8.99 11/06/2025 A( 2 )( 3 ) 90,000 ( 6 ) 01/14/2035 Common Stock 90,000 $ 0 90,000 D

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Dubow Adam 1800 SIERRA POINT PARKWAY, SUITE 200 BRISBANE, CA94005 Gen Counsel & Secretary Signatures /s/ Charles N. York II, as Attorney-in-Fact 11/07/2025 Signature of Reporting Person Date

Explanation of Responses:

( 1 )The option vests as to 100% of the total shares on November 6, 2026, subject to the Reporting Person's provision of service to the Issuer on such vesting date. ( 2 )On October 7, 2025, the Issuer's Board of Directors approved an option repricing (the "Repricing") whereby the Reporting Person's options were repriced on November 6, 2025 (the "Effective Date") with a new exercise price of $8.99 (if lower than the original exercise price), the closing price on Nasdaq as of the Effective Date. In order to exercise the repriced options at the new exercise price, the Reporting Person is required to remain in service with the Issuer through the Retention Period; provided that the additional premium payment will not be required if the Reporting Person's service to the Issuer is terminated by reason of death or Disability (as defined in the Company's 2021 Equity Incentive Plan ("the Plan")). ( 3 )(Continued from Footnote 2) The "Retention Period" commenced on the Effective Date and ends upon the earliest of (i) the 12-month anniversary of the Effective Date and (ii) a Corporate Transaction (as defined in the Plan). All of the other terms of the options remain unchanged. Such transactions were exempt pursuant to Rule 16b-6(d) and Rule 16b-3 of the Exchange Act, as applicable. ( 4 )The option vests as to 1/48th of the total shares monthly, commencing February 17, 2023, subject to the Reporting Person's provision of service to the Issuer on each vesting date. ( 5 )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. ( 6 )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.

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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