Sec Form 4 Filing - Nicholson Garry A @ Day One Biopharmaceuticals, Inc. - 2025-06-02

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 Nicholson Garry A 2. Issuer Name and Ticker or Trading Symbol Day One Biopharmaceuticals, Inc. [ DAWN] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) X __ Director _____ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) 1800 SIERRA POINT PARKWAY, SUITE 200 3. Date of Earliest Transaction (MM/DD/YY) 06/02/2025 (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) $ 7.01 06/02/2025 A 22,500 ( 1 ) 06/01/2035 Common Stock 22,500 $ 0 22,500 D Deferred Restricted Stock Unit (RSU) $ 0 ( 2 ) 06/02/2025 A 15,000 ( 3 ) ( 4 ) Common Stock 15,000 $ 0 15,000 D

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Nicholson Garry A 1800 SIERRA POINT PARKWAY, SUITE 200 BRISBANE, CA94005 X Signatures /s/ Charles N. York II, as Attorney-in-Fact 06/04/2025 Signature of Reporting Person Date

Explanation of Responses:

( 1 )The option vests as to 1/12th of the total grant on each monthly anniversary, beginning on July 2, 2025, subject to the Reporting Person's provision of service to the Issuer on each option vesting date. ( 2 )Each restricted stock unit ("RSU") represents a contingent right to receive one share of the Issuer's Common Stock upon settlement for no consideration. ( 3 )The RSUs will vest as to 100% of the award on the earlier of (i) June 2, 2026 and (ii) the date of the Issuer's 2026 annual meeting of stockholders (in each case, the "RSU Vesting Date") , subject to the Reporting Person's provision of services to the Issuer on each vesting date. On the RSU Vesting Date, pursuant to the Reporting Person's election, the RSUs will automatically convert into an equal number of deferred stock units, which will be settled for an equal number of shares of the Issuer's Common Stock on the earlier of the calendar year 2030 or the Reporting Person's separation with the Issuer. Notwithstanding the aforementioned deferral period, from and after the RSU Vesting Date, the deferred stock units may settle earlier upon the Reporting Person's death, disability, or separation from service with the Issuer, or upon the occurrence of an "unforeseeable emergency," as that term is defined under the Issuer's deferred compensation plan. ( 4 )RSUs do not expire; they either vest or are canceled prior to the RSU 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).

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