Sec Form 3 Filing - Dubow Adam @ Day One Biopharmaceuticals, Inc. - 2023-11-06

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 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) General Counsel (Last) (First) (Middle) 2000 SIERRA POINT PARKWAY, SUITE 501 3. Date of Earliest Transaction (MM/DD/YY) 11/06/2023 (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 3,141 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) $ 21.14 ( 1 ) 10/30/2032 Common Stock 309,000 D Stock Option (Right to Buy) $ 23.41 ( 2 ) 01/16/2033 Common Stock 90,000 D Restricted Stock Units ( 4 ) ( 3 ) ( 3 ) Common Stock 47,400 D Restricted Stock Units ( 4 ) ( 5 ) ( 5 ) Common Stock 22,750 D

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Dubow Adam 2000 SIERRA POINT PARKWAY, SUITE 501 BRISBANE, CA94005 General Counsel Signatures /s/ Charles N. York II, Attorney-in-Fact 11/16/2023 Signature of Reporting Person Date

Explanation of Responses:

( 1 )The option vests as to 25% of the total shares on October 31, 2023, and then 1/48th of the total shares vest monthly thereafter, 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 17, 2023, subject to the Reporting Person's provision of service to the Issuer on each vesting date. ( 3 )The Restricted Stock Units ("RSUs") vest as to 25% of the total award on November 15, 2023, and 1/12th of the remaining amount vests 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. ( 4 )Each RSU represents a contingent right to receive one share of the Issuer's Common Stock upon settlement for noconsideration. ( 5 )The RSUs will 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.

Remarks: Exhibit 24 - Power of Attorney

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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