Sec Form 4 Filing - Grant Julie Papanek @ Day One Biopharmaceuticals, Inc. - 2022-09-13

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 Grant Julie Papanek 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) 395 OYSTER POINT BLVD, SUITE 217 3. Date of Earliest Transaction (MM/DD/YY) 09/13/2022 (Street) SOUTH SAN FRANCISCO, CA94080 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 09/13/2022 S 86,644 D $ 24.39( 1 ) 9,577,001 I See Footnote( 2 ) Common Stock 09/13/2022 S 1,003 D $ 25.01( 3 ) 9,575,998 I See Footnote( 2 ) Common Stock 09/14/2022 S 105,727 D $ 23.68( 4 ) 9,470,271 I See Footnote( 2 ) Common Stock 09/14/2022 S 384 D $ 24.29( 5 ) 9,469,887 I See Footnote( 2 ) Common Stock 09/15/2022 S 47,967 D $ 22.79( 6 ) 9,421,920 I See Footnote( 2 ) Common Stock 09/15/2022 S 3,314 D $ 23.73( 7 ) 9,418,606 I See Footnote( 2 ) Common Stock 439,358 I See Footnote( 8 ) Common Stock 362 I See Footnote( 9 )

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

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Grant Julie Papanek 395 OYSTER POINT BLVD, SUITE 217 SOUTH SAN FRANCISCO, CA94080 X Signatures /s/ Nancy Levenson, Attorney-in-Fact 09/15/2022 Signature of Reporting Person Date

Explanation of Responses:

( 1 )The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $24.00 - $24.99, inclusive. The Reporting Person undertakes to provide to the issuer, any security holder of the issuer, or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote. ( 2 )Represents shares held by Canaan XI L.P. The Reporting Person is a non-managing member of Canaan Partners XI LLC, the general partner of Canaan XI L.P. The Reporting Person does not have voting or dispositive power over any of the shares held directly by Canaan XI L.P. and disclaims beneficial ownership of the shares except to the extent of her pecuniary interest therein, if any, and this report shall not be deemed an admission that the Reporting Person is the beneficial owner of the shares for purposes of Section 16 of the Securities Exchange Act of 1934, as amended (the "Act"), or for any other purpose. ( 3 )The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $25.00 - $25.06, inclusive. The Reporting Person undertakes to provide to the issuer, any security holder of the issuer, or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote. ( 4 )The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $23.25 - $24.23, inclusive. The Reporting Person undertakes to provide to the issuer, any security holder of the issuer, or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote. ( 5 )The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $24.28 - $24.29, inclusive. The Reporting Person undertakes to provide to the issuer, any security holder of the issuer, or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote. ( 6 )The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $22.54 - $23.52, inclusive. The Reporting Person undertakes to provide to the issuer, any security holder of the issuer, or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote. ( 7 )The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $23.55 - $24.01, inclusive. The Reporting Person undertakes to provide to the issuer, any security holder of the issuer, or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote. ( 8 )Represents shares held by The Grant Family Delaware Trust Agreement of which the Reporting Person is trustee. The Reporting Person disclaims beneficial ownership of the shares except to the extent of her pecuniary interest therein, if any, and this report shall not be deemed an admission that the Reporting Person is the beneficial owner of the shares for purposes of Section 16 of the Act, or for any other purpose. ( 9 )Represents shares held by The Adam and Julie Grant Revocable Trust dtd October 13, 2017 of which the Reporting Person and her spouse are co-trustees. The Reporting Person disclaims beneficial ownership of the shares except to the extent of her pecuniary interest therein, if any, and this report shall not be deemed an admission that the Reporting Person is the beneficial owner of the shares for purposes of Section 16 of the Act, or for any other purpose.

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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