Sec Form 4 Filing - Grant Papanek Julie @ Day One Biopharmaceuticals, Inc. - 2021-02-01

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 Papanek Julie 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) 02/01/2021 (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 06/01/2021 C 7,328,497 A 10,574,366 I By Canaan XI, L.P. ( 2 ) Common Stock 06/01/2021 C 148,279 A 10,722,645 I By Canaan XI, L.P. ( 2 ) Common Stock 06/01/2021 P 13,000 A $ 16 10,735,645 I By Canaan XI, L.P. ( 2 ) Common Stock 465,000 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 Series B Preferred Stock ( 4 ) 02/01/2021 A 148,279 ( 5 ) ( 4 ) ( 4 ) Common Stock ( 4 ) 148,279 ( 5 ) ( 4 ) 148,279 ( 5 ) I By Canaan XI, L.P. ( 2 ) Stock Option (right to buy Common Stock) ( 6 ) $ 16 05/26/2021 A 63,000 ( 7 ) 05/25/2031 Common Stock 63,000 ( 6 ) $ 0 63,000 D Series A Preferred Stock ( 1 ) 06/01/2021 C 7,328,497 ( 1 ) ( 1 ) Common Stock 7,328,497 $ 0 0 I By Canaan XI, L.P. ( 2 ) Series B Preferred Stock ( 3 ) 06/01/2021 C 148,279 ( 3 ) ( 3 ) Common Stock 148,279 $ 0 0 I By Canaan XI, L.P. ( 2 )

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Grant Papanek Julie 395 OYSTER POINT BLVD, SUITE 217 SOUTH SAN FRANCISCO, CA94080 X Signatures /s/ Julie Papanek Grant 06/03/2021 Signature of Reporting Person Date

Explanation of Responses:

( 1 )The shares of the Issuer's Series A Preferred Stock automatically converted on a one-for-one basis into shares of the Issuer's Common Stock on May 26, 2021 in connection with the closing of the Issuer's sale of its Common Stock in its firm commitment initial public offering (the "IPO") pursuant to a registration statement on Form S-1 (File No. 333-255754) under the Securities Act of 1933, as amended (the "Registration Statement"), and had no expiration date. ( 2 )Represents securities held by Canaan XI, L.P. ("Canaan LP"). The Reporting Person, as a non-managing member of Canaan LP's general partner, Canaan Partners XI LLC, disclaims beneficial ownership of such securities except to the extent of her pecuniary interest therein, if any. ( 3 )The shares of the Issuer's Series B Preferred Stock automatically converted on a one-for-one basis into shares of the Issuer's Common Stock on May 26, 2021 in connection with the closing of the Issuer's sale of its Common Stock in its IPO pursuant to the Registration Statement, and had no expiration date. ( 4 )The Reporting Person is reporting the purchase of these securities pursuant to Rule 16a-2(a) of the Securities Exchange Act of 1934, as amended (the "Exchange Act"). Each share of Series B Preferred Stock automatically converted into one share of Common Stock at the close of the Issuer's IPO and has no expiration date. ( 5 )The number of securities reported reflects the 1 to 2.325 stock split of the Issuer's Common Stock and convertible preferred stock that was effected on May 23, 2021. ( 6 )The Reporting Person is reporting the acquisition of these securities pursuant to Rule 16a-2(a) of the Exchange Act. ( 7 )The option vests as to 1/36th of the total shares on each monthly anniversary, beginning on June 23, 2021, 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).

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