Sec Form 4 Filing - Canaan XI L.P. @ Day One Biopharmaceuticals, Inc. - 2021-12-08

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 Canaan XI L.P. 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 X __ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) 285 RIVERSIDE AVENUE, SUITE 250 3. Date of Earliest Transaction (MM/DD/YY) 12/08/2021 (Street) WESTPORT, CT06880 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) _____ Form filed by One Reporting Person __ X __ 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 12/08/2021 J( 1 ) 536,000 D 10,199,645 D( 2 )

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 Canaan XI L.P. 285 RIVERSIDE AVENUE, SUITE 250 WESTPORT, CT06880 X Canaan Partners XI LLC 285 RIVERSIDE AVENUE, SUITE 250 WESTPORT, CT06880 X Signatures Canaan XI L.P., By: Canaan Partners XI LLC, its general partner, By: /s/ Nancy Levenson, Attorney-in-Fact 12/09/2021 Signature of Reporting Person Date Canaan Partners XI LLC, By: /s/ Nancy Levenson, Attorney-in-Fact 12/09/2021 Signature of Reporting Person Date

Explanation of Responses:

( 1 )On December 8, 2021, Canaan XI L.P. (the "Canaan Fund") distributed, for no consideration, 536,000 shares of Common Stock of the Issuer (the "Shares") to its limited partners and to Canaan Partners XI LLC ("Canaan XI"), the general partner of the Canaan Fund, representing each such partner's pro rata interest in the shares held by the Canaan Fund. On the same date, Canaan XI distributed, for no consideration, the Shares it received from the distribution to its members in an amount equal to each such member's pro rata interest in the Shares. The aforementioned distributions were made in accordance with the exemptions afforded by Rule 16a-13 and Rule 16a-9 of the Securities Exchange Act of 1934, as amended. ( 2 )These shares are held directly by the Canaan Fund. Canaan XI is the sole general partner of the Canaan Fund and may be deemed to have sole voting, investment and dispositive power with respect to the shares held by the Canaan Fund. Canaan XI disclaims Section 16 beneficial ownership of the securities held by the Canaan Fund, except to the extent of its pecuniary interest therein, if any.

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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