SEC Form 4

FORM 4		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 or Section 30(h) of the Investment Company Act of 1940	OMB APPROVAL OMB Number: 3235-0287 Estimated average burden hours per response: 0.5
	Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

1. Name and Address of Reporting Person* Atlas Venture Fund XI, L.P. (Last) (First) (Middle) 300 TECHNOLOGY SQUARE, 8TH FLOOR (Street) CAMBRIDGE MA 02139 (City) (State) (Zip)	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)
	3. Date of Earliest Transaction (Month/Day/Year) 02/01/2021
	4. If Amendment, Date of Original Filed (Month/Day/Year)	6. Individual or Joint/Group Filing (Check Applicable Line) Form filed by One Reporting Person X Form filed by More than One Reporting Person

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned
1. Title of Security (Instr. 3)	2. Transaction Date (Month/Day/Year)	2A. Deemed Execution Date, if any (Month/Day/Year)	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		7,761,380	A	(1)	7,761,380	I	See Footnote(2)
Common Stock	06/01/2021		C		741,396	A	(3)	741,396	I	See Footnote(4)
Common Stock	06/01/2021		P		250,000(5)	A	$16	250,000	I	See Footnote(2)
Common Stock	06/01/2021		P		250,000(6)	A	$16	250,000	I	See Footnote(4)

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 (Month/Day/Year)	3A. Deemed Execution Date, if any (Month/Day/Year)	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 (Month/Day/Year)		7. Title and Amount of Securities Underlying Derivative Security (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: 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	(7)	02/01/2021		P		741,396		(7)	(7)	Common Stock	741,396	$0.00	741,396	I	See Footnote(4)
Series A Preferred Stock	(1)	06/01/2021		C			7,761,380	(1)	(1)	Common Stock	7,761,380	$0.00	0	I	See Footnote(2)
Series B Preferred Stock	(3)	06/01/2021		C			741,396	(3)	(3)	Common Stock	741,396	$0.00	0	I	See Footnote(4)

1. Name and Address of Reporting Person* Atlas Venture Fund XI, L.P. (Last) (First) (Middle) 300 TECHNOLOGY SQUARE, 8TH FLOOR (Street) CAMBRIDGE MA 02139 (City) (State) (Zip)

1. Name and Address of Reporting Person* Atlas Venture Associates Opportunity I, LLC (Last) (First) (Middle) 300 TECHNOLOGY SQUARE, 8TH FLOOR (Street) CAMBRIDGE MA 02139 (City) (State) (Zip)

1. Name and Address of Reporting Person* Atlas Venture Opportunity Fund I, L.P. (Last) (First) (Middle) 300 TECHNOLOGY SQUARE, 8TH FLOOR (Street) CAMBRIDGE MA 02139 (City) (State) (Zip)

1. Name and Address of Reporting Person* Atlas Venture Associates Opportunity I, L.P. (Last) (First) (Middle) 300 TECHNOLOGY SQUARE, 8TH FLOOR (Street) CAMBRIDGE MA 02139 (City) (State) (Zip)

1. Name and Address of Reporting Person* Atlas Venture Associates XI, L.P. (Last) (First) (Middle) 300 TECHNOLOGY SQUARE, 8TH FLOOR (Street) CAMBRIDGE MA 02139 (City) (State) (Zip)

1. Name and Address of Reporting Person* Atlas Venture Associates XI, LLC (Last) (First) (Middle) 300 TECHNOLOGY SQUARE, 8TH FLOOR (Street) CAMBRIDGE MA 02139 (City) (State) (Zip)

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. The shares are held directly by Atlas Venture Fund XI, L.P. ("Fund XI"). The general partner of Fund XI is Atlas Venture Associates XI, L.P. ("AVA XI LP"). Atlas Venture Associates XI, LLC ("AVA XI LLC") is the general partner of AVA XI LP. Each of AVA XI LP and AVA XI LLC disclaims Section 16 beneficial ownership of the securities held by Fund XI, except to the extent of its 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 shares are held directly by Atlas Venture Opportunity Fund I, L.P. ("Atlas Venture Opportunity Fund I"). The general partner of Atlas Venture Opportunity Fund I is Atlas Venture Associates Opportunity I, L.P. ("AVAO I LP"). Atlas Venture Associates Opportunity I, LLC ("AVAO I LLC") is the general partner of AVAO I LP. Each of AVAO I LP and AVAO I LLC disclaims Section 16 beneficial ownership of the securities held by Atlas Venture Opportunity Fund I, except to the extent of its pecuniary interest therein, if any.

5. On June 1, 2021, Fund XI purchased 250,000 shares of common stock of the Issuer at a purchase price of $16.00 per share pursuant to an underwritten public offering.

6. On June 1, 2021, Atlas Venture Opportunity Fund I purchased 250,000 shares of common stock of the Issuer at a purchase price of $16.00 per share pursuant to an underwritten public offering.

7. 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. 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.

Remarks:

	/s/ Ommer Chohan	06/02/2021
	** Signature of Reporting Person	Date
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