DAWN Day One Biopharmaceuticals Inc

⁽¹⁾ As described in Item 4 below, Atlas Venture Fund XI, L.P., a Delaware limited partnership (“Atlas XI”), Atlas Venture Associates XI, L.P., a Delaware limited partnership (“AVA XI LP”) and Atlas Venture Associates XI, LLC, a Delaware limited liability company (“AVA XI LLC” and together with Atlas XI and AVA XI LP, the “Fund XI Reporting Persons”) beneficially own 6,008,534 shares of the Issuer’s Common Stock. All of these shares are directly held by Atlas XI. AVA XI LP is the general partner of Atlas XI and AVA XI LLC is the general partner of AVA XI LP. Each of AVA XI LP and AVA XI LLC has voting and dispositive power over the shares held by Atlas XI. As such, each of the Fund XI Reporting Persons share voting and dispositive power with respect to the shares held by Atlas XI.

⁽²⁾ This percentage is calculated based upon 73,549,526 outstanding shares of Common Stock of the Issuer as of
November 2, 2022, as reported in the Issuer’s Form 10-Q for the quarter ending September 30, 2022 and filed with the Securities and Exchange Commission on November 7, 2022.

⁽¹⁾ As described in Item 4 below, Atlas Venture Fund XI, L.P., a Delaware limited partnership (“Atlas XI”), Atlas Venture Associates XI, L.P., a Delaware limited partnership (“AVA XI LP”) and Atlas Venture Associates XI, LLC, a Delaware limited liability company (“AVA XI LLC” and together with Atlas XI and AVA XI LP, the “Fund XI Reporting Persons”) beneficially own 6,008,534 shares of the Issuer’s Common Stock. All of these shares are directly held by Atlas XI. AVA XI LP is the general partner of Atlas XI and AVA XI LLC is the general partner of AVA XI LP. Each of AVA XI LP and AVA XI LLC has voting and dispositive power over the shares held by Atlas XI. As such, each of the Fund XI Reporting Persons share voting and dispositive power with respect to the shares held by Atlas XI.

⁽²⁾ This percentage is calculated based upon 73,549,526 outstanding shares of Common Stock of the Issuer as of
November 2, 2022, as reported in the Issuer’s Form 10-Q for the quarter ending September 30, 2022 and filed with the Securities and Exchange Commission on November 7, 2022.

⁽¹⁾ As described in Item 4 below, Atlas Venture Fund XI, L.P., a Delaware limited partnership (“Atlas XI”), Atlas Venture Associates XI, L.P., a Delaware limited partnership (“AVA XI LP”) and Atlas Venture Associates XI, LLC, a Delaware limited liability company (“AVA XI LLC” and together with Atlas XI and AVA XI LP, the “Fund XI Reporting Persons”) beneficially own 6,008,534 shares of the Issuer’s Common Stock. All of these shares are directly held by Atlas XI. AVA XI LP is the general partner of Atlas XI and AVA XI LLC is the general partner of AVA XI LP. Each of AVA XI LP and AVA XI LLC has voting and dispositive power over the shares held by Atlas XI. As such, each of the Fund XI Reporting Persons share voting and dispositive power with respect to the shares held by Atlas XI.

⁽²⁾ This percentage is calculated based upon 73,549,526 outstanding shares of Common Stock of the Issuer as of
November 2, 2022, as reported in the Issuer’s Form 10-Q for the quarter ending September 30, 2022 and filed with the Securities and Exchange Commission on November 7, 2022.

⁽¹⁾ As described in Item 4 below, Atlas Venture Opportunity Fund I, L.P., a Delaware limited partnership (“AVO I”), Atlas Venture Associates Opportunity I, L.P., a Delaware limited partnership (“AVAO LP”) and Atlas Venture Associates Opportunity I, LLC, a Delaware limited liability company (“AVAO LLC” and together with AVO I and AVAO LP, the “Opportunity Fund Reporting Persons”) beneficially own 793,116 shares of the Issuer’s Common Stock. All of these shares are directly held by AVO I. AVAO LP is the general partner of AVO I and AVAO LLC is the general partner of AVAO LP. Each of AVAO LP and AVAO LLC has voting and dispositive power over the shares held by AVO I. As such, each of the Opportunity Fund Reporting Persons share voting and dispositive power with respect to the shares held by AVO I.

⁽²⁾ This percentage is calculated based upon 73,549,526 outstanding shares of Common Stock of the Issuer as of
November 2, 2022, as reported in the Issuer’s Form 10-Q for the quarter ending September 30, 2022 and filed with the Securities and Exchange Commission on November 7, 2022.

⁽¹⁾ As described in Item 4 below, Atlas Venture Opportunity Fund I, L.P., a Delaware limited partnership (“AVO I”), Atlas Venture Associates Opportunity I, L.P., a Delaware limited partnership (“AVAO LP”) and Atlas Venture Associates Opportunity I, LLC, a Delaware limited liability company (“AVAO LLC” and together with AVO I and AVAO LP, the “Opportunity Fund Reporting Persons”) beneficially own 793,116 shares of the Issuer’s Common Stock. All of these shares are directly held by AVO I. AVAO LP is the general partner of AVO I and AVAO LLC is the general partner of AVAO LP. Each of AVAO LP and AVAO LLC has voting and dispositive power over the shares held by AVO I. As such, each of the Opportunity Fund Reporting Persons share voting and dispositive power with respect to the shares held by AVO I.

⁽²⁾ This percentage is calculated based upon 73,549,526 outstanding shares of Common Stock of the Issuer as of
November 2, 2022, as reported in the Issuer’s Form 10-Q for the quarter ending September 30, 2022 and filed with the Securities and Exchange Commission on November 7, 2022.

⁽¹⁾ As described in Item 4 below, Atlas Venture Opportunity Fund I, L.P., a Delaware limited partnership (“AVO I”), Atlas Venture Associates Opportunity I, L.P., a Delaware limited partnership (“AVAO LP”) and Atlas Venture Associates Opportunity I, LLC, a Delaware limited liability company (“AVAO LLC” and together with AVO I and AVAO LP, the “Opportunity Fund Reporting Persons”) beneficially own 793,116 shares of the Issuer’s Common Stock. All of these shares are directly held by AVO I. AVAO LP is the general partner of AVO I and AVAO LLC is the general partner of AVAO LP. Each of AVAO LP and AVAO LLC has voting and dispositive power over the shares held by AVO I. As such, each of the Opportunity Fund Reporting Persons share voting and dispositive power with respect to the shares held by AVO I.

⁽²⁾ This percentage is calculated based upon 73,549,526 outstanding shares of Common Stock of the Issuer as of
November 2, 2022, as reported in the Issuer’s Form 10-Q for the quarter ending September 30, 2022 and filed with the Securities and Exchange Commission on November 7, 2022.

⁽¹⁾ As described in Item 4 below, Atlas Venture Opportunity Fund II, L.P., a Delaware limited partnership (“AVO II”), Atlas Venture Associates Opportunity II, L.P., a Delaware limited partnership (“AVAO II LP”) and Atlas Venture Associates Opportunity II, LLC, a Delaware limited liability company (“AVAO II LLC” and together with AVO II and AVAO II LP, the “Opportunity Fund II Reporting Persons”) beneficially own 766,667 shares of the Issuer’s Common Stock. All of these shares are directly held by AVO II. AVAO II LP is the general partner of AVO II and AVAO II LLC is the general partner of AVAO II LP. Each of AVAO II LP and AVAO II LLC has voting and dispositive power over the shares held by AVO II. As such, each of the Opportunity Fund II Reporting Persons share voting and dispositive power with respect to the shares held by AVO II.

⁽²⁾ This percentage is calculated based upon 73,549,526 outstanding shares of Common Stock of the Issuer as of
November 2, 2022, as reported in the Issuer’s Form 10-Q for the quarter ending September 30, 2022 and filed with the Securities and Exchange Commission on November 7, 2022.

⁽¹⁾ As described in Item 4 below, Atlas Venture Opportunity Fund II, L.P., a Delaware limited partnership (“AVO II”), Atlas Venture Associates Opportunity II, L.P., a Delaware limited partnership (“AVAO II LP”) and Atlas Venture Associates Opportunity II, LLC, a Delaware limited liability company (“AVAO II LLC” and together with AVO II and AVAO II LP, the “Opportunity Fund II Reporting Persons”) beneficially own 766,667 shares of the Issuer’s Common Stock. All of these shares are directly held by AVO II. AVAO II LP is the general partner of AVO II and AVAO II LLC is the general partner of AVAO II LP. Each of AVAO II LP and AVAO II LLC has voting and dispositive power over the shares held by AVO II. As such, each of the Opportunity Fund II Reporting Persons share voting and dispositive power with respect to the shares held by AVO II.

⁽²⁾ This percentage is calculated based upon 73,549,526 outstanding shares of Common Stock of the Issuer as of
November 2, 2022, as reported in the Issuer’s Form 10-Q for the quarter ending September 30, 2022 and filed with the Securities and Exchange Commission on November 7, 2022.

⁽¹⁾ As described in Item 4 below, Atlas Venture Opportunity Fund II, L.P., a Delaware limited partnership (“AVO II”), Atlas Venture Associates Opportunity II, L.P., a Delaware limited partnership (“AVAO II LP”) and Atlas Venture Associates Opportunity II, LLC, a Delaware limited liability company (“AVAO II LLC” and together with AVO II and AVAO II LP, the “Opportunity Fund II Reporting Persons”) beneficially own 766,667 shares of the Issuer’s Common Stock. All of these shares are directly held by AVO II. AVAO II LP is the general partner of AVO II and AVAO II LLC is the general partner of AVAO II LP. Each of AVAO II LP and AVAO II LLC has voting and dispositive power over the shares held by AVO II. As such, each of the Opportunity Fund II Reporting Persons share voting and dispositive power with respect to the shares held by AVO II.

⁽²⁾ This percentage is calculated based upon 73,549,526 outstanding shares of Common Stock of the Issuer as of
November 2, 2022, as reported in the Issuer’s Form 10-Q for the quarter ending September 30, 2022 and filed with the Securities and Exchange Commission on November 7, 2022.

Day One Biopharmaceuticals, Inc. (the “Issuer”)

2000 Sierra Point Parkway Suite 501
Brisbane, CA, 94005

This Schedule 13G is filed by (i) Atlas Venture Fund XI, L.P., a Delaware limited partnership (“Atlas XI”), (ii) Atlas Venture Associates XI, L.P., a Delaware limited partnership (“AVA XI LP”), (iii) Atlas Venture Associates XI, LLC, a Delaware limited liability company (“AVA XI LLC” and together with Atlas XI and AVA XI LP, the “Fund XI Reporting Persons”), (iv) Atlas Venture Opportunity Fund I, L.P., a Delaware limited partnership (“AVO I”), (v) Atlas Venture Associates Opportunity I, L.P., a Delaware limited partnership (“AVAO LP”), (vi) Atlas Venture Associates Opportunity I, LLC, a Delaware limited liability company (“AVAO LLC” and together with AVO I and AVAO LP, the “Opportunity Fund Reporting Persons”), (vii) Atlas Venture Opportunity Fund II, L.P., a Delaware limited partnership (“AVO II”), (viii) Atlas Venture Associates Opportunity II, L.P., a Delaware limited partnership (“AVAO II LP”) and (ix) Atlas Venture Associates Opportunity II, LLC, a Delaware limited liability company (“AVAO II LLC” and together with AVO II and AVAO II LP, the “Opportunity Fund II Reporting Persons” and together with the Fund XI Reporting Persons and Opportunity Fund Reporting Persons, the "Reporting Persons”).

300 Technology Square, 8th Floor
Cambridge, Massachusetts 02139

Each of Atlas XI, AVA XI LP, AVO I, AVAO LP, AVO II and AVAO II LP is a Delaware limited partnership. Each of AVA XI LLC, AVAO LLC and AVAO II LLC is a Delaware limited liability company.

Common Stock, $0.0001 par value per share

23954D 109

Item 3

Not applicable.

Provide the following information regarding the aggregate number and percentage of the class of securities of the issuer identified in Item 1

(a) Amount beneficially owned: Atlas XI is the record owner of 6,008,534 shares of Common Stock. AVA XI LP is the general partner of Atlas XI and AVA XI LLC is the general partner of AVA XI LP. Each of Atlas XI, AVA XI LP and AVA XI LLC has shared voting and dispositive power over the shares held by Atlas XI. As such, each of Atlas XI, AVA XI LP and AVA XI LLC may be deemed to beneficially own the shares held by Atlas XI.

Amount beneficially owned: AVO I is the record owner of 793,116 shares of Common Stock. AVAO LP is the general partner of AVO I and AVAO LLC is the general partner of AVAO LP. Each of AVO I, AVAO LP and AVAO LLC has shared voting and dispositive power over the shares held by AVO I. As such, each of AVO I, AVAO LP and AVAO LLC may be deemed to beneficially own the shares held by AVO I.

Amount beneficially owned: AVO II is the record owner of 766,667 shares of Common Stock. AVAO II LP is the general partner of AVO II and AVAO II LLC is the general partner of AVAO II LP. Each of AVO II, AVAO II LP and AVAO II LLC has shared voting and dispositive power over the shares held by AVO II. As such, each of AVO II, AVAO II LP and AVAO II LLC may be deemed to beneficially own the shares held by AVO II.

(b) Percent of class: Fund XI Reporting Persons, Opportunity Fund Reporting Persons and Opportunity Fund II Reporting Persons may be deemed to beneficially own 8.2%, 1.1%, and 1.0%, respectively, of the Issuer’s outstanding Common Stock, which percentages are calculated based upon 73,549,526 outstanding shares of Common Stock of the Issuer as of November 2, 2022, as reported in the Issuer’s Form 10-Q for the quarter ending September 30, 2022 and filed with the Securities and Exchange Commission on November 7, 2022.

Collectively, the Reporting Persons beneficially own an aggregate of 7,568,317 shares of Common Stock, which represents 10.3% of the Issuer's outstanding Common Stock. The Fund XI Reporting Persons, the Opportunity Fund Reporting Persons and the Opportunity Fund II Reporting Persons are under common control and as a result, the Reporting Persons may be deemed to be members of a group. However, the Reporting Persons disclaim such group membership, and this Schedule 13G shall not be deemed an admission that the Reporting Persons are members of a group for purposes of Section 13 or for any other purposes.

(c) Number of shares as to which the person has:

(i) Sole power to vote or to direct the vote: None of the Reporting Persons have the sole power to direct the vote of the Common Stock.

 (ii) Shared power to vote or to direct the vote: Each Fund XI Reporting Person shares power to vote or direct the vote of 6,008,534 shares of Common Stock, each Opportunity Fund Reporting Person shares power to vote or direct the vote of 793,116 shares of Common Stock and each Opportunity Fund II Reporting Person shares power to vote or direct the vote of 766,667 shares of Common Stock.

 (iii) Sole power to dispose or to direct the disposition of: None of the Reporting Persons have the sole power to dispose or to direct the disposition of the Common Stock.

(iv) Shared power to dispose or to direct the disposition of: Each Fund XI Reporting Person shares power to dispose or to direct the disposition of 6,008,534 shares of Common Stock, each Opportunity Fund Reporting Person shares power to dispose or to direct the disposition of 793,116 shares of Common Stock and each Opportunity Fund II Reporting Person shares power to dispose or to direct the disposition of 766,667 shares of Common Stock.

If this statement is being filed to report the fact that as of the date hereof the reporting person has ceased to be the beneficial owner of more than five percent of the class of securities, check the following. ☐

Not applicable.

Not applicable.

Not applicable.

Not applicable.

By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were acquired and are held in the ordinary course of business and were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect, other than activities solely in connection with a nomination under § 240.14a-11.

SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

Dated: February 14, 2023

EXHIBITS