Exhibit 99.1

JOINT FILING AGREEMENT

In accordance with Rule 13d-1(k) under the Securities Exchange Act of 1934, as amended, the undersigned agree to the joint filing on behalf of each of them of a statement on Schedule 13G (including amendments thereto) with respect to the Common Stock of Day One Biopharmaceuticals, Inc. and further agree that this agreement be included as an exhibit to such filing. Each party to the agreement expressly authorizes each other party to file on its behalf any and all amendments to such statement. Each party to this agreement agrees that this joint filing agreement may be signed in counterparts.

Date: May 15, 2026


	Atlas Venture Fund XI, L.P.

	By:	Atlas Venture Associates XI, L.P., its general partner
	By:	Atlas Venture Associates XI, LLC, its general partner

	By: /s/ Ommer Chohan
	Name:	Ommer Chohan
	Title:	Chief Financial Officer

	Atlas Venture Associates XI, L.P.

	By:	Atlas Venture Associates XI, LLC, its general partner

	By: /s/ Ommer Chohan
	Name:	Ommer Chohan
	Title:	Chief Financial Officer

	Atlas Venture Associates XI, LLC

	By: /s/ Ommer Chohan
	Name:	Ommer Chohan
	Title:	Chief Financial Officer

	Atlas Venture Opportunity Fund I, L.P.

	By:	Atlas Venture Associates Opportunity I, L.P., its general partner
	By:	Atlas Venture Associates Opportunity I, LLC, its general partner

	By: /s/ Ommer Chohan
	Name:	Ommer Chohan
	Title:	Chief Financial Officer

	Atlas Venture Associates Opportunity I, L.P.

	By:	Atlas Venture Associates Opportunity I, LLC, its general partner

	By: /s/ Ommer Chohan
	Name:	Ommer Chohan
	Title:	Chief Financial Officer

	Atlas Venture Associates Opportunity I, LLC

	By: /s/ Ommer Chohan
	Name:	Ommer Chohan
	Title:	Chief Financial Officer



	Atlas Venture Opportunity Fund II, L.P.

	By:	Atlas Venture Associates Opportunity II, L.P., its general partner
	By:	Atlas Venture Associates Opportunity II, LLC, its general partner

	By: /s/ Ommer Chohan
	Name:	Ommer Chohan
	Title:	Chief Financial Officer

	Atlas Venture Associates Opportunity II, L.P.

	By:	Atlas Venture Associates Opportunity II, LLC, its general partner

	By: /s/ Ommer Chohan
	Name:	Ommer Chohan
	Title:	Chief Financial Officer

	Atlas Venture Associates Opportunity II, LLC

	By: /s/ Ommer Chohan
	Name:	Ommer Chohan
	Title:	Chief Financial Officer

Day One Biopharmaceuticals, Inc. SCHEDULE 13G/A