Exhibit 99.1

LOCK-UP AGREEMENT

March 18, 2021

J.P. MORGAN SECURITIES LLC

COWEN AND COMPANY, LLC

PIPER SANDLER & CO.

As Representatives of

the several Underwriters listed in

Schedule 1 to the Underwriting

Agreement referred to below

c/o J.P. Morgan Securities LLC

383 Madison Avenue

New York, NY 10179

c/o Cowen and Company, LLC

599 Lexington Avenue

New York, NY 10022

c/o Piper Sandler & Co.

345 Park Avenue

New York, NY 10154

Re: Day One Biopharmaceuticals Holding Company LLC — Initial Public Offering Ladies and Gentlemen:

The undersigned, currently an owner of membership interests in Day One Biopharmaceuticals Holding Company LLC, who will become an owner of equity interests in [Day One Biopharmaceuticals], Inc., as the successor entity thereto (together with Day One Biopharmaceuticals Holding LLC, the “Company”), understands that you, as representatives (the “Representatives”), of the several Underwriters, propose to enter into an underwriting agreement (the “Underwriting Agreement”) with the Company, providing for the initial public offering (the “Public Offering”) by the several Underwriters named in Schedule 1 to the Underwriting Agreement (the “Underwriters”), of common stock (the “Common Stock”), of the Company (the “Securities”). Capitalized terms used herein and not otherwise defined shall have the meanings set forth in the Underwriting Agreement.

In consideration of the Underwriters’ agreement to purchase and make the Public Offering of the Securities, and for other good and valuable consideration receipt of which is hereby acknowledged, the undersigned hereby agrees that, without the prior written consent of J.P. Morgan Securities LLC on behalf of the several Underwriters, the undersigned will not, and will not cause any direct or indirect affiliate to, during the period beginning on the date of this letter agreement (this “Letter Agreement”) and ending at the close of business 180 days after the date of the final prospectus relating to the Public Offering (the “Prospectus”) (such period, the “Restricted Period”), (1) offer, pledge, sell, contract to sell, sell any option or contract to purchase, purchase any option or contract to sell, grant any option, right or warrant to purchase, lend, or otherwise transfer or dispose of, directly or indirectly, any shares of Common Stock or any securities convertible into or exercisable or exchangeable for Common Stock (including without limitation, Common Stock or such other securities which may be deemed to be beneficially owned by the undersigned in accordance with the rules and regulations of the Securities and Exchange Commission and securities which may be issued upon exercise of a stock option or warrant) (collectively with the Common Stock, the “Lock-Up Securities”), (2) enter into any hedging, swap or other agreement or transaction that transfers, in whole or in part, any of the economic consequences of ownership of the Lock-Up Securities, whether any such transaction described in clause (1) or (2) above is to be settled by delivery of Lock-Up Securities, in cash or otherwise, (3) make any demand for or exercise any right with respect to the registration of any Lock-Up Securities, or (4) publicly disclose the intention to do any of the foregoing. The undersigned acknowledges and agrees that the foregoing precludes the undersigned from engaging in any hedging or other transactions or arrangements (including, without limitation, any short sale or the purchase or sale of, or entry into, any put or call option, or combination thereof, forward, swap or any other derivative transaction or instrument, however described or defined) designed or intended, or which could reasonably be expected to lead to or result in, a sale or disposition or transfer (whether by the undersigned or any other person) of any economic consequences of ownership, in whole or in part, directly or indirectly, of any Lock-Up Securities, whether any such transaction or arrangement (or instrument provided for thereunder) would be settled by delivery of Lock-Up Securities, in cash or otherwise. The undersigned further confirms that it has furnished the Representatives with the details of any transaction the undersigned, or any of its affiliates, is a party to as of the date hereof, which transaction would have been restricted by this Letter Agreement if it had been entered into by the undersigned during the Restricted Period.

Notwithstanding the foregoing, the undersigned may:

(a) transfer or dispose of the undersigned’s Lock-Up Securities:

(i) as a bona fide gift or gifts, or for bona fide estate planning purposes,

(ii) by will or intestacy,

(iii) to any trust for the direct or indirect benefit of the undersigned or the immediate family of the undersigned, or if the undersigned is a trust, to a trustor or beneficiary of the trust or to the estate of a beneficiary of such trust (for purposes of this Letter Agreement, “immediate family” shall mean any relationship by blood, current or former marriage, domestic partnership or adoption, not more remote than first cousin),

(iv) to a corporation, partnership, limited liability company or other entity of which the undersigned or the immediate family of the undersigned are the legal and beneficial owner of all of the outstanding equity securities or similar interests,

(v) to a nominee or custodian of a person or entity to whom a disposition or transfer would be permissible under clauses (i) through (iv) above,

(vi) if the undersigned is a corporation, partnership, limited liability company, trust or other business entity, (A) to another corporation, partnership, limited liability company, trust or other business entity that is an affiliate (as defined in Rule 405 promulgated under the Securities Act of 1933, as amended) of the undersigned, or to any investment fund or other entity controlling, controlled by, managing or managed by or under common control with the undersigned or affiliates of the undersigned

(including, for the avoidance of doubt, where the undersigned is a partnership, to its general partner or a successor partnership or fund, or any other funds managed by such partnership), or (B) as part of a distribution to members, partners, shareholders or other equityholders of the undersigned,

(vii) by operation of law, such as pursuant to a qualified domestic order, divorce settlement, divorce decree or separation agreement, or related court order,

(viii) to the Company (A) from an employee of the Company upon death, disability or termination of employment, in each case, of such employee, or (B) pursuant to a right of first refusal that the Company has with respect to transfers of such Common Stock or other securities, provided that such right is described in the Registration Statement, the Pricing Disclosure Package and the Prospectus,

(ix) as part of a sale of the undersigned’s Lock-Up Securities acquired in the Public Offering or in open market transactions after the closing date for the Public Offering,

(x) to the Company in connection with the vesting, settlement, or exercise of restricted stock units, options, warrants or other rights to purchase shares of Common Stock (including, in each case, by way of “net” or “cashless” exercise), including for the payment of exercise price and tax and remittance payments due as a result of the vesting, settlement, or exercise of such restricted stock units, options, warrants or rights, provided that any such shares of Common Stock received upon such exercise, vesting or settlement (other than such shares as are transferred or surrendered to the Company in connection with such vesting, settlement or exercise event) shall be subject to the terms of this Letter Agreement, and provided further that any such restricted stock units, options, warrants or rights are held by the undersigned pursuant to an agreement or equity awards granted under a stock incentive plan or other equity award plan, each such agreement or plan which is described in the Registration Statement, the Pricing Disclosure Package and the Prospectus; provided no public filing, report or announcement reporting a reduction in beneficial ownership of shares of Common

Stock shall be required or shall be voluntarily made during the Restricted Period within 30 days after the date of the Prospectus, and after such 30th day, if the undersigned is required to file a report reporting a reduction in beneficial ownership of shares of Common Stock during the Restricted Period, the undersigned shall clearly indicate in the footnotes thereto that the filing relates to the circumstances described in this clause and that the shares of Common Stock received upon exercise of the stock option or warrant or restricted stock unit or other right or vesting event are subject to this agreement, and no public filing, report or announcement shall be voluntarily made, or

(xi) pursuant to a bona fide third-party tender offer, merger, consolidation or other similar transaction that is approved by the Board of Directors of the Company and made to all holders of the Company’s capital stock involving a Change of Control (as defined below) of the Company (for purposes hereof, “Change of Control” shall mean the transfer (whether by tender offer, merger, consolidation or other similar transaction), in one transaction or a series of related transactions, to a person or group of affiliated persons (other than an Underwriter pursuant to the Public Offering), of shares of capital stock if, after such transfer, such person or group of affiliated persons would hold more than 75% of the outstanding voting securities of the Company (or the surviving entity)); provided that in the event that such tender offer, merger, consolidation or other similar transaction is not completed, the undersigned’s Lock-Up Securities shall remain subject to the provisions of this Letter Agreement;

provided that (A) in the case of any transfer or distribution pursuant to clauses (a)(i), (ii), (iii), (iv) and (v), such transfer shall not involve a disposition for value, (B) in the case of any transfer or distribution pursuant to clauses (a)(i), (ii), (iii), (iv), (v), (vi) and (vii), each donee, devisee, transferee or distributee shall execute and deliver to the Representatives a lock-up letter in the form of this Letter Agreement, (C) in the case of any transfer or distribution pursuant to clause (a) (i), (ii), (iii), (iv), (v) and (vi), no filing by any party (donor, donee, devisee, transferor, transferee, distributer or distributee) under the Securities Exchange Act of 1934, as amended (the “Exchange Act”), or other public announcement reporting a reduction in beneficial ownership of shares of Common Stock shall be required or shall be made voluntarily in connection with such transfer or distribution (other than a filing on a Form 5 made after the expiration of the Restricted Period referred to above, the filing of a required Schedule 13F, 13G or 13D or, in the case of a clause (vi) only, a required filing under Section 16 of the Exchange Act, provided that any filing does not report an overall reduction in beneficial ownership of the Lock-up Securities among the transferor and transferee(s) and shall clearly indicate in the footnotes thereto that the filing relates to the circumstances described clause (vi)) and (D) in the case of any transfer or distribution pursuant to clauses (a)(vii), (viii) and (ix), it shall be a condition to such transfer that no public filing, report or announcement shall be voluntarily made and if any filing under Section 16(a) of the Exchange Act, or other public filing, report or announcement reporting a reduction in beneficial ownership of shares of Common Stock in connection with such transfer or distribution shall be legally required during the Restricted Period, such filing, report or announcement shall clearly indicate in the footnotes thereto the nature and conditions of such transfer;

(b) exercise outstanding options, settle restricted stock units or other equity awards or exercise outstanding warrants pursuant to plans described in the Registration Statement, the Pricing Disclosure Package and the Prospectus; provided that any Lockup Securities received upon such exercise, vesting or settlement shall be subject to the terms of this Letter Agreement;

(c) convert outstanding preferred stock, warrants to acquire preferred stock or convertible securities into shares of Common Stock or warrants to acquire shares of Common Stock; provided that any such shares of Common Stock or warrants received upon such conversion shall be subject to the terms of this Letter Agreement;

(d) convert or transfer membership interests in Day One Biopharmaceuticals Holding Company, LLC for equity interests in [Day One Biopharmaceuticals, Inc.] in connection with the consummation of the Public Offering and disclosed in the Prospectus, it being understood that any such shares of Common Stock or other securities of [Day One Biopharmaceuticals, Inc] received by the undersigned upon such transfer shall be subject to the restrictions on transfer set forth in this Lock-Up Agreement; and

(e) establish trading plans pursuant to Rule 10b5-1 under the Exchange Act for the transfer of shares of Lock-Up Securities; provided that (1) such plans do not provide for the transfer of Lock-Up Securities during the Restricted Period and (2) no filing by any party under the Exchange Act or other public announcement shall be required or made voluntarily in connection with such trading plan during the Restricted Period in contravention of this Lock-Up Agreement.

If the undersigned is not a natural person, the undersigned represents and warrants that no single natural person, entity or “group” (within the meaning of Section 13(d)(3) of the Exchange Act other than a natural person, entity or “group” (as described above) that has executed a Letter Agreement in substantially the same form as this Letter Agreement, beneficially owns, directly or indirectly, 50% or more of the common equity interests, or 50% or more of the voting power, in the undersigned.

If the undersigned is an officer or director of the Company, the undersigned further agrees that the foregoing provisions shall be equally applicable to any Company-directed Securities the undersigned may purchase in the Public Offering.

If the undersigned is an officer or director of the Company, (i) J.P. Morgan Securities LLC on behalf of the several Underwriters agree that, at least three business days before the effective date of any release or waiver of the foregoing restrictions in connection with a transfer of shares of Lock-Up Securities, J.P. Morgan Securities LLC on behalf of the several Underwriters will notify the Company of the impending release or waiver, and (ii) the Company has agreed in the Underwriting Agreement to announce the impending release or waiver through a major news service at least two business days before the effective date of the release or waiver. Any release or waiver granted by J.P. Morgan Securities LLC on behalf of the several Underwriters hereunder to any such officer or director shall only be effective two business days after the publication date of such announcement. The provisions of this paragraph will not apply if (a) the release or waiver is effected solely to permit a transfer not for consideration or that is to an immediate family member as defined in FINRA Rule 5130(i)(5) and (b) the transferee has agreed in writing to be bound by the same terms described in this Letter Agreement to the extent and for the duration that such terms remain in effect at the time of the transfer.

In furtherance of the foregoing, the Company, and any duly appointed transfer agent for the registration or transfer of the securities described herein, are hereby authorized to decline to make any transfer of securities if such transfer would constitute a violation or breach of this Letter Agreement.

The undersigned hereby represents and warrants that the undersigned has full power and authority to enter into this Letter Agreement. All authority herein conferred or agreed to be conferred and any obligations of the undersigned shall be binding upon the successors, assigns, heirs or personal representatives of the undersigned.

The undersigned acknowledges and agrees that the Underwriters have not provided any recommendation or investment advice nor have the Underwriters solicited any action from the undersigned with respect to the Public Offering of the Securities and the undersigned has consulted their own legal, accounting, financial, regulatory and tax advisors to the extent deemed appropriate. The undersigned further acknowledges and agrees that, although the Representatives may be required or choose to provide certain Regulation Best Interest and Form CRS disclosures to you in connection with the Public Offering, the Representatives and the other Underwriters are not making a recommendation to you to enter into this Letter Agreement and nothing set forth in such disclosures is intended to suggest that the Representatives or any Underwriter is

making such a recommendation.

The undersigned understands that, (i) if the Underwriting Agreement does not become effective by September 30, 2021 (provided, however, that the undersigned agrees that this Letter Agreement shall be automatically extended by three months if the Company provides written notice to the undersigned that the Company is still pursuing the Public Offering contemplated by the Underwriting Agreement), (ii) if the Underwriting Agreement (other than the provisions thereof which survive termination) shall terminate or be terminated prior to payment for and delivery of the Common Stock to be sold thereunder, (iii) the Company notifies the Representatives in writing prior to the execution of the Underwriting Agreement that it does not intend to proceed with the Public Offering, or (iv) prior to payment for the Securities, the registration statement filed with the Securities and Exchange Commission in connection with the Public Offering is withdrawn prior to the execution of the Underwriting Agreement, the Letter Agreement shall automatically terminate and be of no further force or effect and undersigned shall be released from all obligations under this Letter Agreement. The undersigned understands that the Underwriters are entering into the Underwriting Agreement and proceeding with the Public Offering in reliance upon this Letter Agreement.

The undersigned hereby consents to receipt of this Letter Agreement in electronic form and understands and agrees that this Letter Agreement may be signed electronically. In the event that any signature is delivered by facsimile transmission, electronic mail, or otherwise by electronic transmission evidencing an intent to sign this Letter Agreement, such facsimile transmission, electronic mail or other electronic transmission shall create a valid and binding obligation of the undersigned with the same force and effect as if such signature were an original. Execution and delivery of this Letter Agreement by facsimile transmission, electronic mail or other electronic transmission is legal, valid and binding for all purposes.

[Signature page follows]

This Letter Agreement and any claim, controversy or dispute arising under or related to this Letter Agreement shall be governed by and construed in accordance with the laws of the State of New York.

Very truly yours,
AI DAY1 LLC
By: Access Industries Management, LLC,
Its Manager
By:	/s/ Suzette Del Giudice
	Name:	Suzette Del Giudice
	Title:	Executive Vice President
By:	/s/ Alejandro Moreno
	Name:	Alejandro Moreno
	Title:	Executive Vice President

[Signature page to Lock-up Agreement]