Day One Biopharmaceuticals, Inc. (the “Company,” “we,” “us” or “our”) is committed to promoting high standards of honest and ethical business conduct and compliance with laws, rules and regulations. Because stock is an important part of the Company’s compensation program, our Board of Directors (“Board”) has adopted this Insider Trading Policy (“Policy”) governing the purchase, sale and other dispositions of the Company’s securities by the individuals and entities covered by this Policy to promote compliance with insider trading laws, rules and regulations, as well as applicable stock exchange listing standards.

Insider trading happens when someone who is in possession of material nonpublic information (“MNPI”) trades securities on the basis of that information or discloses MNPI to someone else who trades on the basis of that information.

If you are considering trading our stock or other securities, please keep these three key points in mind:

• Never buy or sell our securities based on MNPI;

• Keep all MNPI confidential, including from your family and friends; and

• When in doubt about whether you have MNPI, ask before trading.

You are responsible for understanding and following this Policy and for the consequences of any actions you may take. Our Chief Compliance Officer or Chief Financial Officer will assist with implementing, interpreting and enforcing this Policy, pre-clearing trading activities of certain people, and pre-approving any Rule 10b5-1 Plans (plans that permit insiders to sell Company securities on a pre-determined schedule that the insider does not control, discussed more fully later in this Policy).

This Policy applies to our employees, contractors, consultants and Board members, as well as to their immediate family members, people sharing their households and anyone subject to their influence or control. It also applies to entities such as venture capital funds, partnerships, trusts and corporations that are associated or affiliated with our employees, contractors or consultants and Board members. An “immediate family member” under this Policy means any child, stepchild, parent, stepparent, spouse, domestic partner, sibling, mother-in-law, father-in- law, son-in-law, daughter-in-law, brother-in-law, or sister-in-law of a person security holder, and includes any person (other than a tenant or employee) sharing the household of that person. We will refer to all of these individuals and entities in this Policy collectively as “Insiders.” Notwithstanding the foregoing, this Policy shall not apply to any venture capital fund or other entity that engages in the investment of securities in the ordinary course of its business (e.g., an investment fund or partnership) other than for an Employee, officer or directors own account if such entity has established its own insider trading controls and procedures in compliance with applicable securities laws.

Additional trading restrictions in this Policy apply to our officers (as defined in Rule 16a-1(f) of the Securities Exchange Act of 1934, as amended (the “Exchange Act”)), and directors (together with the officers, the “Section 16 Insiders”) and to the individuals listed on Exhibit A (“Designated Insiders”) who are not Section 16 Insiders but who have regular access to MNPI in the normal course of their job. The list of Designated Insiders may be modified by our Chief Compliance Officer or Chief Financial Officer.

If you are aware of MNPI when your employment or service relationship with the Company ends, you still may not trade our securities until that MNPI has become public or is no longer material.

Additionally, the Company will not transact in its securities unless in compliance with U.S. securities laws.

The primary purpose of this Policy is to prevent people who are in possession of MNPI from trading in our stock or other securities on the basis of that MNPI or disclosing MNPI to someone else who trades on the basis of that information.

“Material information” is information about the Company, positive or negative, that a reasonable stockholder would consider important in making a decision to purchase or sell the Company’s securities. Material information can be positive or negative and can relate to virtually any aspect of the Company’s business or its securities.

Examples of material information may include:

• financial information (especially cash balance, burn and runway);

• significant regulatory communications;

• timing and achievement of major development milestones;

• results of studies and trials;

• entry into a new commercial agreement or termination of an existing commercial agreement;

• mergers or acquisitions;

• important pipeline expansion;

• significant cybersecurity incidents or data breaches;

• significant new litigation or regulatory inquires or developments in existing litigation or inquires;

• significant developments in borrowings, or financings or capital investments;

• significant changes in corporate strategy;

• stock offerings or stock splits; and

• changes in senior executive management or our Board.

This list is illustrative only and is not intended to provide a comprehensive list of circumstances that could result in material information. Determination of what may constitute material information will depend upon the facts and circumstances in each particular situation.

“Nonpublic” means that the confidential information has not yet been shared broadly outside the Company. Please remember as well that we may possess confidential information relating to or belonging to our collaborators, partners or other third parties and that it is equally important that we treat this information with the same care with which we treat our own information. If you are not sure whether information is considered public, you should either consult with our Chief Compliance Officer or Chief Financial Officer or assume that the information is nonpublic and treat it as confidential.

This Policy applies to all transactions involving our securities, including common stock, restricted stock units (“RSUs”), options and warrants to purchase common stock and any other debt or equity securities the Company may issue from time to time, such as bonds, preferred stock, convertible notes, as well as to derivative securities relating to the Company’s securities, whether or not issued by the Company, such as exchange-traded options.

Responsible roles identified in this policy may delegate an activity to a qualified delegate but retain accountability for the outcome.

Refer to GUI-004 Day One Glossary

The following is a list of prohibited activities for all Insiders:

All of the restrictions noted above for Insiders and elsewhere in this Policy also apply to our Section 16 Insiders and Designated Insiders.

Prior to trading our securities other than pursuant to a 10b5-1 Plan, Section 16 Officers and Designated Insiders must obtain pre- approval from our Chief Compliance Officer or Chief Financial Officer (or in the case of the Chief Compliance Officer or Chief Financial Officer, the Chief Executive Officer) by: (a) providing written notification of the amount and nature of the proposed trade, (b) certifying no earlier than two business days prior to the proposed trade that you have no MNPI and, to your knowledge, you will have no MNPI as of the proposed trade, and (c) receiving email confirmation from our Chief Compliance Officer or Chief Financial Officer approving the trade, which approval can be granted or denied at the Chief Compliance Officer’s or Chief Financial Officer’s discretion. You may satisfy (a) and (b) by emailing the required information and certification to our Chief Compliance Officer or Chief Financial Officer and must notify our Chief Compliance Officer or Chief Financial Officer promptly via email of any changes to the certification in (b) prior to the proposed trade.

The trading restrictions of this Policy do not apply to the following:

The trading prohibitions of this Policy are not the only stock-trading rules and regulations you need to follow. You should be aware of additional prohibitions and restrictions set by contract or by federal and state securities laws and regulations (e.g., contractual restrictions on the resale of securities, rules on short swing trading by Section 16 Insiders, compliance with Rule 144 under the Securities Act of 1933, as amended, and others). Any Insider who is uncertain whether other prohibitions or restrictions apply should ask our Chief Compliance Officer or Chief Financial Officer.

You can only Trade in a Trading Window. Other than pursuant to an approved 10b5-1 Plan, Insiders are allowed to trade our securities only during a trading window period, which opens after the close of trading on the next full trading day following the widespread public release of our quarterly or year end operating results, and closes at the close of trading on the last day of the month of the then current quarter (the “Trading Window”). If the last day of the month falls on a weekend or U.S. federal holiday, the Blackout Period will begin after the closing of trading on the immediately preceding business day. For example, if we publicly announce our quarterly financial results after the close of trading on a Monday (or before trading begins on a Tuesday), then the first time an Insider can trade our securities is after the close of market on Tuesday (effectively at the opening of the market on Wednesday for regular trading). However, if we announce quarterly financial results after trading begins on that Tuesday, then the first time the Insider can trade is after the close of market on Wednesday (effectively at the opening of the market on Thursday for regular trading).

Even During a Trading Window, You Are Not Allowed to Trade While in Possession of MNPI. Even during a Trading Window, you still may not trade our securities if you possess MNPI at that time. An Insider who possesses MNPI during a Trading Window may only trade our securities after the close of trading on the next full trading day following our widespread public release of that MNPI.

You Cannot Trade During a Blackout Period. Even during a Trading Window, our Chief Compliance Officer or Chief Financial Officer, at his or her discretion, may designate special trading Blackout Periods that apply to specific individuals or groups of people (including all Insiders) for as long as our Chief Compliance Officer or Chief Financial Officer determines. No Insider subject to a Blackout Period may trade our securities during any such Blackout Period. Additionally, no Insider subject to a Blackout Period may tell anyone (other than an advisor authorized to effectuate trades on such Insider’s behalf) not subject to the Blackout Period that a Blackout Period has been designated or that one previously was in place because that also is confidential information that cannot be disclosed internally or externally; provided that each of the Chief Compliance Officer and Chief Financial Officer may, at his or her discretion, disclose the existence of a Blackout Period to third parties not subject to the Blackout Period.

We allow Insiders to trade in our securities while in possession of MNPI, outside of a Trading Window or during a Blackout Period, only pursuant to a “10b5-1 Plan.”

What Is a 10b5-1 Plan? A 10b5-1 Plan is a written plan for selling or purchasing a predetermined number of shares that is entered into while an Insider is not in possession of MNPI as contemplated in Rule 10b5-1.

How Do I Adopt a 10b5-1 Plan? We have engaged a broker to administer our 10b5-1 Plans, and any 10b5-1 Plan that you adopt must be adopted through that broker unless otherwise approved by our Chief Compliance Officer or Chief Financial Officer. If you are interested in setting up a 10b5-1 Plan, you should consult with our Chief Compliance Officer or Chief Financial Officer and make sure that:

An individual may have no more than one 10b5-1 Plan adopted at any point in time (i.e., multiple concurrent or overlapping plans are prohibited), subject to the exceptions noted in Rule 10b5-1, which are provided for you in the Appendix. One of these exceptions is for plans authorizing certain “sell-to-cover” transactions.

Approval of a 10b5-1 Plan by our Chief Compliance Officer or Chief Financial Officer and/or acknowledgment of a 10b5-1 Plan by the Company shall not be considered a determination by us or our Chief Compliance Officer or Chief Financial Officer that the 10b5-1 Plan satisfies the requirements of Rule 10b5-1.

How Do I Modify a 10b5-1 Plan? Once you have an approved 10b5-1 Plan in place, you will need approval from our Chief Compliance Officer or Chief Financial Officer to make certain changes to it. Modifying or changing the amount, price or timing of the purchase or sale of our securities underlying the 10b5-1 Plan (or a modification or change to a written formula or algorithm, or computer program that affects the amount, price or timing of the purchase or sale of such securities) (any such modification or change, a “Plan Modification”) will be deemed to be the same as terminating your existing 10b5-1 Plan and entering into a new 10b5-1 Plan. As a result, the approval process for a Plan Modification is the same as the approval process for initially adopting a 10b5-1 Plan, including being subject to a new Cooling-Off Period. We discourage you from making multiple Plan Modifications, as that may give the appearance that you are trading on MNPI under the guise of that plan. Plan Modifications can only be made during a Trading Window and not during any Blackout Period and can be made only when you are not in possession of MNPI. For other modifications to a 10b5-1 Plan, you must notify our Chief Compliance Officer or Chief Financial Officer of such modification in writing at least two business days prior to the modification and such modification must be approved by the Chief Compliance Officer or Chief Financial Officer.

How Do I Terminate a 10b5-1 Plan? Once you have an approved 10b5-1 Plan in place, you will need approval from our Chief Compliance Officer or Chief Financial Officer to terminate it.

Insiders are not allowed to enter into “non-Rule 10b5-1 trading arrangements” (as defined in Regulation S-K Item 408(c)) unless otherwise approved in advance by the Chief Compliance Officer or Chief Financial Officer.

The consequences of violating the insider trading laws can be severe. People who violate insider trading laws may be required to disgorge profits made or losses avoided by trading, pay the loss suffered by the persons who purchased securities from or sold securities to the insider tippee, pay civil fines of up to three times the profit made or loss avoided, pay a criminal penalty of up to $5 million for individuals and $25 million for entities and serve a prison term of up to 20 years. In addition, individual directors, officers and other supervisory personnel may also be required to pay major civil or criminal penalties for failure to take appropriate steps to prevent insider trading by those under their supervision, influence or control.

We may impose discipline on anyone violating this Policy, up to and including termination of employment, and we may issue stop transfer orders to our transfer agent to prevent any attempted trades that would violate this Policy.

The Chief Compliance Officer or Chief Financial Officer will administer and interpret this Policy and enforce compliance as needed. The Chief Compliance Officer or Chief Financial Officer may consult with the Company’s outside legal counsel as needed. The Chief Compliance Officer or Chief Financial Officer may designate other individuals to perform the Chief Compliance Officer’s or Chief Financial Officer’s duties under this Policy.

Neither the Company nor the Chief Compliance Officer nor the Chief Financial Officer will be liable for any act made under this Policy. Neither the Company nor the Chief Compliance Officer nor the Chief Financial Officer is responsible for any failure to approve a trade or for imposing any Blackout Period.

Any Insider who violates this Policy or any federal or state laws governing insider trading or tipping, or who knows of any such violation by any other Insider, must report the violation immediately to our Chief Compliance Officer. If you want to submit a concern or complaint regarding a possible violation of this Policy anonymously, you should follow the procedures outlined in our Whistleblower Policy (POL-016). Anyone who violates this Policy may be subject to disciplinary measures, which may include termination of employment.

Our Board reserves the right in its sole discretion to modify or grant waivers to this Policy. Any amendments or waiver may be publicly disclosed if required by applicable laws, rules and regulations. For the avoidance of doubt, unless explicitly stated by the Board, any waiver, amendment or modification of the Policy by the Board shall not be considered a waiver of the Company’s Code of Business Conduct and Ethics (CODE-001).

We will not transact in our securities unless in compliance with applicable U.S. securities laws, rules and regulations and applicable Nasdaq listing standards.

Exhibit A: Designated Insiders

Appendix : Exceptions to the Multiple, Overlapping 10b5-1 Plan Restriction

EXHIBIT A: Designated Insiders

All full-time employees

Appendix

Exceptions to the Multiple, Overlapping 10b5-1 Plan Restriction

Such exceptions are:

Exceptions to the Single-Trade 10b5-1 Plan Restriction

There is an exception for eligible “sell-to-cover” 10b5-1 Plans where the plan authorizes an agent to sell only such securities as are necessary to satisfy tax withholding obligations arising exclusively from the vesting of a compensatory award, such as restricted stock or stock appreciation rights, and the Insider does not otherwise exercise control over the timing of such sales.