# EDGAR Filing Document

**Accession Number:** 0001655962
**File Stem:** 0001655962-25-000006
**Filing Date:** 2025-11
**Character Count:** 7960
**Document Hash:** 8da1cc7c822083f152919f0043eb495f
**Contains OCR:** False
**Source Format:** 

## Filing Content

## Filing Summary
**0001655962-25-000006.hdr.sgml**: 20251118

**ACCESSION NUMBER**: 0001655962-25-000006

**CONFORMED SUBMISSION TYPE**: 4

**PUBLIC DOCUMENT COUNT**: 1

**CONFORMED PERIOD OF REPORT**: 20251115

**FILED AS OF DATE**: 20251118

**DATE AS OF CHANGE**: 20251118

**REPORTING-OWNER**: 

**OWNER DATA:**
- **COMPANY CONFORMED NAME:** York Charles N II
- **CENTRAL INDEX KEY:** 0001655962

**ORGANIZATION NAME:**

**FILING VALUES:**
- **FORM TYPE:** 4
- **SEC ACT:** 1934 Act
- **SEC FILE NUMBER:** 001-40431
- **FILM NUMBER:** 251496317

**MAIL ADDRESS:**
- **STREET 1:** C/O DAYONE BIOPHARMACEUTICALS, INC.
- **STREET 2:** 1800 SIERRA POINT PARKWAY, SUITE 200
- **CITY:** BRISBANE
- **STATE:** CA
- **ZIP:** 94005
**ISSUER**: 

**COMPANY DATA:**
- **COMPANY CONFORMED NAME:** Day One Biopharmaceuticals, Inc.
- **CENTRAL INDEX KEY:** 0001845337
- **STANDARD INDUSTRIAL CLASSIFICATION:** PHARMACEUTICAL PREPARATIONS [2834]
- **ORGANIZATION NAME:** 03 Life Sciences
- **EIN:** 832415215
- **STATE OF INCORPORATION:** DE
- **FISCAL YEAR END:** 1231

**BUSINESS ADDRESS:**
- **STREET 1:** 1800 SIERRA POINT PARKWAY, SUITE 200
- **CITY:** BRISBANE
- **STATE:** CA
- **ZIP:** 94005
- **BUSINESS PHONE:** 650 484-0899

**MAIL ADDRESS:**
- **STREET 1:** 1800 SIERRA POINT PARKWAY, SUITE 200
- **CITY:** BRISBANE
- **STATE:** CA
- **ZIP:** 94005

**FORMER COMPANY:**
- **FORMER CONFORMED NAME:** Day One Biopharmaceuticals Holding Co LLC
- **DATE OF NAME CHANGE:** 20210209

### UNITED STATES SECURITIES AND EXCHANGE COMMISSION
**Washington, D.C. 20549**

## FORM 4

### STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP

[ ] Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

[ ] Check this box to indicate that a transaction was made pursuant to a contract, instruction or written plan for the purchase or sale of equity securities of the issuer that is intended to satisfy the affirmative defense conditions of Rule 10b5-1(c). See Instruction 10.

---

| | | |
|:---|:---|:---|
| **1. Name and Address of Reporting Person**<sup>*</sup><br>York Charles N II<br><sub>(Last) (First) (Middle)</sub><br>1800 SIERRA POINT PARKWAY, SUITE 200<br><sub>(Street)</sub><br>BRISBANE, CA 94005<br><sub>(City) (State) (Zip)</sub> | **3. Date of Earliest Transaction (Month/Day/Year)**<br>2025-11-15 | **5. Relationship of Reporting Person(s) to Issuer**<br>(Check all applicable)<br>[ ] Director [ ] 10% Owner<br>[X] Officer (give title below) [ ] Other (specify below)<br>_COO and CFO_ |
| **2. Issuer Name and Ticker or Trading Symbol**<br>Day One Biopharmaceuticals, Inc. [ DAWN ] | **4. If Amendment, Date of Original Filed (Month/Day/Year)**<br>  | **6. Individual or Joint/Group Filing (Check Applicable Line)**<br>[X] Form filed by One Reporting Person<br>[ ] Form filed by More than One Reporting Person |

---

## Table I - Non-Derivative Securities

---

|  |  |  |  |  |  |  |  |  |  |  |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| 1. Title of Security | 2. Transaction Date | 2A. Deemed Execution Date | 3. Transaction Code (V) | 3. Transaction Code (V) | 4. Securities Acquired (A) or Disposed of (D) | 4. Securities Acquired (A) or Disposed of (D) | 4. Securities Acquired (A) or Disposed of (D) | 5. Amount of Securities Beneficially Owned | 6. Ownership Form | 7. Nature of Indirect Beneficial Ownership |
| 1. Title of Security | 2. Transaction Date | 2A. Deemed Execution Date | Code | V | Amount | (A) or (D) | Price | 5. Amount of Securities Beneficially Owned | 6. Ownership Form | 7. Nature of Indirect Beneficial Ownership |
| Common Stock | 2025-11-15 |  | M |  | 1375 | A | <sup>(1)</sup> | 283277<sup>(2)</sup> | D |  |
| Common Stock | 2025-11-15 |  | M |  | 2250 | A | <sup>(1)</sup> | 285527 | D |  |
| Common Stock | 2025-11-15 |  | M |  | 6625 | A | <sup>(1)</sup> | 292152 | D |  |
| Common Stock | 2025-11-15 |  | M |  | 6625 | A | <sup>(1)</sup> | 298777 | D |  |
| Common Stock | 2025-11-17 |  | S<sup>(3)</sup> |  | 4062 | D | 8.9147<sup>(4)</sup> | 294715 | D |  |

---

## Table II - Derivative Securities

---

|  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| 1. Title of Derivative Security | 2. Conversion or Exercise Price | 3. Transaction Date | 3A. Deemed Execution Date | 4. Transaction Code (V) | 4. Transaction Code (V) | 5. Number of Derivative Securities Acquired (A) or Disposed of (D) | 5. Number of Derivative Securities Acquired (A) or Disposed of (D) | 6. Date Exercisable and Expiration Date | 6. Date Exercisable and Expiration Date | 7. Title and Amount of Underlying Securities | 7. Title and Amount of Underlying Securities | 8. Price of Derivative Security | 9. Number of Derivative Securities Beneficially Owned | 10. Ownership Form | 11. Nature of Indirect Beneficial Ownership |
| 1. Title of Derivative Security | 2. Conversion or Exercise Price | 3. Transaction Date | 3A. Deemed Execution Date | Code | V | (A) | (D) | Date Exercisable | Expiration Date | Title | Amount or Number of Shares | 8. Price of Derivative Security | 9. Number of Derivative Securities Beneficially Owned | 10. Ownership Form | 11. Nature of Indirect Beneficial Ownership |
| Restricted Stock Unit (RSU) | <sup>(1)</sup> | 2025-11-15 |  | M |  |  | 1375 | <sup>(5)</sup> | <sup>(6)</sup> | Common Stock | 1375 | $0 | 0 | D |  |
| Restricted Stock Unit (RSU) | <sup>(1)</sup> | 2025-11-15 |  | M |  |  | 2250 | <sup>(5)</sup> | <sup>(6)</sup> | Common Stock | 2250 | $0 | 9000 | D |  |
| Restricted Stock Unit (RSU) | <sup>(1)</sup> | 2025-11-15 |  | M |  |  | 6625 | <sup>(5)</sup> | <sup>(6)</sup> | Common Stock | 6625 | $0 | 53000 | D |  |
| Restricted Stock Unit (RSU) | <sup>(1)</sup> | 2025-11-15 |  | M |  |  | 6625 | <sup>(5)</sup> | <sup>(6)</sup> | Common Stock | 6625 | $0 | 79500 | D |  |

---

### Footnotes:

(1) Each Restricted Stock Unit ("RSU") represents a contingent right to receive one share of the Issuer's Common Stock upon settlement for no consideration.

(2) Includes 3,902 shares of Common Stock acquired by the Reporting Person between May 12, 2023 and November 14, 2025 pursuant to the Issuer's employee stock purchase plan.

(3) The sale of shares is for the sole purpose of covering the Reporting Person's tax liability with respect to the settlement of RSUs.

(4) The price reported in Column 4 is a weighted average price. These shares were sold as part of block trades for multiple security holders of the Issuer at prices ranging from $8.545 to $9.305, inclusive. The Reporting Person undertakes to provide to the Issuer, any securityholder of the Issuer or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the range set forth herein with regard to the block trade.

(5) The RSUs will vest as to 1/16th of the total award in quarterly installments on February 15, May 15, August 15 and November 15, subject to the Reporting Person's provision of service to the Issuer on each vesting date.

(6) RSUs do not expire; they either vest or are canceled prior to the vesting date.

**Signature:** /s/ Charles N. York II  
**Date:** 2025-11-18

### Remarks:

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.

* If the form is filed by more than one reporting person, see Instruction 4 (b)(v).

** Intentional misstatements or omissions of facts constitute Federal Criminal Violations See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).

Note: File three copies of this Form, one of which must be manually signed. If space is insufficient, see Instruction 6 for procedure.

**Persons who respond to the collection of information contained in this form are not required to respond unless the form displays a currently valid OMB Number.**