Statement of Changes in Beneficial Ownership (Form 4)

FORM 4 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 that is intended to satisfy the affirmative defense conditions of Rule 10b5-1(c). See Instruction 10.

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP OF SECURITIES Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934 or Section 30(h) of the Investment Company Act of 1940

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(Print or Type Responses)

1. Name and Address of Reporting Person * Tsao David 2. Issuer Name and Ticker or Trading Symbol BillionToOne, Inc. [BLLN] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) __X__ Director _____ 10% Owner __X__ Officer (give title below) _____ Other (specify below) Chief Technology Officer (Last) (First) (Middle) C/O BILLIONTOONE, INC., 1035 O'BRIEN DRIVE 3. Date of Earliest Transaction (Month/Day/Year) 11/07/2025 (Street) MENLO PARK, CA 94025 4. If Amendment, Date Original Filed (Month/Day/Year) 6. Individual or Joint/Group Filing (Check Applicable Line) _X_ Form filed by One Reporting Person ___ Form filed by More than One Reporting Person (City) (State) (Zip) Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned

1.Title of Security (Instr. 3) 2. Transaction Date (Month/Day/Year) 2A. Deemed Execution Date, if any (Month/Day/Year) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price Common Stock 11/07/2025 J(1) 2,325,108 D (1) 0 D Class A Common Stock 11/07/2025 J(1) 2,325,108 A (1) 2,325,108 D Class A Common Stock 11/07/2025 J(2) 2,325,108 D (2) 0 D Class A Common Stock 11/07/2025 P(3) 1,000 A $60 1,000 D

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.
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Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned
(e.g., puts, calls, warrants, options, convertible securities)

1. Title of Derivative Security (Instr. 3)

2. Conversion or Exercise Price of Derivative Security

3. Transaction Date (Month/Day/Year)

3A. Deemed Execution Date, if any (Month/Day/Year)

4. Transaction Code (Instr. 8)

5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5)

6. Date Exercisable and Expiration Date (Month/Day/Year)

7. Title and Amount of Underlying Securities (Instr. 3 and 4)

8. Price of Derivative Security (Instr. 5)

9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4)

10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4)

11. Nature of Indirect Beneficial Ownership (Instr. 4)

Code

V

(A)

(D)

Date Exercisable

Expiration Date

Title

Amount or Number of Shares

Class B Common Stock

(4)

11/07/2025

J(2)

2,325,108

(4)

(4)

Class A Common Stock

2,325,108

$ 0

2,325,108

D

Stock Option (right to buy)

$2.80

11/07/2025

J(1)

640,000

(5)

06/07/2031

Common Stock

640,000

(1)

0

D

Stock Option (right to buy)

$2.80

11/07/2025

J(1)

640,000

(5)

06/07/2031

Class A Common Stock

640,000

(1)

640,000

D

Stock Option (right to buy)

$20.04

11/07/2025

J(1)

501,551

(6)

06/18/2035

Common Stock

501,551

(1)

0

D

Stock Option (right to buy)

$20.04

11/07/2025

J(1)

501,551

(6)

06/18/2035

Class A Common Stock

501,551

(1)

501,551

D

Reporting Owners

Reporting Owner Name / Address	Relationships
	Director	10% Owner	Officer	Other
Tsao David C/O BILLIONTOONE, INC. 1035 O'BRIEN DRIVE MENLO PARK, CA 94025	X		Chief Technology Officer

Signatures

/s/ Thomas P. Lynch, Attorney-in-Fact		11/12/2025
**Signature of Reporting Person		Date

Explanation of Responses:

*	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).
(1)	Pursuant to a reclassification exempt under Rule 16b-7, each share of Common Stock was automatically reclassified into one share of Class A common stock immediately prior to the completion of the Issuer's initial public offering of Class A common stock (the "Offering").
(2)	Following the reclassification of Common Stock into Class A Common Stock, the shares of Class A common stock were exchanged at a 1:1 ratio for shares of Class B common stock in a transaction previously approved by the Issuer's board of directors.
(3)	These shares of Class A common stock were purchased by the Reporting Person through a directed share program in connection with the Offering.
(4)	Each share of Class B Common Stock is convertible into one share of Class A common stock at the option of the holder. Class B common stock will convert automatically on a one-for-one basis into shares of the Issuer's Class A common stock upon the earliest of (i) seven years from the date of filing of the amended and restated certificate of incorporation, in connection with the Offering and (ii) the date specified by a vote of the holders of Class B common stock representing a majority of the outstanding shares of Class B common stock.
(5)	The options are fully vested and exercisable.
(6)	The options are subject to a service-based vesting requirement, which shall be satisfied over a six-year period. The options vest and become exercisable in equal monthly installments commencing on June 11, 2025, subject to the Reporting Person's continuous service with the Issuer through each such vesting date.

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