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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1. Name and Address of Reporting Person * Lynch Thomas P. 2. Issuer Name and Ticker or Trading Symbol BillionToOne, Inc. [BLLN] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director _____ 10% Owner __X__ Officer (give title below) _____ Other (specify below) See Remarks (Last) (First) (Middle) C/O BILLIONTOONE, INC., 1035 O'BRIEN DRIVE 3. Date of Earliest Transaction (Month/Day/Year) 09/08/2026 (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 Class A Common Stock 09/08/2026 M(1) 11,042 A $8.65 11,742 D Class A Common Stock 09/08/2026 M(1) 3,969 A $11.55 15,711 D Class A Common Stock 09/08/2026 M(1) 1,663 A $17.12 17,374 D Class A Common Stock 09/08/2026 M(1) 3,326 A $30.78 20,700 D Class A Common Stock 09/08/2026 S(1) 38 D $96.4739 20,662 D Class A Common Stock 09/08/2026 S(1) 1,944 D $98.9662(2) 18,718 D Class A Common Stock 09/08/2026 S(1) 8,285 D $100.1228(3) 10,433 D Class A Common Stock 09/08/2026 S(1) 775 D $100.8289(4) 9,658 D Class A Common Stock 09/08/2026 S(1) 614 D $98.9986(5) 9,044 D Class A Common Stock 09/08/2026 S(1) 2,997 D $100.1135(3) 6,047 D Class A Common Stock 09/08/2026 S(1) 358 D $100.8234(6) 5,689 D Class A Common Stock 09/08/2026 S(1) 385 D $99.223(7) 5,304 D Class A Common Stock 09/08/2026 S(1) 1,131 D $100.3261(8) 4,173 D Class A Common Stock 09/08/2026 S(1) 147 D $101.4077(9) 4,026 D Class A Common Stock 09/08/2026 S(1) 576 D $98.8882(2) 3,450 D Class A Common Stock 09/08/2026 S(1) 2,518 D $100.1959(10) 932 D Class A Common Stock 09/08/2026 S(1) 232 D $100.9242(11) 700 D

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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

Stock Option (right to buy)

$8.65

09/08/2026

M

11,042

(12)

01/18/2033

Class A Common Stock

11,042

$8.65

68,358

D

Stock Option (right to buy)

$11.55

09/08/2026

M

3,969

(12)

10/17/2033

Class A Common Stock

3,969

$11.55

11,031

D

Stock Option (right to buy)

$17.12

09/08/2026

M

1,663

(12)

10/16/2034

Class A Common Stock

1,663

$17.12

8,337

D

Stock Option (right to buy)

$30.78

09/08/2026

M

3,326

(12)

09/30/2035

Class A Common Stock

3,326

$30.78

36,674

D

Reporting Owners

Reporting Owner Name / Address	Relationships
	Director	10% Owner	Officer	Other
Lynch Thomas P. C/O BILLIONTOONE, INC. 1035 O'BRIEN DRIVE MENLO PARK, CA 94025			See Remarks

Signatures

Thomas P. Lynch		09/10/2026
**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)	The transactions reported on this Form 4 were effected pursuant to a Rule 10b5-1 trading plan adopted by the reporting person on June 8, 2026.
(2)	The reported price in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $98.340 to $99.230 per share, inclusive. The holder undertakes to provide, upon written request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote.
(3)	The reported price in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $99.585 to $100.565 per share, inclusive. The holder undertakes to provide, upon written request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote.
(4)	The reported price in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $100.620 to $100.9125 per share, inclusive. The holder undertakes to provide, upon written request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote.
(5)	The reported price in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $98.340 to $99.195 per share, inclusive. The holder undertakes to provide, upon written request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote.
(6)	The reported price in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $100.720 to $100.9025 per share, inclusive. The holder undertakes to provide, upon written request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote.
(7)	The reported price in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $98.830 to $99.825 per share, inclusive. The holder undertakes to provide, upon written request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote.
(8)	The reported price in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $99.830 to $100.790 per share, inclusive. The holder undertakes to provide, upon written request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote.
(9)	The reported price in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $100.890 to $101.870 per share, inclusive. The holder undertakes to provide, upon written request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote.
(10)	The reported price in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $99.670 to $100.550 per share, inclusive. The holder undertakes to provide, upon written request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote.
(11)	The reported price in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $100.725 to $101.650 per share, inclusive. The holder undertakes to provide, upon written request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote.
(12)	The options are fully vested and exercisable. The terms of the underlying option grant are as previously reported on the Reporting Person's Form 3 filed on November 5, 2025.
Remarks: General Counsel, Chief Compliance Officer and Secretary

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