SEC FORM 4

SEC Form 4

FORM 4

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

STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP

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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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^*

Lynch Thomas P.

(Last)

(First)

(Middle)

C/O BILLIONTOONE, INC.

1035 O'BRIEN DRIVE

(Street)

MENLO PARK

CALIFORNIA

94025

(City)

(State)

(Zip)

UNITED STATES

(Country)

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

2a. Foreign Trading Symbol

3. Date of Earliest Transaction (Month/Day/Year)
09/08/2026

6. Individual or Joint/Group Filing (Check Applicable Line)

X	Form filed by One Reporting Person
	Form filed by More than One Reporting Person

4. If Amendment, Date of Original Filed (Month/Day/Year)

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)
1. Title of Security (Instr. 3)	2. Transaction Date (Month/Day/Year)	2A. Deemed Execution Date, if any (Month/Day/Year)	Code	V	Amount	(A) or (D)	Price		6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4)	7. Nature of Indirect Beneficial Ownership (Instr. 4)
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

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 Securities Underlying Derivative Security (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: Direct (D) or Indirect (I) (Instr. 4)	11. Nature of Indirect Beneficial Ownership (Instr. 4)
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)	Code	V	(A)	(D)	Date Exercisable	Expiration Date	Title	Amount or Number of Shares	8. Price of Derivative Security (Instr. 5)		10. Ownership Form: Direct (D) or Indirect (I) (Instr. 4)	11. Nature of Indirect Beneficial Ownership (Instr. 4)
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

Explanation of Responses:

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

	Thomas P. Lynch	09/10/2026
	** Signature of Reporting Person	Date
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.
* Form 4: SEC 1474 (03-26)