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 * Atay Oguzhan 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) See Remarks (Last) (First) (Middle) C/O BILLIONTOONE, INC., 1035 O'BRIEN DRIVE 3. Date of Earliest Transaction (Month/Day/Year) 06/08/2026 (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 06/08/2026 M(1) 20,000 A $2.8 20,000 D Class A Common Stock 06/08/2026 M(1) 6,250 A $2.8 26,250 D Class A Common Stock 06/08/2026 S(1) 79 D $100.87 26,171 D Class A Common Stock 06/08/2026 S(1) 5,466 D $94.7627(2) 20,705 D Class A Common Stock 06/08/2026 S(1) 1,856 D $95.5898(3) 18,849 D Class A Common Stock 06/08/2026 S(1) 4,372 D $96.8921(4) 14,477 D Class A Common Stock 06/08/2026 S(1) 4,066 D $98.1209(5) 10,411 D Class A Common Stock 06/08/2026 S(1) 3,605 D $99.063(6) 6,806 D Class A Common Stock 06/08/2026 S(1) 556 D $99.9688(7) 6,250 D Class A Common Stock 06/08/2026 S(1) 6,250 D $100 0 D Class A Common Stock 06/08/2026 C(8) 10,000 A $ 0 (8) 10,000 D Class A Common Stock 06/09/2026 G 10,000 D $ 0 0 D Class A Common Stock 06/08/2026 S(1) 6,250 D $96.45 193,750 I By spouse(9) Class A Common Stock 06/08/2026 S(1) 6,250 D $100 187,500 I By spouse(9)

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

Stock Option (right to buy)

$2.8

06/08/2026

M

20,000

(10)

06/07/2031

Class A Common Stock

20,000

$2.8

620,000

D

Stock Option (right to buy)

$2.8

06/08/2026

M

6,250

(10)

06/07/2031

Class A Common Stock

6,250

$2.8

613,750

D

Class B Common Stock

(8)(11)

06/08/2026

C

10,000

(11)

(11)

Class A Common Stock

10,000

$ 0 (8)

2,217,542

D

Reporting Owners

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

Signatures

/s/ Thomas P. Lynch, Attorney-in-Fact		06/10/2026
**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 March 6, 2026.
(2)	The reported price in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $94.300 to $95.280 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 $95.320 to $96.225 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 $96.530 to $97.2113 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 $97.560 to $98.5428 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 $98.560 to $99.550 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.
(7)	The reported price in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $99.640 to $100.540 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)	These shares of Class B common stock were converted at a 1:1 ratio for shares of Class A common stock at the option of the holder.
(9)	Represents shares held by the Reporting Person's spouse.
(10)	The options are fully vested and exercisable.
(11)	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.
Remarks: Chairman and Chief Executive Officer

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