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 * Ileri Firat 2. Issuer Name and Ticker or Trading Symbol BillionToOne, Inc. [BLLN] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) __X__ Director __X__ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) SQUARE VICTORIA REGAIN 1 3. Date of Earliest Transaction (Month/Day/Year) 11/06/2025 (Street) SINT-JOOST-TEN-NODE, C9 1210 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 11/06/2025 A 8,333(1) A $ 0 8,333 D Common Stock 11/07/2025 C(2)(3) 28,981 A (3) 37,314 D Common Stock 11/07/2025 C(2)(3) 640,727 A (3) 640,727 I See Footnotes(4)(9) Common Stock 11/07/2025 C(2)(3) 18,845 A (3) 18,845 I See Footnotes(5)(9) Common Stock 11/07/2025 C(2)(3) 2,015,118 A (3) 2,015,118 I See Footnotes(6)(9) Common Stock 11/07/2025 C(2)(3) 2,451,665 A (3) 2,631,664 I See Footnotes(7)(9) Common Stock 11/07/2025 C(2)(3) 2,580,833 A (3) 2,580,833 I See Footnotes(8)(9) Common Stock 11/07/2025 J(10) 37,314 D (10) 0 D Common Stock 11/07/2025 J(10) 640,727 D (10) 0 I See Footnotes(4)(9) Common Stock 11/07/2025 J(10) 18,845 D (10) 0 I See Footnotes(5)(9) Common Stock 11/07/2025 J(10) 2,015,118 D (10) 0 I See Footnotes(6)(9) Common Stock 11/07/2025 J(10) 2,631,664 D (10) 0 I See Footnotes(7)(9) Common Stock 11/07/2025 J(10) 2,580,833 D (10) 0 I See Footnotes(8)(9) Class A Common Stock 11/07/2025 J(10) 37,314 A (10) 37,314 D Class A Common Stock 11/07/2025 J(10) 640,727 A (10) 640,727 I See Footnotes(4)(9) Class A Common Stock 11/07/2025 J(10) 18,845 A (10) 18,845 I See Footnotes(5)(9) Class A Common Stock 11/07/2025 J(10) 2,015,118 A (10) 2,015,118 I See Footnotes(6)(9) Class A Common Stock 11/07/2025 J(10) 2,631,664 A (10) 2,631,664 I See Footnotes(7)(9) Class A Common Stock 11/07/2025 J(10) 2,580,833 A (10) 2,580,833 I See Footnotes(8)(9)

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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
Series A-6 Convertible Preferred Stock	(3)	11/07/2025		C			28,981	(3)	(3)	Common Stock	28,981	(3)	0	D
Series B-2 Convertible Preferred Stock	(3)	11/07/2025		C			640,727	(3)	(3)	Common Stock	640,727	(3)	0	I	See Footnotes(4)(9)
Series B-1 Convertible Preferred Stock	(3)	11/07/2025		C			18,845	(3)	(3)	Common Stock	18,845	(3)	0	I	See Footnotes(5)(9)
Series C Convertible Preferred Stock	(3)	11/07/2025		C			1,569,015	(3)	(3)	Common Stock	1,569,015	(3)	0	I	See Footnotes(6)(9)
Series D Convertible Preferred Stock	(3)	11/07/2025		C			446,103	(3)	(3)	Common Stock	446,103	(3)	0	I	See Footnotes(6)(9)
Series A-1 Convertible Preferred Stock	(3)	11/07/2025		C			20,000	(3)	(3)	Common Stock	20,000	(3)	0	I	See Footnotes(7)(9)
Series A-2 Convertible Preferred Stock	(3)	11/07/2025		C			33,000	(3)	(3)	Common Stock	33,000	(3)	0	I	See Footnotes(7)(9)
Series B-1 Convertible Preferred Stock	(3)	11/07/2025		C			2,261,377	(3)	(3)	Common Stock	2,261,377	(3)	0	I	See Footnotes(7)(9)
Series C Convertible Preferred Stock	(3)	11/07/2025		C			137,288	(3)	(3)	Common Stock	137,288	(3)	0	I	See Footnotes(7)(9)
Series A-6 Convertible Preferred Stock	(3)	11/07/2025		C			2,260,520	(3)	(3)	Common Stock	2,260,520	(3)	0	I	See Footnotes(8)(9)
Series B-2 Convertible Preferred Stock	(3)	11/07/2025		C			320,363	(3)	(3)	Common Stock	320,363	(3)	0	I	See Footnotes(8)(9)

Reporting Owners

Reporting Owner Name / Address	Relationships
	Director	10% Owner	Officer	Other
Ileri Firat SQUARE VICTORIA REGAIN 1 SINT-JOOST-TEN-NODE, C9 1210	X	X

Signatures

/s/ Thomas P. Lynch, Attorney-in-Fact		11/10/2025
**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)	Each restricted stock unit ("RSU") represents a contingent right to receive one share of the Issuer's Common Stock. The RSUs shall vest over three years in equal annual installments on the anniversary of November 6, 2025, subject to the Reporting Person's continuous service to the Issuer through each such vesting date.
(2)	The total represents shares received upon conversion of shares of Series A-1 Convertible Preferred Stock, Series A-2 Convertible Preferred Stock, Series A-6 Convertible Preferred Stock, Series B-1 Convertible Preferred Stock, Series B-2 Convertible Preferred Stock, Series C Convertible Preferred Stock and/or Series D Convertible Preferred Stock.
(3)	The reported security converted into the Issuer's Common Stock on a one-for-one basis automatically immediately prior to the closing of the Issuer's initial public offering of Class A common stock (the "Offering") without payment of further consideration. The shares have no expiration date.
(4)	The reported security is directly held by HB&Q NV (HB&Q COMPARTMENT III).
(5)	The reported security is directly held by Hummingbird Collective CommV.
(6)	The reported security is directly held by Hummingbird Dragons CommV.
(7)	The reported security is directly held by Hummingbird Opportunity Fund II CommV.
(8)	The reported security is directly held by Hummingbird Ventures III CommV.
(9)	Humming Bird Ventures Management NV is the managing member of Hummingbird Opportunity Fund II CommV, Hummingbird Ventures III CommV, Hummingbird Dragons CommV, Hummingbird Collective CommV, and HB&Q NV (HB&Q COMPARTMENT III) (the Hummingbird Funds) and, as a result, may be deemed to beneficially own shares held by the Hummingbird Funds. Mr. Ileri, Barend Van den Brande, and Lukas Decoster are partners of Hummingbird Ventures Management NV and may be deemed to have shared voting and investment power over the shares held by the Hummingbird Funds. The Reporting Person disclaims beneficial ownership of these shares except to the extent of his pecuniary interest therein.
(10)	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 Offering.

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