BillionToOne CTO Exercises Options, Sells 3,333 Shares

The option row reports 556,667 options following the exercise, and the trading plan was adopted June 5, 2026.

Rhea-AI Filing Summary

BillionToOne, Inc. (BLLN) Chief Technology Officer and director David Tsao exercised options to acquire 3,333 Class A common shares on September 24, 2026, at an exercise price of $2.80 per share, then sold 3,333 shares at $125 per share. The transactions were made under a Rule 10b5-1 trading plan adopted June 5, 2026. The derivative row reports 556,667 stock options following the exercise; the options were fully vested and exercisable.

Insider Trade Summary 10b5-1

Exercise and Sale: 3,333 shares ($407K approx. pre-tax spread)

Exercise and Sale 3 txns

Insider Tsao David

Role Chief Technology Officer

Sold 3,333 shs ($417K)

Approx. gross sale proceeds $417K

Approx. exercise cost $9K

Approx. pre-tax spread $407K

Type	Security	Shares	Price	Value
Exercise	Stock Option (right to buy) F2	3,333	$2.80	$9K
Exercise	Class A Common Stock F1	3,333	$2.80	$9K
Sale	Class A Common Stock F1	3,333	$125.00	$417K

Holdings After Transaction: Stock Option (right to buy) — 556,667 contracts (Direct); Class A Common Stock — 1,000 shares (Direct)

Footnotes (2)

1. F1. The transactions reported on this Form 4 were effected pursuant to a Rule 10b5-1 trading plan adopted by the reporting person on June 5, 2026.
2. F2. The options are fully vested and exercisable.

Key Figures

Options exercised: 3,333 options Exercise price: $2.80 per share Class A common shares sold: 3,333 shares +2 more

5 metrics

Options exercised 3,333 options September 24, 2026

Exercise price $2.80 per share Options exercised September 24, 2026

Class A common shares sold 3,333 shares September 24, 2026

Sale price $125 per share Sale on September 24, 2026

Stock options following exercise 556,667 options Reported following the September 24, 2026 transaction

Key Terms

Rule 10b5-1 trading plan, fully vested and exercisable, Stock Option (right to buy)

3 terms

Rule 10b5-1 trading plan regulatory

"pursuant to a Rule 10b5-1 trading plan adopted by the reporting person"

A Rule 10b5-1 trading plan is a pre-arranged schedule that allows company insiders to buy or sell stock at specific times, even if they have inside information. It helps prevent accusations of unfair trading by making these transactions look planned and transparent, rather than sneaky or illegal.

fully vested and exercisable financial

"The options are fully vested and exercisable."

Stock Option (right to buy) financial

"Stock Option (right to buy)"

FAQ

AI-generated questions and answers. How Rhea-AI works. Not financial advice.

How many BLLN shares did David Tsao sell, and at what price?

David Tsao sold 3,333 Class A common shares at $125 per share on September 24, 2026, under a Rule 10b5-1 trading plan adopted June 5, 2026.

Were David Tsao's BLLN options vested?

The options were fully vested and exercisable.

AI-generated analysis. How Rhea-AI works. Not financial advice.

Form 4 Insider Transactions: What Each Field Means →

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Available on EDGAR 09/28/2026 - 05:57 PM Accepted by SEC EDGAR 09/28/2026 - 05:57 PM Learn about SEC filing dates

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

OMB APPROVAL

OMB Number:			3235-0287
Estimated average burden
hours per response:			0.5

Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

X

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^*

Tsao David

(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)

X	Director		10% Owner
X	Officer (give title below)		Other (specify below)
	Chief Technology Officer

2a. Foreign Trading Symbol

3. Date of Earliest Transaction (Month/Day/Year)
09/24/2026

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)
			Code	V	Amount	(A) or (D)	Price
Class A Common Stock	09/24/2026		M(1)		3,333	A	$2.8	4,333	D
Class A Common Stock	09/24/2026		S(1)		3,333	D	$125	1,000	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)
				Code	V	(A)	(D)	Date Exercisable	Expiration Date	Title	Amount or Number of Shares
Stock Option (right to buy)	$2.8	09/24/2026		M			3,333	(2)	06/07/2031	Class A Common Stock	3,333	$2.8	556,667	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 5, 2026.

2. The options are fully vested and exercisable.

Remarks:

	/s/ Thomas P. Lynch, Attorney-in-Fact	09/28/2026
	** 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)