Tax-withholding share sale by Upstream Bio (UPB) General Counsel disclosed

Rhea-AI Filing Summary

Upstream Bio, Inc. General Counsel Allison Ambrose reported an automatic sale of 475 shares of common stock at $9.29 per share. The company has a “sell-to-cover” policy, so these shares were sold solely to cover tax withholding on vested restricted stock units, not at her discretion. After this transaction, she directly holds 16,525 shares, indicating this was a small, tax-related adjustment to her position rather than a discretionary open-market trade.

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

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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^*

Ambrose Allison

(Last) (First) (Middle)

UPSTREAM BIO, INC.

890 WINTER STREET, SUITE 200

(Street)

WALTHAM MA 02451

(City) (State) (Zip)

2. Issuer Name and Ticker or Trading Symbol
Upstream Bio, Inc. [ UPB ]

5. Relationship of Reporting Person(s) to Issuer
(Check all applicable)

	Director		10% Owner
X	Officer (give title below)		Other (specify below)
	General Counsel

3. Date of Earliest Transaction (Month/Day/Year)
03/16/2026

4. If Amendment, Date of 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

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	03/16/2026		S(1)		475	D	$9.29	16,525	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

Explanation of Responses:

1. The Issuer has adopted a "sell-to-cover" policy to satisfy the tax withholding obligations of the Reporting Person. The sales reported on this Form 4 represent the number of shares required to be sold by the Reporting Person to cover tax withholding obligations in connection with the vesting of restricted stock units. Such sales were automatic and not at the discretion of the Reporting Person.

	/s/ Allison Ambrose	03/17/2026
	** 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).
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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.

FAQ

What insider transaction did Upstream Bio (UPB) report for Allison Ambrose?

Upstream Bio reported that General Counsel Allison Ambrose sold 475 common shares at $9.29 each. The sale was automatic under a company “sell-to-cover” policy to satisfy tax withholding on vested restricted stock units, rather than a discretionary market trade.

Why did Upstream Bio (UPB) General Counsel’s shares get sold in this Form 4?

The shares were sold to cover tax withholding obligations tied to restricted stock units vesting. Under Upstream Bio’s adopted “sell-to-cover” policy, the required number of shares is sold automatically, so the transaction did not occur at the General Counsel’s discretion.

How many Upstream Bio (UPB) shares did the insider sell and at what price?

Allison Ambrose reported the sale of 475 shares of Upstream Bio common stock at a price of $9.29 per share. This sale was executed solely to cover tax withholding from RSU vesting, according to the company’s automatic “sell-to-cover” policy disclosed in the filing footnote.

How many Upstream Bio (UPB) shares does Allison Ambrose hold after the transaction?

Following the tax-related sale, Allison Ambrose directly holds 16,525 shares of Upstream Bio common stock. This indicates she retains a substantially larger equity stake than the number of shares sold, reinforcing that the reported transaction was a limited, tax-driven adjustment to her holdings.

Was the Upstream Bio (UPB) insider sale made under a pre-set policy or plan?

Yes. The filing explains that Upstream Bio has adopted a “sell-to-cover” policy for tax withholding on equity awards. The 475-share sale was automatic under this policy and not at the reporting person’s discretion, making it a routine compensation-related transaction rather than a strategic sale.