Tax-related stock sale by Upstream Bio (UPB) chief business officer

Rhea-AI Filing Summary

Upstream Bio, Inc. Chief Business Officer Adam Houghton reported an automatic sale of 699 shares of common stock at $9.29 per share. According to the company’s sell-to-cover policy, these shares were sold solely to satisfy tax withholding on vested restricted stock units. After this tax-related sale, Houghton directly holds 24,301 shares of Upstream Bio common stock.

Insider Trade Summary

Net Seller: 699 shares ($6,494)

Net Sell 1 txn

Insider Houghton Adam

Role Chief Business Officer

Sold 699 shs ($6K)

Type	Security	Shares	Price	Value
Sale	Common Stock	699	$9.29	$6K

Holdings After Transaction: Common Stock — 24,301 shares (Direct)

Footnotes (1)

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Form 4 Insider Transactions: What Each Field Means →

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).

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^*

Houghton Adam

(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)
	Chief Business Officer

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)		699	D	$9.29	24,301	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, Attorney-in-Fact	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).
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.

FAQ

What insider transaction did Upstream Bio (UPB) report for Adam Houghton?

Upstream Bio reported that Chief Business Officer Adam Houghton sold 699 shares of common stock at $9.29 per share. The company states this was an automatic sale under its sell-to-cover policy to satisfy tax withholding from restricted stock unit vesting.

Was the Upstream Bio (UPB) insider stock sale discretionary or automatic?

The sale was automatic, not discretionary. Upstream Bio explains it has a sell-to-cover policy, and the 699 shares sold represented the amount needed to cover Houghton’s tax withholding obligations from restricted stock unit vesting.

How many Upstream Bio (UPB) shares does Adam Houghton hold after the reported sale?

Following the 699-share tax-related sale, Adam Houghton directly holds 24,301 shares of Upstream Bio common stock. This shows the transaction involved a relatively small portion of his reported holdings and primarily addressed tax obligations on vesting equity.

What price was received in the Upstream Bio (UPB) insider stock sale?

The reported transaction shows 699 shares of Upstream Bio common stock sold at a price of $9.29 per share. The company indicates these shares were sold automatically to cover tax withholding tied to restricted stock unit vesting, under its sell-to-cover policy.

Why did Upstream Bio (UPB) classify this insider sale as sell-to-cover?

Upstream Bio notes it has adopted a sell-to-cover policy for tax withholding on equity awards. The shares reported on this Form 4 were sold specifically to satisfy tax obligations from restricted stock unit vesting and were not sold at Houghton’s discretion.