Upstream Bio executive sells 700 shares at $5.49

Upstream Bio’s chief business officer executed an automatic sell-to-cover stock sale tied to RSU tax withholding and now holds 22,901 shares.

Rhea-AI Filing Summary

Upstream Bio, Inc. (UPB) reported that Chief Business Officer Adam Houghton sold 700 shares of common stock on September 16, 2026 at $5.49 per share in an open-market or private transaction. After this sale, he held 22,901 shares directly.

According to the company’s disclosed sell-to-cover policy, these sales were made automatically to satisfy tax withholding obligations arising from the vesting of restricted stock units, and were not at Houghton’s discretion. No Rule 10b5-1 trading plan is reported for this transaction.

Insider Trade Summary

Net Seller: 700 shares

Net Sell 1 txn

Insider Houghton Adam

Role Chief Business Officer

Sold 700 shs ($4K)

Type	Security	Shares	Price	Value
Sale	Common Stock F1	700	$5.49	$4K

Holdings After Transaction: Common Stock — 22,901 shares (Direct)

Footnotes (1)

1. F1. 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.

Key Figures

Shares sold: 700 shares Sale price per share: $5.49 per share Shares held after transaction: 22,901 shares

3 metrics

Shares sold 700 shares Common stock sale by Chief Business Officer on September 16, 2026

Sale price per share $5.49 per share Price for the 700 shares sold on September 16, 2026

Shares held after transaction 22,901 shares Direct holdings of Adam Houghton following the sale

Key Terms

sell-to-cover policy, tax withholding obligations, restricted stock units

3 terms

sell-to-cover policy financial

"The Issuer has adopted a "sell-to-cover" policy to satisfy the tax"

tax withholding obligations financial

"required to be sold by the Reporting Person to cover tax withholding"

restricted stock units financial

"cover tax withholding obligations in connection with the vesting of restricted stock units"

Restricted stock units are a type of company reward where employees are promised shares of stock, but they only fully own these shares after meeting certain conditions, like staying with the company for a set time. They matter because they can become valuable assets and are often used to motivate employees to help the company succeed.

FAQ

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

What insider transaction did UPB report for Adam Houghton?

Upstream Bio reported that Chief Business Officer Adam Houghton sold 700 shares of common stock on September 16, 2026. The sale was associated with tax withholding on vesting restricted stock units under the company’s sell-to-cover policy and was made automatically.

At what price were the 700 UPB shares sold by the chief business officer?

The 700 Upstream Bio (UPB) shares were sold at $5.49 per share. The sale is described as a sale in an open market or private transaction and is linked to tax withholding obligations from restricted stock unit vesting.

How many UPB shares does Adam Houghton hold after this Form 4 transaction?

After the reported transaction, Adam Houghton directly holds 22,901 shares of Upstream Bio common stock. This figure is stated as the total shares following the September 16, 2026 sale of 700 shares.

Why did the Upstream Bio chief business officer sell 700 shares of UPB stock?

The 700 shares were sold to cover tax withholding obligations triggered by the vesting of restricted stock units. Upstream Bio states it has adopted a sell-to-cover policy, and these sales represent the number of shares required to satisfy those tax obligations.

Were the recent UPB insider sales by Adam Houghton discretionary trades?

No. The company states the sales were automatic under its sell-to-cover policy and were not at the discretion of Adam Houghton. They were executed solely to cover tax withholding obligations related to restricted stock unit vesting.

Was the September 16, 2026 UPB insider transaction under a Rule 10b5-1 plan?

No. The filing’s Rule 10b5-1 checkbox is not marked, and no Rule 10b5-1 trading plan is referenced in the footnotes. Instead, the sale is tied to Upstream Bio’s automatic sell-to-cover policy for tax withholding on RSU vesting.

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

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Available on EDGAR 09/17/2026 - 04:05 PM Accepted by SEC EDGAR 09/17/2026 - 04:05 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).

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 MASSACHUSETTS 02451

(City) (State) (Zip)

UNITED STATES

(Country)

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

2a. Foreign Trading Symbol

3. Date of Earliest Transaction (Month/Day/Year)
09/16/2026

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
Common Stock	09/16/2026		S(1)		700	D	$5.49	22,901	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	09/17/2026
	** 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.
* Form 4: SEC 1474 (03-26)