Upstream Bio counsel sells 485 shares at $5.49

Upstream Bio’s General Counsel completed an automatic sell-to-cover stock sale tied to RSU vesting, with 15,563 shares remaining held directly.

Rhea-AI Filing Summary

Upstream Bio, Inc. (UPB) reported that its General Counsel, Allison Ambrose, sold 485 shares of common stock on September 16, 2026 at $5.49 per share. The company states this was an automatic sell-to-cover transaction to satisfy tax withholding on vesting restricted stock units, not at the executive’s discretion, leaving 15,563 shares held directly.

Insider Trade Summary

Net Seller: 485 shares

Net Sell 1 txn

Insider Ambrose Allison

Role General Counsel

Sold 485 shs ($3K)

Type	Security	Shares	Price	Value
Sale	Common Stock F1	485	$5.49	$3K

Holdings After Transaction: Common Stock — 15,563 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: 485 shares Sale price per share: $5.49 per share Shares held after transaction: 15,563 shares +1 more

4 metrics

Shares sold 485 shares Common stock sale on September 16, 2026 by General Counsel Allison Ambrose

Sale price per share $5.49 per share Price for the 485 UPB common shares sold on September 16, 2026

Shares held after transaction 15,563 shares Direct UPB common stock holdings of Allison Ambrose following the sale

Net shares sold 485 shares Net-sell direction across all reported transactions in this Form 4

Key Terms

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

3 terms

sell-to-cover financial

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

Sell-to-cover is when part of newly issued or exercised company stock is immediately sold to pay required taxes and fees, so the recipient keeps the remaining shares. For investors this matters because it reduces the number of shares insiders or employees actually hold after a grant, can create small, routine share sales that aren’t signal of cashing out, and slightly increases share supply on the market—like selling a portion of a paycheck to cover the tax bill.

restricted stock units financial

"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.

tax withholding obligations financial

"number of shares required to be sold by the Reporting Person to cover tax withholding obligations"

FAQ

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

What insider transaction did UPB disclose in this Form 4?

UPB disclosed that General Counsel Allison Ambrose sold 485 shares of common stock on September 16, 2026 at $5.49 per share in an automatic sell-to-cover transaction related to RSU vesting.

Why did the UPB General Counsel sell 485 shares of stock?

The filing states the 485-share sale was to cover tax withholding obligations arising from the vesting of restricted stock units. The sales were automatic under the issuer’s sell-to-cover policy and not at the discretion of General Counsel Allison Ambrose.

How many UPB shares does the General Counsel hold after this transaction?

After the September 16, 2026 transaction, General Counsel Allison Ambrose directly holds 15,563 shares of Upstream Bio, Inc. common stock, according to the Form 4 disclosure.

Was the UPB insider sale under a Rule 10b5-1 trading plan?

The Form 4 indicates no Rule 10b5-1 plan is reported. Instead, the sale was made under the issuer’s sell-to-cover policy to automatically satisfy tax withholding obligations tied to RSU vesting.

What was the sale price in the UPB insider transaction?

The Form 4 reports that the 485 shares of UPB common stock were sold at $5.49 per share on September 16, 2026 in an open-market or private transaction categorized as a sale.

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

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Available on EDGAR 09/17/2026 - 04:06 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^*

Ambrose Allison

(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)
	General Counsel

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)		485	D	$5.49	15,563	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	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)