Upstream Bio (UPB) GC auto-sells shares to cover RSU taxes

Rhea-AI Filing Summary

Upstream Bio, Inc. General Counsel Allison Ambrose reported an automatic sale of common stock tied to tax withholding on vested restricted stock units. On this transaction date, 477 shares were sold at $6.10 per share under a company "sell-to-cover" policy, and Ambrose held 16,048 shares of common stock afterward.

Insider Trade Summary

Net Seller: 477 shares ($2,910)

Net Sell 1 txn

Insider Ambrose Allison

Role General Counsel

Sold 477 shs ($3K)

Type	Security	Shares	Price	Value
Sale	Common Stock	477	$6.10	$3K

Holdings After Transaction: Common Stock — 16,048 shares (Direct, null)

Footnotes (1)

1. [object Object]

Key Figures

Shares sold: 477 shares Sale price: $6.10 per share Shares held after transaction: 16,048 shares +2 more

5 metrics

Shares sold 477 shares Common Stock sold in tax-related transaction

Sale price $6.10 per share Price for the 477 shares sold

Shares held after transaction 16,048 shares Post-transaction direct holdings of common stock

Net buy/sell shares -477 shares Net effect of reported transactions in this Form 4

Transaction count (sales) 1 sale Number of sell transactions reported

Key Terms

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

4 terms

sell-to-cover policy financial

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

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.

open-market sale financial

"transaction_action": "open-market sale""

An open-market sale is when a shareholder sells existing shares directly on a public exchange to any willing buyer, rather than through a private deal. Think of it like putting goods on a busy market stall where price is set by supply and demand; for investors it matters because such sales increase available supply, can put short-term downward pressure on the stock price, and signal changes in liquidity or investor confidence.

tax withholding obligations financial

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

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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^*

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)
06/16/2026

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	06/16/2026		S(1)		477	D	$6.1	16,048	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	06/17/2026
	** 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)

FAQ

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

Upstream Bio reported that General Counsel Allison Ambrose sold 477 shares of common stock at $6.10 per share. The sale was linked to tax withholding on vested restricted stock units under the company’s sell-to-cover policy and occurred automatically.

Was the Upstream Bio (UPB) insider sale by Allison Ambrose discretionary?

No, the filing states the sale was automatic under a company "sell-to-cover" policy. Shares were sold only to satisfy tax withholding obligations from vesting restricted stock units, meaning the timing and amount were not at Ambrose’s discretion.

How many Upstream Bio (UPB) shares did Allison Ambrose sell and at what price?

Allison Ambrose sold 477 shares of Upstream Bio common stock at $6.10 per share. The transaction was reported as an open-market sale but designated as tax-related, tied to the vesting of restricted stock units and associated withholding needs.

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

After the reported transaction, Allison Ambrose directly held 16,048 shares of Upstream Bio common stock. This post-transaction holding figure is disclosed in the Form 4 and reflects ownership following the automatic tax-related sale of 477 shares.

What is Upstream Bio’s sell-to-cover policy mentioned in the Form 4?

The company’s sell-to-cover policy requires selling a portion of vested shares to cover tax withholding obligations. In this case, the reported sale reflects only the number of Upstream Bio shares necessary to satisfy taxes on vested restricted stock units, executed automatically.