Upstream Bio (UPB) CMO auto-sells 894 shares to cover RSU taxes

Rhea-AI Filing Summary

Upstream Bio, Inc. reported that Chief Medical Officer Aaron Deykin executed an open-market sale of 894 shares of common stock at $9.29 per share on March 16, 2026. According to the company’s sell-to-cover policy, these shares were sold automatically to satisfy tax withholding obligations from vesting restricted stock units and were not sold at his discretion.

Following this tax-related sale, Deykin directly holds 32,687 shares of Upstream Bio common stock.

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^*

Deykin Aaron

(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 Medical 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)		894	D	$9.29	32,687	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) disclose for its Chief Medical Officer?

Upstream Bio disclosed that Chief Medical Officer Aaron Deykin sold 894 common shares at $9.29 each. The shares were sold automatically under a sell-to-cover policy to pay tax withholding on vesting restricted stock units, not as a discretionary sale.

Why did Upstream Bio (UPB) Chief Medical Officer Aaron Deykin sell 894 shares?

The 894 shares were sold to cover tax withholding obligations from vesting restricted stock units under Upstream Bio’s sell-to-cover policy. The filing states the sales were automatic and not made at Deykin’s discretion, indicating a routine tax-related transaction.

At what price were the 894 Upstream Bio (UPB) shares sold by the CMO?

The 894 Upstream Bio shares were sold at an average price of $9.29 per share. This open-market sale occurred on March 16, 2026, and was executed solely to satisfy tax withholding obligations linked to restricted stock unit vesting.

How many Upstream Bio (UPB) shares does the Chief Medical Officer hold after the reported sale?

After selling 894 shares for tax withholding, Chief Medical Officer Aaron Deykin directly holds 32,687 shares of Upstream Bio common stock. This remaining position is disclosed in the Form 4 as his direct ownership following the automatic sell-to-cover transaction.

Was the Upstream Bio (UPB) CMO’s sale of 894 shares a discretionary trade?

No. The filing states the sale was automatic under Upstream Bio’s sell-to-cover policy. The 894 shares were sold only to meet tax withholding obligations from restricted stock unit vesting, and the transaction was not at the reporting person’s discretion.