Upstream Bio CMO sells 895 shares at $5.49

Upstream Bio’s chief medical officer reported an automatic sell-to-cover sale tied to RSU vesting, leaving him with over thirty‑two thousand UPB shares.

Rhea-AI Filing Summary

Upstream Bio, Inc. (UPB) reported that its Chief Medical Officer, Aaron Deykin, sold 895 shares of common stock on September 16, 2026 at $5.49 per share. According to the company’s disclosure, this was an automatic “sell-to-cover” transaction to satisfy tax withholding obligations from vesting restricted stock units and was not at his discretion. After the sale, he directly holds 32,302 shares of Upstream Bio common stock.

Insider Trade Summary

Net Seller: 895 shares

Net Sell 1 txn

Insider Deykin Aaron

Role Chief Medical Officer

Sold 895 shs ($5K)

Type	Security	Shares	Price	Value
Sale	Common Stock F1	895	$5.49	$5K

Holdings After Transaction: Common Stock — 32,302 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: 895 shares Sale price per share: $5.49 per share Shares owned after transaction: 32,302 shares +1 more

4 metrics

Shares sold 895 shares Common stock sold by Chief Medical Officer on September 16, 2026

Sale price per share $5.49 per share Price for the 895 shares of common stock sold

Shares owned after transaction 32,302 shares Direct common stock holdings of Aaron Deykin after the sale

Net shares sold 895 shares Net selling activity reported in this Form 4

Key Terms

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

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.

tax withholding obligations financial

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

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.

FAQ

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

What insider transaction did UPB report for Chief Medical Officer Aaron Deykin?

Upstream Bio reported that Chief Medical Officer Aaron Deykin sold 895 shares of common stock on September 16, 2026. The company states this was an automatic sell-to-cover transaction related to tax withholding on vesting restricted stock units.

How many UPB shares did the insider sell and at what price?

The filing shows a sale of 895 shares of Upstream Bio common stock at a price of $5.49 per share on September 16, 2026, classified as a sale in the open market or a private transaction.

How many Upstream Bio (UPB) shares does Aaron Deykin hold after this transaction?

After the reported transaction, Chief Medical Officer Aaron Deykin directly owns 32,302 shares of Upstream Bio common stock, as stated in the ownership line following the sale entry.

Why were the 895 UPB shares sold by the Chief Medical Officer?

The company states it has adopted a “sell-to-cover” policy, and the 895 shares were sold to cover tax withholding obligations arising from the vesting of restricted stock units. The sales were automatic and not at the reporting person’s discretion.

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

The filing indicates no Rule 10b5‑1 trading plan; the related checkbox is not marked. Instead, a company sell-to-cover policy governed the sale to satisfy tax withholding obligations tied to restricted stock unit vesting.

Is the reported UPB insider transaction a market sale or compensation-related?

The transaction is coded as a sale of common stock, but the footnote explains it was executed under a company sell-to-cover policy solely to satisfy tax withholding obligations from vesting restricted stock units, rather than a discretionary portfolio sale.

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^*

Deykin Aaron

(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 Medical 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)		895	D	$5.49	32,302	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)