Statement of Changes in Beneficial Ownership - Form 4

Ownership Submission

FORM 4 Check this box if no longer subject to Section 16, Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP OF SECURITIES Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person * AI Upstream LLC 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 _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) C/O ACCESS INDUSTRIES, INC. , 40 WEST 57TH STREET, 28TH FLOOR 3. Date of Earliest Transaction (Month/Day/Year) (Street) NEW YORK NY 10019 4. If Amendment, Date Original Filed(Month/Day/Year) 6. Individual or Join/Group Filing(Check Applicable Line) ___ Form filed by One Reporting Person _X_ Form filed by More than One Reporting Person (City) (State) (Zip) Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned

1.Title of Security 2. Transaction Date (Month/Day/Year) 2A. Deemed Execution Date, if any (Month/Day/Year) 3. Transaction Code 4. Securities Acquired (A) or Disposed of (D) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) 6. Ownership Form: Direct (D) or Indirect (I) 7. Nature of Indirect Beneficial Ownership Code V Amount (A) or (D) Price

Table II - Derivative Securities Beneficially Owned (e.g., puts, calls, warrants, options, convertible securities)
(e.g., puts, calls, warrants, options, convertible securities)

1. Title of Derivate Security	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	5. Number of Derivative Securities Acquired (A) or Disposed of (D)	6. Date Exercisable and Expiration Date	7. Title and Amount of Securities Underlying Derivative Security	8. Price of Derivative Security	9. Number of derivative Securities Beneficially Owned Following Reported Transaction(s)	10. Ownership Form of Derivative Security: Direct (D) or Indirect (I)	11. Nature of Indirect Beneficial Ownership
Code	V	(A)	(D)	Date Exercisable	Expriation Date	Title	Amount or Number of Shares

Reporting Owners

Reporting Owner Name / Address	Relationships
Director	10% Owner	Officer	Other
AI Upstream LLC C/O ACCESS INDUSTRIES, INC. 40 WEST 57TH STREET, 28TH FLOOR NEW YORK, NY10019
AI Biotechnology LLC C/O ACCESS INDUSTRIES, INC. 40 WEST 57TH STREET, 28TH FLOOR NEW YORK, NY10019
Access Industries Holdings LLC C/O ACCESS INDUSTRIES, INC. 40 WEST 57TH STREET, 28TH FLOOR NEW YORK, NY10019
ACCESS INDUSTRIES MANAGEMENT, LLC C/O ACCESS INDUSTRIES, INC. 40 WEST 57TH STREET, 28TH FLOOR NEW YORK, NY10019
Blavatnik Len C/O ACCESS INDUSTRIES, INC. 40 WEST 57TH STREET, 28TH FLOOR NEW YORK, NY10019

Signatures

/s/ Alejandro Moreno for AI Upstream LLC	2024-10-17
**Signature of Reporting Person	Date
/s/ Alejandro Moreno for AI Biotechnology LLC	2024-10-17
**Signature of Reporting Person	Date
/s/ Alejandro Moreno for Access Industries Holdings LLC	2024-10-17
**Signature of Reporting Person	Date
/s/ Alejandro Moreno for Access Industries Management, LLC	2024-10-17
**Signature of Reporting Person	Date
/s/ Alejandro Moreno as Attorney-in-Fact for Mr. Blavatnik	2024-10-17
**Signature of Reporting Person	Date

Explanation of Responses:

(*)	If the form is filed by more than one reporting person, see Instruction 5(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).
(1)	Each share of the Issuer's Series A preferred stock and Series B preferred stock (collectively, the "Preferred Stock") converted into Common Stock on a 1.049-for-one basis upon closing of the Issuer's initial public offering without payment of consideration. The Preferred Stock had no expiration date.
(2)	The securities reported are held directly by AI Upstream LLC ("AI Upstream") and may be deemed to be beneficially owned by AI Biotechnology LLC ("AI Biotechnology"), Access Industries Holdings LLC ("AIH"), Access Industries Management, LLC ("AIM") and Len Blavatnik because (i) Mr. Blavatnik controls AIM and AIH, (ii) AIM controls AIH, (iii) AIH owns all of the voting units of AI Biotechnology and (iv) AI Biotechnology owns all of the voting units of AI Upstream. Each of the reporting persons (other than AI Upstream) disclaims beneficial ownership of these securities, except to the extent of its or his pecuniary interest therein, and this form shall not be construed as an admission that any such reporting person is the beneficial owner of any of the securities reported on this form.

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