Exhibit 32.1
CERTIFICATION PURSUANT TO
18 U.S.C. SECTION 1350, AS ADOPTED PURSUANT TO
SECTION 906 OF THE SARBANES-OXLEY ACT OF 2002
In connection with the Quarterly Report of Upstream Bio, Inc. (the “Company”) on Form 10-Q for the period ending September 30, 2024 as filed with the Securities and Exchange Commission on the date hereof (the “Report”), the undersigned officers of the Company hereby certify, pursuant to 18 U.S.C. § 1350, as adopted pursuant to § 906 of the Sarbanes-Oxley Act of 2002, that:
(1)The Report fully complies with the requirements of section 13(a) or 15(d) of the Securities Exchange Act of 1934; and
(2)The information contained in the Report fairly presents, in all material respects, the financial condition and result of operations of the Company.
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Date: November 7, 2024 | | By: | /s/ E. Rand Sutherland |
| | | E. Rand Sutherland, M.D., M.P.H. |
| | | Chief Executive Officer (Principal Executive Officer) |
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Date: November 7, 2024 | | By: | /s/ Michael Paul Gray |
| | | Michael Paul Gray, M.B.A. |
| | | Chief Financial and Operating Officer (Principal Financial Officer and Principal Accounting Officer) |
