Amendments to Bylaws Form 8 K

Item 5.03 Amendments to Articles of Incorporation or Bylaws.

As previously disclosed in the Registration Statement on Form S-1, as amended (File No. 333-282197) (the "Registration Statement"), of Upstream Bio, Inc. (the "Company"), and in connection with the completion of the initial public offering of the Company's common stock (the "IPO"), on October 15, 2024, the Company filed its third amended and restated certificate of incorporation (the "Amended and Restated Certificate") with the Secretary of State of the State of Delaware. The Company's board of directors (the "Board") and the Company's stockholders previously approved the Amended and Restated Certificate to be filed in connection with, and to be effective immediately prior to, the completion of the IPO. The Amended and Restated Certificate amends and restates the Company's existing second amended and restated certificate of incorporation, as amended, in its entirety to, among other things: (i) authorize 500,000,000 shares of common stock; (ii) eliminate all references to the previously-existing series of preferred stock; and (iii) authorize 10,000,000 shares of undesignated preferred stock that may be issued from time to time by the Board in one or more series.

The foregoing description of the Amended and Restated Certificate is qualified by reference to the full Amended and Restated Certificate, a copy of which is attached hereto as Exhibit 3.1 and is incorporated herein by reference.

In addition, as previously disclosed in the Registration Statement, the second amended and restated bylaws of the Company (the "Amended and Restated Bylaws"), previously approved by the Board and the Company's stockholders, became effective as of the effectiveness of the Registration Statement on October 10, 2024. The Amended and Restated Bylaws amend and restate the Company's bylaws in their entirety to, among other things: (i) eliminate the ability of the Company's stockholders to take action by written consent in lieu of a meeting and call special meetings of stockholders; (ii) establish an advance notice procedure for stockholder proposals to be brought before an annual meeting of our stockholders, including proposed nominations of persons for election to our Board; and (iii) conform to the amended provisions of the Amended and Restated Certificate.

The foregoing description of the Amended and Restated Bylaws is qualified by reference to the full Amended and Restated Bylaws, a copy of which is attached hereto as Exhibit 3.2 and is incorporated herein by reference.