<SEC-DOCUMENT>0001193125-24-239740.txt : 20241018
<SEC-HEADER>0001193125-24-239740.hdr.sgml : 20241018
<ACCEPTANCE-DATETIME>20241018181056
ACCESSION NUMBER:		0001193125-24-239740
CONFORMED SUBMISSION TYPE:	SC 13G
PUBLIC DOCUMENT COUNT:		1
FILED AS OF DATE:		20241018
DATE AS OF CHANGE:		20241018

SUBJECT COMPANY:	

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			Upstream Bio, Inc.
		CENTRAL INDEX KEY:			0002022626
		STANDARD INDUSTRIAL CLASSIFICATION:	PHARMACEUTICAL PREPARATIONS [2834]
		ORGANIZATION NAME:           	03 Life Sciences
		IRS NUMBER:				000000000
		STATE OF INCORPORATION:			DE
		FISCAL YEAR END:			1231

	FILING VALUES:
		FORM TYPE:		SC 13G
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	005-94651
		FILM NUMBER:		241381615

	BUSINESS ADDRESS:	
		STREET 1:		890 WINTER STREET, SUITE 200
		CITY:			WALTHAM
		STATE:			MA
		ZIP:			02451
		BUSINESS PHONE:		781-208-2466

	MAIL ADDRESS:	
		STREET 1:		890 WINTER STREET, SUITE 200
		CITY:			WALTHAM
		STATE:			MA
		ZIP:			02451

FILED BY:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			BCLS Fund III Investments, LP
		CENTRAL INDEX KEY:			0001952107
		ORGANIZATION NAME:           	
		IRS NUMBER:				873064173
		STATE OF INCORPORATION:			DE
		FISCAL YEAR END:			1231

	FILING VALUES:
		FORM TYPE:		SC 13G

	BUSINESS ADDRESS:	
		STREET 1:		200 CLARENDON STREET
		CITY:			BOSTON
		STATE:			MA
		ZIP:			02116
		BUSINESS PHONE:		6175162000

	MAIL ADDRESS:	
		STREET 1:		200 CLARENDON STREET
		CITY:			BOSTON
		STATE:			MA
		ZIP:			02116
</SEC-HEADER>
<DOCUMENT>
<TYPE>SC 13G
<SEQUENCE>1
<FILENAME>d814421dsc13g.htm
<DESCRIPTION>SC 13G
<TEXT>
<HTML><HEAD>
<TITLE>SC 13G</TITLE>
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<Center><DIV STYLE="width:8.5in" align="left">
 <P STYLE="line-height:1.0pt;margin-top:0pt;margin-bottom:0pt;border-bottom:1px solid #000000">&nbsp;</P>
<P STYLE="line-height:3.0pt;margin-top:0pt;margin-bottom:2pt;border-bottom:1px solid #000000">&nbsp;</P> <P STYLE="margin-top:4pt; margin-bottom:0pt; font-size:18pt; font-family:Times New Roman" ALIGN="center"><B>UNITED STATES </B></P>
<P STYLE="margin-top:0pt; margin-bottom:0pt; font-size:18pt; font-family:Times New Roman" ALIGN="center"><B>SECURITIES AND EXCHANGE COMMISSION </B></P>
<P STYLE="margin-top:0pt; margin-bottom:0pt; font-size:12pt; font-family:Times New Roman" ALIGN="center"><B>Washington, DC 20549 </B></P> <P STYLE="font-size:12pt;margin-top:0pt;margin-bottom:0pt">&nbsp;</P><center>
<P STYLE="line-height:6.0pt;margin-top:0pt;margin-bottom:2pt;border-bottom:1.00pt solid #000000;width:21%">&nbsp;</P></center> <P STYLE="margin-top:12pt; margin-bottom:0pt; font-size:18pt; font-family:Times New Roman" ALIGN="center"><B>SCHEDULE 13G
</B></P> <P STYLE="margin-top:0pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman" ALIGN="center"><B>(Rule <FONT STYLE="white-space:nowrap">13d-102)</FONT> </B></P>
<P STYLE="margin-top:12pt; margin-bottom:0pt; font-size:12pt; font-family:Times New Roman" ALIGN="center"><B>Information to be Included in Statements Filed Pursuant </B></P>
<P STYLE="margin-top:0pt; margin-bottom:0pt; font-size:12pt; font-family:Times New Roman" ALIGN="center"><B>to &#167; <FONT STYLE="white-space:nowrap">240.13d-1(b),</FONT> (c) and (d)&nbsp;and Amendments Thereto Filed </B></P>
<P STYLE="margin-top:0pt; margin-bottom:0pt; font-size:12pt; font-family:Times New Roman" ALIGN="center"><B>Pursuant to &#167; <FONT STYLE="white-space:nowrap">240.13d-2</FONT> </B></P>
<P STYLE="margin-top:12pt; margin-bottom:0pt; font-size:12pt; font-family:Times New Roman" ALIGN="center"><B>Under the Securities Exchange Act of 1934 </B></P>
<P STYLE="margin-top:0pt; margin-bottom:0pt; font-size:12pt; font-family:Times New Roman" ALIGN="center"><B>(Amendment No. ) </B></P> <P STYLE="font-size:12pt;margin-top:0pt;margin-bottom:0pt">&nbsp;</P><center>
<P STYLE="line-height:6.0pt;margin-top:0pt;margin-bottom:2pt;border-bottom:1.00pt solid #000000;width:21%">&nbsp;</P></center> <P STYLE="margin-top:12pt; margin-bottom:0pt; font-size:24pt; font-family:Times New Roman" ALIGN="center"><B>UPSTREAM BIO,
INC. </B></P> <P STYLE="margin-top:0pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman" ALIGN="center"><B>(Name of Issuer) </B></P> <P STYLE="font-size:12pt;margin-top:0pt;margin-bottom:0pt">&nbsp;</P><center>
<P STYLE="line-height:6.0pt;margin-top:0pt;margin-bottom:2pt;border-bottom:1.00pt solid #000000;width:21%">&nbsp;</P></center> <P STYLE="margin-top:12pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman" ALIGN="center"><B>Common Stock,
$0.001 </B></P> <P STYLE="margin-top:0pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman" ALIGN="center"><B>par value per share </B></P>
<P STYLE="margin-top:0pt; margin-bottom:0pt; font-size:8pt; font-family:Times New Roman" ALIGN="center"><B>(Title of Class&nbsp;of Securities) </B></P>
<P STYLE="margin-top:12pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman" ALIGN="center"><B>91678A107 </B></P> <P STYLE="margin-top:0pt; margin-bottom:0pt; font-size:8pt; font-family:Times New Roman" ALIGN="center"><B>(CUSIP Number)
</B></P> <P STYLE="margin-top:12pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman" ALIGN="center"><B>October&nbsp;15, 2024 </B></P>
<P STYLE="margin-top:0pt; margin-bottom:0pt; font-size:8pt; font-family:Times New Roman" ALIGN="center"><B>(Date of Event Which Requires Filing of This Statement) </B></P> <P STYLE="font-size:12pt;margin-top:0pt;margin-bottom:0pt">&nbsp;</P><center>
<P STYLE="line-height:6.0pt;margin-top:0pt;margin-bottom:2pt;border-bottom:1.00pt solid #000000;width:21%">&nbsp;</P></center> <P STYLE="margin-top:12pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman">Check the appropriate box to
designate the rule pursuant to which this Schedule is filed: </P> <P STYLE="margin-top:12pt; margin-bottom:0pt; margin-left:9%; font-size:10pt; font-family:Times New Roman">&#9744; Rule <FONT STYLE="white-space:nowrap">13d-1(b)</FONT> </P>
<P STYLE="margin-top:12pt; margin-bottom:0pt; margin-left:9%; font-size:10pt; font-family:Times New Roman">&#9746; Rule <FONT STYLE="white-space:nowrap">13d-1(c)</FONT> </P>
<P STYLE="margin-top:12pt; margin-bottom:0pt; margin-left:9%; font-size:10pt; font-family:Times New Roman">&#9744; Rule <FONT STYLE="white-space:nowrap">13d-1(d)</FONT> </P> <P STYLE="font-size:12pt;margin-top:0pt;margin-bottom:0pt">&nbsp;</P>
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<TD WIDTH="4%" VALIGN="top" ALIGN="left">*</TD>
<TD ALIGN="left" VALIGN="top"> <P STYLE=" margin-top:0pt ; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman; " ALIGN="left">The remainder of this cover page shall be filled out for a reporting person&#146;s initial filing on this form
with respect to the subject class of securities, and for any subsequent amendment containing information which would alter the disclosures provided in a prior cover page. </P></TD></TR></TABLE>
<P STYLE="margin-top:12pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman">The information required in the remainder of this cover page shall not be deemed to be &#147;filed&#148; for the purpose of Section&nbsp;18 of the Securities
Exchange Act of 1934, as amended (&#147;Act&#148;), or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes). </P>
<P STYLE="font-size:10pt;margin-top:0pt;margin-bottom:0pt">&nbsp;</P> <P STYLE="line-height:1.0pt;margin-top:0pt;margin-bottom:0pt;border-bottom:1px solid #000000">&nbsp;</P>
<P STYLE="line-height:3.0pt;margin-top:0pt;margin-bottom:2pt;border-bottom:1px solid #000000">&nbsp;</P>
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<TD VALIGN="top">CUSIP No. 91678A107</TD>
<TD VALIGN="bottom">&nbsp;&nbsp;</TD>
<TD VALIGN="top" ALIGN="center">13G</TD>
<TD VALIGN="bottom">&nbsp;&nbsp;</TD>
<TD VALIGN="top" ALIGN="right"><B></B> Page
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<TD VALIGN="top" STYLE="BORDER-LEFT:1px solid #000000; BORDER-TOP:1px solid #000000; BORDER-BOTTOM:1px solid #000000; padding-left:8pt">&#8199;1&#8194;</TD>
<TD VALIGN="bottom" STYLE=" BORDER-LEFT:1px solid #000000; BORDER-TOP:1px solid #000000; BORDER-BOTTOM:1px solid #000000">&nbsp;</TD>
<TD VALIGN="top" COLSPAN="5" STYLE="BORDER-TOP:1px solid #000000; BORDER-RIGHT:1px solid #000000; BORDER-BOTTOM:1px solid #000000; padding-right:2pt">
<P STYLE="margin-top:0pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman">&#8194;Names of Reporting Persons</P> <P STYLE="font-size:12pt; margin-top:0pt; margin-bottom:0pt">&nbsp;</P>
<P STYLE="margin-top:0pt; margin-bottom:1pt; font-size:10pt; font-family:Times New Roman">&#8194;BCLS Fund III Investments, LP</P></TD></TR>
<TR STYLE="page-break-inside:avoid ; font-family:Times New Roman; font-size:10pt">
<TD VALIGN="top" STYLE="BORDER-LEFT:1px solid #000000; BORDER-BOTTOM:1px solid #000000; padding-left:8pt">&#8199;2</TD>
<TD VALIGN="bottom" STYLE=" BORDER-LEFT:1px solid #000000; BORDER-BOTTOM:1px solid #000000">&nbsp;</TD>
<TD VALIGN="top" COLSPAN="5" STYLE="BORDER-RIGHT:1px solid #000000; BORDER-BOTTOM:1px solid #000000; padding-right:2pt"> <P STYLE="margin-top:0pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman">&#8194;Check the Appropriate Box if a
Member of a Group</P> <P STYLE="font-size:12pt; margin-top:0pt; margin-bottom:0pt">&nbsp;</P>
<P STYLE="margin-top:0pt; margin-bottom:1pt; font-size:10pt; font-family:Times New Roman">&#8194;(a)&#8194;&#9744;&#8195;&#8195;(b)&#8194;&#9744;</P></TD></TR>
<TR STYLE="page-break-inside:avoid ; font-family:Times New Roman; font-size:10pt">
<TD VALIGN="top" STYLE="BORDER-LEFT:1px solid #000000; BORDER-BOTTOM:1px solid #000000; padding-left:8pt">&#8199;3</TD>
<TD VALIGN="bottom" STYLE=" BORDER-LEFT:1px solid #000000; BORDER-BOTTOM:1px solid #000000">&nbsp;</TD>
<TD VALIGN="top" COLSPAN="5" STYLE="BORDER-RIGHT:1px solid #000000; BORDER-BOTTOM:1px solid #000000; padding-right:2pt"> <P STYLE="margin-top:0pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman">&#8194;SEC Use Only</P>
<P STYLE="font-size:12pt;margin-top:0pt;margin-bottom:1pt">&nbsp;</P></TD></TR>
<TR STYLE="page-break-inside:avoid ; font-family:Times New Roman; font-size:10pt">
<TD VALIGN="top" STYLE="BORDER-LEFT:1px solid #000000; BORDER-BOTTOM:1px solid #000000; padding-left:8pt">&#8199;4</TD>
<TD VALIGN="bottom" STYLE=" BORDER-LEFT:1px solid #000000; BORDER-BOTTOM:1px solid #000000">&nbsp;</TD>
<TD VALIGN="top" COLSPAN="5" STYLE="BORDER-RIGHT:1px solid #000000; BORDER-BOTTOM:1px solid #000000; padding-right:2pt"> <P STYLE="margin-top:0pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman">&#8194;Citizenship or Place of
Organization</P> <P STYLE="font-size:12pt; margin-top:0pt; margin-bottom:0pt">&nbsp;</P>
<P STYLE="margin-top:0pt; margin-bottom:1pt; font-size:10pt; font-family:Times New Roman">&#8194;Delaware</P></TD></TR></TABLE>
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<TD WIDTH="2%"></TD>

<TD VALIGN="bottom" WIDTH="1%"></TD>
<TD WIDTH="6%"></TD>

<TD VALIGN="bottom" WIDTH="1%"></TD>
<TD WIDTH="2%"></TD>

<TD VALIGN="bottom" WIDTH="1%"></TD>
<TD WIDTH="87%"></TD></TR>

<TR STYLE="page-break-inside:avoid ; font-family:Times New Roman; font-size:10pt">
<TD VALIGN="middle" COLSPAN="3" ROWSPAN="4" ALIGN="center" STYLE="BORDER-LEFT:1px solid #000000; padding-left:8pt"> <P STYLE="margin-top:0pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman" ALIGN="center">Number&nbsp;of</P>
<P STYLE="margin-top:0pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman" ALIGN="center">Shares &#8201;Beneficially&#8201;</P>
<P STYLE="margin-top:0pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman" ALIGN="center">Owned&nbsp;by</P> <P STYLE="margin-top:0pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman" ALIGN="center">Each</P>
<P STYLE="margin-top:0pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman" ALIGN="center">Reporting</P> <P STYLE="margin-top:0pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman" ALIGN="center">Person</P>
<P STYLE="margin-top:0pt; margin-bottom:1pt; font-size:10pt; font-family:Times New Roman" ALIGN="center">With</P></TD>
<TD VALIGN="bottom" STYLE=" BORDER-LEFT:1px solid #000000; BORDER-BOTTOM:1px solid #000000">&nbsp;&nbsp;</TD>
<TD VALIGN="top" STYLE="BORDER-BOTTOM:1px solid #000000">5&#8194;</TD>
<TD VALIGN="bottom" STYLE=" BORDER-LEFT:1px solid #000000; BORDER-BOTTOM:1px solid #000000">&nbsp;</TD>
<TD VALIGN="top" STYLE="BORDER-RIGHT:1px solid #000000; BORDER-BOTTOM:1px solid #000000; padding-right:2pt"> <P STYLE="margin-top:0pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman">&#8194;Sole Voting Power</P>
<P STYLE="font-size:12pt; margin-top:0pt; margin-bottom:0pt">&nbsp;</P> <P STYLE="margin-top:0pt; margin-bottom:1pt; font-size:10pt; font-family:Times New Roman">&#8194;0 shares of Common Stock</P></TD></TR>
<TR STYLE="page-break-inside:avoid ; font-family:Times New Roman; font-size:10pt">
<TD VALIGN="bottom" STYLE=" BORDER-LEFT:1px solid #000000; BORDER-BOTTOM:1px solid #000000">&nbsp;&nbsp;</TD>
<TD VALIGN="top" STYLE="BORDER-BOTTOM:1px solid #000000">6</TD>
<TD VALIGN="bottom" STYLE=" BORDER-LEFT:1px solid #000000; BORDER-BOTTOM:1px solid #000000">&nbsp;</TD>
<TD VALIGN="top" STYLE="BORDER-RIGHT:1px solid #000000; BORDER-BOTTOM:1px solid #000000; padding-right:2pt"> <P STYLE="margin-top:0pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman">&#8194;Shared Voting Power</P>
<P STYLE="font-size:12pt; margin-top:0pt; margin-bottom:0pt">&nbsp;</P> <P STYLE="margin-top:0pt; margin-bottom:1pt; font-size:10pt; font-family:Times New Roman">&#8194;2,295,322 shares of Common Stock</P></TD></TR>
<TR STYLE="page-break-inside:avoid ; font-family:Times New Roman; font-size:10pt">
<TD VALIGN="bottom" STYLE=" BORDER-LEFT:1px solid #000000; BORDER-BOTTOM:1px solid #000000">&nbsp;&nbsp;</TD>
<TD VALIGN="top" STYLE="BORDER-BOTTOM:1px solid #000000">7</TD>
<TD VALIGN="bottom" STYLE=" BORDER-LEFT:1px solid #000000; BORDER-BOTTOM:1px solid #000000">&nbsp;</TD>
<TD VALIGN="top" STYLE="BORDER-RIGHT:1px solid #000000; BORDER-BOTTOM:1px solid #000000; padding-right:2pt"> <P STYLE="margin-top:0pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman">&#8194;Sole Dispositive Power</P>
<P STYLE="font-size:12pt; margin-top:0pt; margin-bottom:0pt">&nbsp;</P> <P STYLE="margin-top:0pt; margin-bottom:1pt; font-size:10pt; font-family:Times New Roman">&#8194;0 shares of Common Stock</P></TD></TR>
<TR STYLE="page-break-inside:avoid ; font-family:Times New Roman; font-size:10pt">
<TD VALIGN="bottom" STYLE=" BORDER-LEFT:1px solid #000000">&nbsp;&nbsp;</TD>
<TD VALIGN="top">8</TD>
<TD VALIGN="bottom" STYLE=" BORDER-LEFT:1px solid #000000">&nbsp;</TD>
<TD VALIGN="top" STYLE="BORDER-RIGHT:1px solid #000000; padding-right:2pt"> <P STYLE="margin-top:0pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman">&#8194;Shared Dispositive Power</P>
<P STYLE="font-size:12pt; margin-top:0pt; margin-bottom:0pt">&nbsp;</P> <P STYLE="margin-top:0pt; margin-bottom:1pt; font-size:10pt; font-family:Times New Roman">&#8194;2,295,322 shares of Common
Stock</P></TD></TR></TABLE>
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<TD WIDTH="4%"></TD>

<TD VALIGN="bottom" WIDTH="1%"></TD>
<TD WIDTH="6%"></TD>

<TD VALIGN="bottom" WIDTH="1%"></TD>
<TD></TD>

<TD VALIGN="bottom" WIDTH="1%"></TD>
<TD WIDTH="87%"></TD></TR>

<TR STYLE="page-break-inside:avoid ; font-family:Times New Roman; font-size:10pt">
<TD VALIGN="top" STYLE="BORDER-LEFT:1px solid #000000; BORDER-TOP:1px solid #000000; BORDER-BOTTOM:1px solid #000000; padding-left:8pt">&#8199;9&#8194;</TD>
<TD VALIGN="bottom" STYLE=" BORDER-LEFT:1px solid #000000; BORDER-TOP:1px solid #000000; BORDER-BOTTOM:1px solid #000000">&nbsp;</TD>
<TD VALIGN="top" COLSPAN="5" STYLE="BORDER-TOP:1px solid #000000; BORDER-RIGHT:1px solid #000000; BORDER-BOTTOM:1px solid #000000; padding-right:2pt">
<P STYLE="margin-top:0pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman">&#8194;Aggregate Amount Beneficially Owned by Each Reporting Person</P> <P STYLE="font-size:12pt; margin-top:0pt; margin-bottom:0pt">&nbsp;</P>
<P STYLE="margin-top:0pt; margin-bottom:1pt; font-size:10pt; font-family:Times New Roman">&#8194;2,295,322 shares of Common Stock</P></TD></TR>
<TR STYLE="page-break-inside:avoid ; font-family:Times New Roman; font-size:10pt">
<TD VALIGN="top" STYLE="BORDER-LEFT:1px solid #000000; BORDER-BOTTOM:1px solid #000000; padding-left:8pt">10</TD>
<TD VALIGN="bottom" STYLE=" BORDER-LEFT:1px solid #000000; BORDER-BOTTOM:1px solid #000000">&nbsp;</TD>
<TD VALIGN="top" COLSPAN="5" STYLE="BORDER-RIGHT:1px solid #000000; BORDER-BOTTOM:1px solid #000000; padding-right:2pt"> <P STYLE="margin-top:0pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman">&#8194;Check if the Aggregate Amount
in Row (9) Excludes Certain Shares</P> <P STYLE="font-size:12pt;margin-top:0pt;margin-bottom:0pt">&nbsp;</P> <P STYLE="margin-top:0pt; margin-bottom:1pt; font-size:10pt; font-family:Times New Roman">&#8194;&#9744;</P></TD></TR>
<TR STYLE="page-break-inside:avoid ; font-family:Times New Roman; font-size:10pt">
<TD VALIGN="top" STYLE="BORDER-LEFT:1px solid #000000; BORDER-BOTTOM:1px solid #000000; padding-left:8pt">11</TD>
<TD VALIGN="bottom" STYLE=" BORDER-LEFT:1px solid #000000; BORDER-BOTTOM:1px solid #000000">&nbsp;</TD>
<TD VALIGN="top" COLSPAN="5" STYLE="BORDER-RIGHT:1px solid #000000; BORDER-BOTTOM:1px solid #000000; padding-right:2pt"> <P STYLE="margin-top:0pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman">&#8194;Percent of Class Represented by
Amount in Row 9</P> <P STYLE="font-size:12pt; margin-top:0pt; margin-bottom:0pt">&nbsp;</P> <P STYLE="margin-top:0pt; margin-bottom:1pt; font-size:10pt; font-family:Times New Roman">&#8194;4.3%</P></TD></TR>
<TR STYLE="page-break-inside:avoid ; font-family:Times New Roman; font-size:10pt">
<TD VALIGN="top" STYLE="BORDER-LEFT:1px solid #000000; BORDER-BOTTOM:1px solid #000000; padding-left:8pt">12</TD>
<TD VALIGN="bottom" STYLE=" BORDER-LEFT:1px solid #000000; BORDER-BOTTOM:1px solid #000000">&nbsp;</TD>
<TD VALIGN="top" COLSPAN="5" STYLE="BORDER-RIGHT:1px solid #000000; BORDER-BOTTOM:1px solid #000000; padding-right:2pt"> <P STYLE="margin-top:0pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman">&#8194;Type of Reporting Person</P>
<P STYLE="font-size:12pt; margin-top:0pt; margin-bottom:0pt">&nbsp;</P> <P STYLE="margin-top:0pt; margin-bottom:1pt; font-size:10pt; font-family:Times New Roman">&#8194;PN</P></TD></TR>
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<TD VALIGN="top">CUSIP No. 91678A107</TD>
<TD VALIGN="bottom">&nbsp;&nbsp;</TD>
<TD VALIGN="top" ALIGN="center">13G</TD>
<TD VALIGN="bottom">&nbsp;&nbsp;</TD>
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<TD WIDTH="10%" VALIGN="top" ALIGN="left"><B>Item&#8201;1(a)</B></TD>
<TD ALIGN="left" VALIGN="top"> <P STYLE=" margin-top:0pt ; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman; " ALIGN="left"><B>Name of Issuer </B></P></TD></TR></TABLE>
<P STYLE="margin-top:6pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman">The name of the issuer to which this filing on Schedule 13G relates is Upstream Bio, Inc. (the &#147;Issuer&#148;). </P>
<P STYLE="font-size:18pt;margin-top:0pt;margin-bottom:0pt">&nbsp;</P>
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<TD WIDTH="10%" VALIGN="top" ALIGN="left"><B>Item&#8201;1(b)</B></TD>
<TD ALIGN="left" VALIGN="top"> <P STYLE=" margin-top:0pt ; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman; " ALIGN="left"><B>Address of Issuer&#146;s Principal Executive Offices </B></P></TD></TR></TABLE>
<P STYLE="margin-top:6pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman">The principal executive offices of the Issuer are located at 890 Winter Street, Suite 200, Waltham, Massachusetts 02451. </P>
<P STYLE="font-size:18pt;margin-top:0pt;margin-bottom:0pt">&nbsp;</P>
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<TD WIDTH="10%" VALIGN="top" ALIGN="left"><B>Item&#8201;2(a)</B></TD>
<TD ALIGN="left" VALIGN="top"> <P STYLE=" margin-top:0pt ; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman; " ALIGN="left"><B>Name of Person Filing </B></P></TD></TR></TABLE>
<P STYLE="margin-top:6pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman">This Schedule 13G is being filed by BCLS Fund III Investments, LP, a Delaware limited partnership (the &#147;Reporting Person&#148;). </P>
<P STYLE="margin-top:12pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman">Bain Capital Life Sciences Investors, LLC, a Delaware limited liability company (&#147;BCLSI&#148;), is the manager of Bain Capital Life Sciences III General
Partner, LLC, a Delaware limited liability company (&#147;BCLS III GP&#148;), which is the general partner of Bain Capital Life Sciences Fund III, L.P., a Delaware limited partnership (&#147;BCLS Fund III&#148;), which is the managing member of BCLS
Fund III Investments GP, LLC, a Delaware limited liability company (together with the Reporting Person, BCLSI, BCLS III GP and BCLS Fund III, the &#147;Bain Capital Life Sciences Entities&#148;), which is the general partner of the Reporting Person.
As a result, BCLSI may be deemed to share voting and dispositive power with respect to the securities held by the Reporting Person. </P> <P STYLE="font-size:18pt;margin-top:0pt;margin-bottom:0pt">&nbsp;</P>
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<TD WIDTH="10%" VALIGN="top" ALIGN="left"><B>Item&#8201;2(b)</B></TD>
<TD ALIGN="left" VALIGN="top"> <P STYLE=" margin-top:0pt ; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman; " ALIGN="left"><B>Address of Principal Business Office or, if None, Residence </B></P></TD></TR></TABLE>
<P STYLE="margin-top:6pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman">The principal business address for each of the Bain Capital Life Sciences Entities is 200 Clarendon Street, Boston, Massachusetts 02116. </P>
<P STYLE="font-size:18pt;margin-top:0pt;margin-bottom:0pt">&nbsp;</P>
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<TD WIDTH="10%" VALIGN="top" ALIGN="left"><B>Item&#8201;2(c)</B></TD>
<TD ALIGN="left" VALIGN="top"> <P STYLE=" margin-top:0pt ; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman; " ALIGN="left"><B>Citizenship </B></P></TD></TR></TABLE>
<P STYLE="margin-top:6pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman">Each of the Bain Capital Life Sciences Entities is organized under the laws of the State of Delaware. </P>
<P STYLE="font-size:18pt;margin-top:0pt;margin-bottom:0pt">&nbsp;</P>
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<TD WIDTH="10%" VALIGN="top" ALIGN="left"><B>Item&#8201;2(d)</B></TD>
<TD ALIGN="left" VALIGN="top"> <P STYLE=" margin-top:0pt ; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman; " ALIGN="left"><B>Title of Class&nbsp;of Securities </B></P></TD></TR></TABLE>
<P STYLE="margin-top:6pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman">The class of securities of the Issuer to which this Schedule 13G relates is Common Stock, $0.001 par value per share (&#147;Common Stock&#148;). </P>
<P STYLE="font-size:18pt;margin-top:0pt;margin-bottom:0pt">&nbsp;</P>
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<TD WIDTH="10%" VALIGN="top" ALIGN="left"><B>Item&#8201;2(e)</B></TD>
<TD ALIGN="left" VALIGN="top"> <P STYLE=" margin-top:0pt ; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman; " ALIGN="left"><B>CUSIP Number </B></P></TD></TR></TABLE>
<P STYLE="margin-top:6pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman">The CUSIP number of the Common Stock is 91678A107. </P> <P STYLE="font-size:18pt;margin-top:0pt;margin-bottom:0pt">&nbsp;</P>
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<TD WIDTH="10%" VALIGN="top" ALIGN="left"><B>Item&#8201;3</B></TD>
<TD ALIGN="left" VALIGN="top"> <P STYLE=" margin-top:0pt ; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman; " ALIGN="left"><B>If this statement is filed pursuant to &#167;&#167; <FONT STYLE="white-space:nowrap">240.13d-1(b),</FONT> or
<FONT STYLE="white-space:nowrap">240.13d-2(b)</FONT> or (c), check whether the person filing is a </B></P></TD></TR></TABLE> <P STYLE="font-size:12pt;margin-top:0pt;margin-bottom:0pt">&nbsp;</P>
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<TD VALIGN="bottom">(a)</TD>
<TD VALIGN="bottom">&nbsp;</TD>
<TD VALIGN="bottom">&#9744;</TD>
<TD VALIGN="bottom">&nbsp;&nbsp;</TD>
<TD VALIGN="bottom">Broker or dealer registered under section 15 of the Act (15 U.S.C. 78o);</TD></TR>
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<TD VALIGN="bottom">(b)</TD>
<TD VALIGN="bottom">&nbsp;</TD>
<TD VALIGN="bottom">&#9744;</TD>
<TD VALIGN="bottom">&nbsp;&nbsp;</TD>
<TD VALIGN="bottom">Bank as defined in section 3(a)(6) of the Act (15 U.S.C. 78c);</TD></TR>
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<TD VALIGN="bottom">(c)</TD>
<TD VALIGN="bottom">&nbsp;</TD>
<TD VALIGN="bottom">&#9744;</TD>
<TD VALIGN="bottom">&nbsp;&nbsp;</TD>
<TD VALIGN="bottom">Insurance company as defined in section 3(a)(19) of the Act (15 U.S.C. 78c);</TD></TR>
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<TD HEIGHT="8"></TD>
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<TD VALIGN="bottom">(d)</TD>
<TD VALIGN="bottom">&nbsp;</TD>
<TD VALIGN="bottom">&#9744;</TD>
<TD VALIGN="bottom">&nbsp;&nbsp;</TD>
<TD VALIGN="bottom">Investment company registered under Section&nbsp;8 of the Investment Company Act of 1940 (15 U.S.C. <FONT STYLE="white-space:nowrap">80a-8);</FONT></TD></TR>
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<TD VALIGN="bottom">(e)</TD>
<TD VALIGN="bottom">&nbsp;</TD>
<TD VALIGN="bottom">&#9744;</TD>
<TD VALIGN="bottom">&nbsp;&nbsp;</TD>
<TD VALIGN="bottom">An investment adviser in accordance with &#167; <FONT STYLE="white-space:nowrap">240.13d-1(b)(1)(ii)(E);</FONT></TD></TR>
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<TD VALIGN="bottom">(f)</TD>
<TD VALIGN="bottom">&nbsp;</TD>
<TD VALIGN="bottom">&#9744;</TD>
<TD VALIGN="bottom">&nbsp;&nbsp;</TD>
<TD VALIGN="bottom">An employee benefit plan or endowment fund in accordance with &#167; <FONT STYLE="white-space:nowrap">240.13d-1(b)(1)(ii)(F);</FONT></TD></TR>
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<TD VALIGN="top">CUSIP No. 91678A107</TD>
<TD VALIGN="bottom">&nbsp;&nbsp;</TD>
<TD VALIGN="top" ALIGN="center">13G</TD>
<TD VALIGN="bottom">&nbsp;&nbsp;</TD>
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<TD VALIGN="top" NOWRAP> <P STYLE=" margin-top:0pt ; margin-bottom:0pt; margin-left:1.00em; text-indent:-1.00em; font-size:10pt; font-family:Times New Roman">(g)</P></TD>
<TD VALIGN="bottom">&nbsp;</TD>
<TD VALIGN="top">&#9744;</TD>
<TD VALIGN="bottom">&nbsp;&nbsp;</TD>
<TD VALIGN="bottom">A parent holding company or control person in accordance with &#167; <FONT STYLE="white-space:nowrap">240.13d-1(b)(1)(ii)(G);</FONT></TD></TR>
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<TD VALIGN="top" NOWRAP> <P STYLE=" margin-top:0pt ; margin-bottom:0pt; margin-left:1.00em; text-indent:-1.00em; font-size:10pt; font-family:Times New Roman">(h)</P></TD>
<TD VALIGN="bottom">&nbsp;</TD>
<TD VALIGN="top">&#9744;</TD>
<TD VALIGN="bottom">&nbsp;&nbsp;</TD>
<TD VALIGN="bottom">A savings association as defined in Section&nbsp;3(b) of the Federal Deposit Insurance Act (12 U.S.C. 1813);</TD></TR>
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<TD VALIGN="top" NOWRAP> <P STYLE=" margin-top:0pt ; margin-bottom:0pt; margin-left:1.00em; text-indent:-1.00em; font-size:10pt; font-family:Times New Roman">(i)</P></TD>
<TD VALIGN="bottom">&nbsp;</TD>
<TD VALIGN="top">&#9744;</TD>
<TD VALIGN="bottom">&nbsp;&nbsp;</TD>
<TD VALIGN="bottom">A church plan that is excluded from the definition of an investment company under section 3(c)(14) of the Investment Company Act (15 U.S.C. <FONT STYLE="white-space:nowrap">80a-3);</FONT></TD></TR>
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<TD VALIGN="top" NOWRAP> <P STYLE=" margin-top:0pt ; margin-bottom:0pt; margin-left:1.00em; text-indent:-1.00em; font-size:10pt; font-family:Times New Roman">(j)</P></TD>
<TD VALIGN="bottom">&nbsp;</TD>
<TD VALIGN="top">&#9744;</TD>
<TD VALIGN="bottom">&nbsp;&nbsp;</TD>
<TD VALIGN="bottom">A <FONT STYLE="white-space:nowrap">non-U.S.</FONT> institution in accordance with &#167; <FONT STYLE="white-space:nowrap">240.13d-1(b)(1)(ii)(J);</FONT></TD></TR>
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<TD VALIGN="top" NOWRAP> <P STYLE=" margin-top:0pt ; margin-bottom:0pt; margin-left:1.00em; text-indent:-1.00em; font-size:10pt; font-family:Times New Roman">(k)</P></TD>
<TD VALIGN="bottom">&nbsp;</TD>
<TD VALIGN="top">&#9744;</TD>
<TD VALIGN="bottom">&nbsp;&nbsp;</TD>
<TD VALIGN="bottom">Group, in accordance with &#167; <FONT STYLE="white-space:nowrap">240.13d-1(b)(1)(ii)(K).</FONT></TD></TR>
</TABLE> <P STYLE="margin-top:12pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman">If filing as a <FONT STYLE="white-space:nowrap">non-U.S.</FONT> institution in accordance with &#167; <FONT
STYLE="white-space:nowrap">240.13d-1(b)(1)(ii)(J),</FONT> please specify the type of institution: </P> <P STYLE="font-size:18pt;margin-top:0pt;margin-bottom:0pt">&nbsp;</P>
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<TD WIDTH="10%" VALIGN="top" ALIGN="left"><B>Item&#8201;4</B></TD>
<TD ALIGN="left" VALIGN="top"> <P STYLE=" margin-top:0pt ; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman; " ALIGN="left"><B>Ownership </B></P></TD></TR></TABLE>
<P STYLE="margin-top:6pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman"><B>(a) Amount beneficially owned: </B></P> <P STYLE="margin-top:6pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman">As of October&nbsp;15,
2024, the Reporting Person held 2,295,322 shares of Common Stock, representing 4.3% of the outstanding shares of Common Stock. </P> <P STYLE="margin-top:12pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman">The percentage of the
outstanding shares of Common Stock held by the Reporting Person is based on 53,591,695 shares of Common Stock issued and outstanding following the closing of the Issuer&#146;s initial public offering (inclusive of the exercise in full of the
underwriters option to purchase additional shares), as set forth in the prospectus filed by the Issuer with the Securities and Exchange Commission pursuant to Rule 424(b) promulgated under the Securities Act of 1933, as amended, on October&nbsp;11,
2024. </P> <P STYLE="margin-top:6pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman"><B>(b) Percent of class: </B></P> <P STYLE="margin-top:6pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman">See Item 4(a) hereof.
</P> <P STYLE="margin-top:6pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman"><B>(c) Number of shares as to which the person has: </B></P> <P STYLE="margin-top:6pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman">(i)
Sole power to vote or to direct the vote: </P> <P STYLE="margin-top:18pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman">0 shares of Common Stock </P>
<P STYLE="margin-top:6pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman">(ii) Shared power to vote or to direct the vote: </P> <P STYLE="margin-top:18pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman">2,295,322
shares of Common Stock </P> <P STYLE="margin-top:6pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman">(iii) Sole power to dispose or to direct the disposition of: </P>
<P STYLE="margin-top:18pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman">0 shares of Common Stock </P> <P STYLE="margin-top:6pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman">(iv) Shared power to dispose or to
direct the disposition of: </P> <P STYLE="margin-top:18pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman">2,295,322 shares of Common Stock </P> <P STYLE="font-size:6pt;margin-top:0pt;margin-bottom:0pt">&nbsp;</P>
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<TD ALIGN="left" VALIGN="top"> <P STYLE=" margin-top:0pt ; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman; " ALIGN="left"><B>Ownership of Five Percent or Less of a Class </B></P></TD></TR></TABLE>
<P STYLE="margin-top:6pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman">If this statement is being filed to report the fact that as of the date hereof the Reporting Person ceased to be the beneficial owner of more than five percent
of the class of securities, check the following: &#9746; </P> <P STYLE="font-size:18pt;margin-top:0pt;margin-bottom:0pt">&nbsp;</P>
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<TD ALIGN="left" VALIGN="top"> <P STYLE=" margin-top:0pt ; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman; " ALIGN="left"><B>Ownership of More Than Five Percent on Behalf of Another Person </B></P></TD></TR></TABLE>
<P STYLE="margin-top:6pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman">Not applicable. </P>
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<TD VALIGN="top">CUSIP No. 91678A107</TD>
<TD VALIGN="bottom">&nbsp;&nbsp;</TD>
<TD VALIGN="top" ALIGN="center">13G</TD>
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<TD ALIGN="left" VALIGN="top"> <P STYLE=" margin-top:0pt ; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman; " ALIGN="left"><B>Identification and Classification of the Subsidiary Which Acquired the Security Being Reported on By the
Parent Holding Company or Control Person </B></P></TD></TR></TABLE> <P STYLE="margin-top:6pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman">Not applicable. </P> <P STYLE="font-size:18pt;margin-top:0pt;margin-bottom:0pt">&nbsp;</P>
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<TD WIDTH="10%" VALIGN="top" ALIGN="left"><B>Item&#8201;8</B></TD>
<TD ALIGN="left" VALIGN="top"> <P STYLE=" margin-top:0pt ; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman; " ALIGN="left"><B>Identification and Classification of Members of the Group </B></P></TD></TR></TABLE>
<P STYLE="margin-top:6pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman">Not applicable. </P> <P STYLE="font-size:18pt;margin-top:0pt;margin-bottom:0pt">&nbsp;</P>
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<TD ALIGN="left" VALIGN="top"> <P STYLE=" margin-top:0pt ; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman; " ALIGN="left"><B>Notice of Dissolution of Group </B></P></TD></TR></TABLE>
<P STYLE="margin-top:6pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman">Not applicable. </P> <P STYLE="font-size:18pt;margin-top:0pt;margin-bottom:0pt">&nbsp;</P>
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<TD WIDTH="10%" VALIGN="top" ALIGN="left"><B>Item&#8201;10</B></TD>
<TD ALIGN="left" VALIGN="top"> <P STYLE=" margin-top:0pt ; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman; " ALIGN="left"><B>Certifications </B></P></TD></TR></TABLE>
<P STYLE="margin-top:6pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman">By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were not acquired and are not held for the purpose of
or with the effect of changing or influencing the control of the Issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect, other than activities solely in
connection with a nomination under &#167; <FONT STYLE="white-space:nowrap">240.14a-11.</FONT> </P>
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<TD VALIGN="top">CUSIP No. 91678A107</TD>
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<TD VALIGN="top" ALIGN="center">13G</TD>
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<TD VALIGN="top" ALIGN="right"><B></B> Page
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 <P STYLE="margin-top:0pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman" ALIGN="center"><B>SIGNATURES </B></P>
<P STYLE="margin-top:12pt; margin-bottom:0pt; font-size:10pt; font-family:Times New Roman">After reasonable inquiry and to the best of its knowledge and belief, the undersigned certifies that the information in this statement is true, complete and
correct. </P> <P STYLE="font-size:12pt;margin-top:0pt;margin-bottom:0pt">&nbsp;</P>
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<TD VALIGN="top">Dated: October&nbsp;18, 2024</TD>
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<TD VALIGN="top" COLSPAN="5"><B>BCLS Fund III Investments, LP</B></TD></TR>
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<TD VALIGN="bottom">&nbsp;</TD>
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<TD VALIGN="bottom">&nbsp;</TD>
<TD VALIGN="top">By:</TD>
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<TD VALIGN="top" COLSPAN="3">BCLS Fund III Investments GP, LLC,</TD></TR>
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<TD VALIGN="top"></TD>
<TD VALIGN="bottom">&nbsp;</TD>
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<TD VALIGN="bottom">&nbsp;</TD>
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<TD VALIGN="top" COLSPAN="3">its general partner</TD></TR>
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<TD VALIGN="bottom">&nbsp;</TD>
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<TD VALIGN="bottom">&nbsp;</TD>
<TD VALIGN="top">By:</TD>
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<TD VALIGN="top" COLSPAN="3">Bain Capital Life Sciences Fund III, L.P.,</TD></TR>
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<TD VALIGN="top"></TD>
<TD VALIGN="bottom">&nbsp;</TD>
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<TD VALIGN="top" COLSPAN="3">its member</TD></TR>
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<TD VALIGN="bottom">&nbsp;</TD>
<TD VALIGN="top">By:</TD>
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<TD VALIGN="top" COLSPAN="3">Bain Capital Life Sciences III General Partner, LLC,</TD></TR>
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<TD VALIGN="top"></TD>
<TD VALIGN="bottom">&nbsp;</TD>
<TD VALIGN="top"></TD>
<TD VALIGN="bottom">&nbsp;</TD>
<TD VALIGN="top"></TD>
<TD VALIGN="bottom">&nbsp;</TD>
<TD VALIGN="top" COLSPAN="3">its general partner</TD></TR>
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<TD VALIGN="bottom">&nbsp;</TD>
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<TD VALIGN="bottom">&nbsp;</TD>
<TD VALIGN="top">By:</TD>
<TD VALIGN="bottom">&nbsp;</TD>
<TD VALIGN="top" COLSPAN="3">Bain Capital Life Sciences Investors, LLC,</TD></TR>
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<TD VALIGN="top"></TD>
<TD VALIGN="bottom">&nbsp;</TD>
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<TD VALIGN="top" COLSPAN="3">its manager</TD></TR>
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<TD VALIGN="top">By:</TD>
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<TD VALIGN="top" COLSPAN="3"> <P STYLE="margin-top:0pt; margin-bottom:1pt; border-bottom:1px solid #000000; font-size:10pt; font-family:Times New Roman">/s/ Ricky Sun</P></TD></TR>
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<TD VALIGN="top"></TD>
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<TD VALIGN="top">Name:</TD>
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<TD VALIGN="top">Ricky Sun</TD></TR>
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<TD VALIGN="top">Title:</TD>
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<TD VALIGN="top">Partner</TD></TR>
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