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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="5" face="Times New Roman" style="font-size:18.0pt;font-weight:bold;">UNITED STATES</font></b></p>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="5" face="Times New Roman" style="font-size:18.0pt;font-weight:bold;">SECURITIES AND EXCHANGE COMMISSION</font></b></p>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">Washington, D.C. 20549</font></b></p>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="5" face="Times New Roman" style="font-size:18.0pt;font-weight:bold;">FORM&nbsp;8-K</font></b></p>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="3" face="Times New Roman" style="font-size:12.0pt;font-weight:bold;">CURRENT REPORT</font></b></p>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="3" face="Times New Roman" style="font-size:12.0pt;font-weight:bold;">Pursuant to Section&nbsp;13 or 15(d)&nbsp;of the Securities Exchange Act of 1934</font></b></p>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">Date of Report (Date of earliest event reported): <b>September&nbsp;18, 2013</b></font></p>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="5" face="Times New Roman" style="font-size:18.0pt;font-weight:bold;">Acceleron Pharma Inc.</font></b></p>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">(Exact name of registrant as specified in its charter)</font></p>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">Delaware</font></b></p>    </td>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">001-36065</font></b></p>    </td>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">27-0072226</font></b></p>    </td>   </tr>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">(State or other jurisdiction of<br>   incorporation)</font></p>    </td>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">(Commission File Number)</font></p>    </td>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">(IRS Employer<br>   Identification No.)</font></p>    </td>   </tr>  </table>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">128 Sidney Street</font></b></p>    </td>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">Cambridge, MA</font></b></p>    </td>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">02139</font></b></p>    </td>   </tr>
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<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&#160;(Registrant&#146;s telephone number, including area code):&#160; <b>(617) 649-9200</b></font></p>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">Not Applicable</font></b></p>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">(Former name or former address, if changed since last report)</font></p>
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<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">Check the appropriate box below if the Form&nbsp;8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:</font></p>
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<p style="margin:0in 0in .0001pt .25in;text-indent:-.25in;"><font size="2" face="Wingdings" style="font-size:10.0pt;">o</font><font size="1" style="font-size:3.0pt;">&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; </font><font size="2" style="font-size:10.0pt;">Written communications pursuant to Rule&nbsp;425 under the Securities Act (17 CFR 230.425)</font></p>
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<p style="margin:0in 0in .0001pt .25in;text-indent:-.25in;"><font size="2" face="Wingdings" style="font-size:10.0pt;">o</font><font size="1" style="font-size:3.0pt;">&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; </font><font size="2" style="font-size:10.0pt;">Soliciting material pursuant to Rule&nbsp;14a-12 under the Exchange Act (17 CFR 240.14a-12)</font></p>
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<p style="margin:0in 0in .0001pt .25in;text-indent:-.25in;"><font size="2" face="Wingdings" style="font-size:10.0pt;">o</font><font size="1" style="font-size:3.0pt;">&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; </font><font size="2" style="font-size:10.0pt;">Pre-commencement communications pursuant to Rule&nbsp;14d-2(b)&nbsp;under the Exchange Act (17 CFR 240.14d-2(b))</font></p>
<p style="margin:0in 0in .0001pt 14.25pt;text-indent:-14.25pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>
<p style="margin:0in 0in .0001pt .25in;text-indent:-.25in;"><font size="2" face="Wingdings" style="font-size:10.0pt;">o</font><font size="1" style="font-size:3.0pt;">&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; </font><font size="2" style="font-size:10.0pt;">Pre-commencement communications pursuant to Rule&nbsp;13e-4(c)&nbsp;under the Exchange Act (17 CFR 240.13e-4(c))</font></p>
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<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>
<p style="margin:0in 0in .0001pt;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">Item 3.02 Unregistered Sales of Equity Securities</font></b></p>
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">On September&nbsp;24, 2013 Acceleron Pharma Inc. (the &#147;Company&#148;) issued and sold 666,667 shares of its common stock $0.001 par value per share (the &#147;Shares&#148;) to Celgene Corporation (&#147;Celgene&#148;) in connection with the private placement which occurred concurrently with the Company&#146;s initial public offering of its common stock (the &#147;IPO&#148;).&#160; The sale and issuance of the Shares was effected pursuant to a Common Stock Purchase Agreement between the Company and Celgene dated September&nbsp;24, 2013.</font></p>
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">The aggregate purchase price of the Shares was approximately $10,000,000 representing a price per share of $15.00, the price at which shares of the Company&#146;s common stock were sold to the public in the IPO.&#160; The purchase price of the Shares was paid for by Celgene in immediately available funds.</font></p>
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<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">The sale and issuance by the Company of the Shares to Celgene described under this Item 3.02 was not registered under the Securities Act.&#160; The Shares were sold in reliance on the exemption from registration provided by Rule&nbsp;506 promulgated under the Securities Act of 1933.</font></p>
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>
<p style="margin:0in 0in .0001pt;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">Item 5.02 Departure of Directors or Certain Officers; Election of Directors; Appointment of Certain Officers; Compensatory Arrangements of Certain Officers</font></b></p>
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">As previously disclosed in the Company&#146;s Registration Statement on Form&nbsp;S-1 (File No.&nbsp;333-190417) (the &#147;Registration Statement&#148;), on September&nbsp;18, 2013 and in connection with the IPO, Carl L. Gordon resigned from the Company&#146;s board of directors. The resignation of Dr.&nbsp;Gordon was not caused by any disagreement with the Company.</font></p>
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<p style="margin:0in 0in .0001pt;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">Item 5.03. Amendments to Articles of Incorporation or Bylaws; Change in Fiscal Year.</font></b></p>
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<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">As previously disclosed in the Registration Statement, on September 23, 2013 and in connection with the consummation of the IPO, the Company filed a restated certificate of incorporation (the &#147;Restated Certificate&#148;) with the Secretary of State of the State of Delaware. The Restated Certificate became effective as of 12:00 p.m. on September 24. The Company&#146;s board of directors and stockholders previously approved the Restated Certificate to be filed in connection with the consummation of the IPO. The Restated Certificate restates the Company&#146;s certificate of incorporation in its entirety to, among other things: (i)&nbsp;increase the authorized number of shares of common stock to 175 million shares; (ii)&nbsp;eliminate all references to previously-existing series of preferred stock; and (iii)&nbsp;authorize 25 million shares of undesignated preferred stock that may be issued from time to time by the Company&#146;s board of directors in one or more series.</font></p>
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">The foregoing description of the Restated Certificate is qualified by reference to the Restated Certificate, a copy of which is attached hereto as Exhibit&nbsp;3.1 and is incorporated herein by reference.</font></p>
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">As previously disclosed in the Registration Statement, on September&nbsp;24, 2013 and in connection with the consummation of the IPO, the amended and restated by-laws of the Company (the &#147;Amended and Restated By-Laws&#148;), previously approved by the Company&#146;s board of directors and stockholders to become effective immediately upon the consummation of the IPO, became effective. The Amended and Restated By-Laws amend and restate the Company&#146;s by-laws in their entirety to, among other things: (i)&nbsp;eliminate the ability of the Company&#146;s stockholders to take action by written consent in lieu of a meeting; (ii)&nbsp;establish procedures relating to the presentation of stockholder proposals at stockholder meetings; (iii)&nbsp;establish procedures relating to the nomination of directors; (iv)&nbsp;provide for the indemnification of the Company&#146;s directors and officers; and (v)&nbsp;conform to the amended provisions of the Restated Certificate.</font></p>
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">The foregoing description of the Amended and Restated By-Laws is qualified by reference to the Amended and Restated By-Laws, a copy of which is attached hereto as Exhibit&nbsp;3.2 and is incorporated herein by reference.</font></p>
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<p style="margin:0in 0in .0001pt;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">Item 8.01. Other Items.</font></b></p>
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<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">As previously disclosed in the Registration Statement, on September&nbsp;24, 2013 and in connection with the consummation of the IPO, an automatic conversion (the &#147;Conversion&#148;) of all previously outstanding shares of preferred stock into common stock, previously approved by the Company&#146;s board of directors and stockholders to become effective immediately upon the consummation of the IPO, became effective.&#160; The Conversion was </font></p>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">2<a name="PB_2_091035_5335"></a></font></p>
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<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">effected on a mandatory basis in accordance with the Company&#146;s certificate of incorporation in effect at the time of its approval and at the applicable conversion rates determined under the certificate of incorporation in effect at such time.</font></p>
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<p style="margin:0in 0in .0001pt;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">Item 9.01. Financial Statements and Exhibits.</font></b></p>
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<p style="margin:0in 0in .0001pt;text-indent:3.0pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">(d)&nbsp;Exhibits</font></p>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="1" face="Times New Roman" style="font-size:8.0pt;font-weight:bold;">Description</font></b></p>    </td>   </tr>
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<td width="9%" valign="top" style="border:none;padding:0in 0in 0in 0in;width:9.46%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>
<td width="3%" valign="bottom" style="padding:0in 0in 0in 0in;width:3.22%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>
<td width="87%" valign="top" style="border:none;padding:0in 0in 0in 0in;width:87.32%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>   </tr>
<tr>
<td width="9%" valign="top" style="padding:0in 0in 0in 0in;width:9.46%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">3.1</font></p>    </td>
<td width="3%" valign="bottom" style="padding:0in 0in 0in 0in;width:3.22%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:1.0pt;">&nbsp;</font></p>    </td>
<td width="87%" valign="top" style="padding:0in 0in 0in 0in;width:87.32%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">Amended   and Restated Certificate of Incorporation of Acceleron Pharma Inc.</font></p>    </td>   </tr>
<tr>
<td width="9%" valign="top" style="padding:0in 0in 0in 0in;width:9.46%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>
<td width="3%" valign="bottom" style="padding:0in 0in 0in 0in;width:3.22%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>
<td width="87%" valign="top" style="padding:0in 0in 0in 0in;width:87.32%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>   </tr>
<tr>
<td width="9%" valign="top" style="padding:0in 0in 0in 0in;width:9.46%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">3.2</font></p>    </td>
<td width="3%" valign="bottom" style="padding:0in 0in 0in 0in;width:3.22%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:1.0pt;">&nbsp;</font></p>    </td>
<td width="87%" valign="top" style="padding:0in 0in 0in 0in;width:87.32%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">Amended   and Restated By-Laws of Acceleron Pharma Inc.</font></p>    </td>   </tr>
<tr>
<td width="9%" valign="top" style="padding:0in 0in 0in 0in;width:9.46%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>
<td width="3%" valign="bottom" style="padding:0in 0in 0in 0in;width:3.22%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>
<td width="87%" valign="top" style="padding:0in 0in 0in 0in;width:87.32%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>   </tr>
<tr>
<td width="9%" valign="top" style="padding:0in 0in 0in 0in;width:9.46%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">10.1</font></p>    </td>
<td width="3%" valign="bottom" style="padding:0in 0in 0in 0in;width:3.22%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:1.0pt;">&nbsp;</font></p>    </td>
<td width="87%" valign="top" style="padding:0in 0in 0in 0in;width:87.32%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">Common   Stock Purchase Agreement dated as of September&nbsp;24, 2013</font></p>    </td>   </tr>  </table>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">3<a name="PB_3_091246_5796"></a></font></p>
<div style="margin:0in 0in .0001pt;"><hr size="3" width="100%" noshade color="#010101" align="left"></div>
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<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">SIGNATURES</font></b></p>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>
<p style="margin:0in 0in .0001pt;text-indent:37.05pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.</font></p>
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>
<table border="0" cellspacing="0" cellpadding="0" width="100%" style="border-collapse:collapse;width:100.0%;">
<tr>
<td width="45%" valign="top" style="padding:0in 0in 0in 0in;width:45.22%;">
<p style="margin:0in 0in .0001pt;"><b><font size="2" face="Times New Roman" style="font-size:1.0pt;font-weight:bold;">&nbsp;</font></b></p>    </td>
<td width="4%" valign="top" style="padding:0in 0in 0in 0in;width:4.28%;">
<p style="margin:0in 0in .0001pt;"><b><font size="2" face="Times New Roman" style="font-size:1.0pt;font-weight:bold;">&nbsp;</font></b></p>    </td>
<td width="50%" valign="top" style="padding:0in 0in 0in 0in;width:50.5%;">
<p style="margin:0in 0in .0001pt;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">ACCELERON PHARMA INC.</font></b></p>    </td>   </tr>
<tr>
<td width="45%" valign="top" style="padding:0in 0in 0in 0in;width:45.22%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>
<td width="4%" valign="top" style="padding:0in 0in 0in 0in;width:4.28%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>
<td width="50%" valign="top" style="padding:0in 0in 0in 0in;width:50.5%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>   </tr>
<tr>
<td width="45%" valign="top" style="padding:0in 0in 0in 0in;width:45.22%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:1.0pt;">&nbsp;</font></p>    </td>
<td width="4%" valign="top" style="padding:0in 0in 0in 0in;width:4.28%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">By:</font></p>    </td>
<td width="50%" valign="top" style="border:none;border-bottom:solid windowtext 1.0pt;padding:0in 0in 0in 0in;width:50.5%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">/s/   John Knopf Ph.D.</font></p>    </td>   </tr>
<tr>
<td width="45%" valign="top" style="padding:0in 0in 0in 0in;width:45.22%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:1.0pt;">&nbsp;</font></p>    </td>
<td width="4%" valign="top" style="padding:0in 0in 0in 0in;width:4.28%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:1.0pt;">&nbsp;</font></p>    </td>
<td width="50%" valign="top" style="border:none;padding:0in 0in 0in 0in;width:50.5%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">John   Knopf Ph.D.<br>   Chief Executive Officer and President</font></p>    </td>   </tr>  </table>
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">Date: September&nbsp;24, 2013</font></p>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">4<a name="PB_4_091341_2897"></a></font></p>
<div style="margin:0in 0in .0001pt;"><hr size="3" width="100%" noshade color="#010101" align="left"></div>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">&nbsp;</font></b></p>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">EXHIBIT&nbsp;INDEX</font></b></p>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>
<table border="0" cellspacing="0" cellpadding="0" width="100%" style="border-collapse:collapse;width:100.0%;">
<tr>
<td width="8%" valign="bottom" style="border:none;border-bottom:solid windowtext 1.0pt;padding:0in 0in 0in 0in;width:8.96%;">
<p style="margin:0in 0in .0001pt;"><b><font size="1" face="Times New Roman" style="font-size:8.0pt;font-weight:bold;">No.</font></b></p>    </td>
<td width="3%" valign="bottom" style="padding:0in 0in 0in 0in;width:3.22%;">
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="1" face="Times New Roman" style="font-size:1.0pt;font-weight:bold;">&nbsp;</font></b></p>    </td>
<td width="87%" valign="bottom" style="border:none;border-bottom:solid windowtext 1.0pt;padding:0in 0in 0in 0in;width:87.82%;">
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="1" face="Times New Roman" style="font-size:8.0pt;font-weight:bold;">Description</font></b></p>    </td>   </tr>
<tr>
<td width="8%" valign="top" style="border:none;padding:0in 0in 0in 0in;width:8.96%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>
<td width="3%" valign="bottom" style="padding:0in 0in 0in 0in;width:3.22%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>
<td width="87%" valign="top" style="border:none;padding:0in 0in 0in 0in;width:87.82%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>   </tr>
<tr>
<td width="8%" valign="top" style="padding:0in 0in 0in 0in;width:8.96%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">3.1</font></p>    </td>
<td width="3%" valign="bottom" style="padding:0in 0in 0in 0in;width:3.22%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:1.0pt;">&nbsp;</font></p>    </td>
<td width="87%" valign="top" style="padding:0in 0in 0in 0in;width:87.82%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">Amended   and Restated Certificate of Incorporation of Acceleron Pharma Inc.</font></p>    </td>   </tr>
<tr>
<td width="8%" valign="top" style="padding:0in 0in 0in 0in;width:8.96%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>
<td width="3%" valign="bottom" style="padding:0in 0in 0in 0in;width:3.22%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>
<td width="87%" valign="top" style="padding:0in 0in 0in 0in;width:87.82%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>   </tr>
<tr>
<td width="8%" valign="top" style="padding:0in 0in 0in 0in;width:8.96%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">3.2</font></p>    </td>
<td width="3%" valign="bottom" style="padding:0in 0in 0in 0in;width:3.22%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:1.0pt;">&nbsp;</font></p>    </td>
<td width="87%" valign="top" style="padding:0in 0in 0in 0in;width:87.82%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">Amended   and Restated By-Laws of Acceleron Pharma Inc.</font></p>    </td>   </tr>
<tr>
<td width="8%" valign="top" style="padding:0in 0in 0in 0in;width:8.96%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>
<td width="3%" valign="bottom" style="padding:0in 0in 0in 0in;width:3.22%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>
<td width="87%" valign="top" style="padding:0in 0in 0in 0in;width:87.82%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>   </tr>
<tr>
<td width="8%" valign="top" style="padding:0in 0in 0in 0in;width:8.96%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">10.1</font></p>    </td>
<td width="3%" valign="bottom" style="padding:0in 0in 0in 0in;width:3.22%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:1.0pt;">&nbsp;</font></p>    </td>
<td width="87%" valign="top" style="padding:0in 0in 0in 0in;width:87.82%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">Common   Stock Purchase Agreement dated as of September&nbsp;24, 2013</font></p>    </td>   </tr>  </table>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">5<a name="PB_5_091404_3020"></a></font></p>
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{"company-data": {"conformed-name": "ACCELERON PHARMA INC", "cik": "0001280600", "assigned-sic": "2836", "irs-number": "000000000", "state-of-incorporation": "DE", "fiscal-year-end": "1231"}, "filing-values": {"form-type": "8-K", "act": "34", "file-number": "001-36065", "film-number": "131112493"}, "business-address": {"street1": "128 SYDNEY STREET", "city": "CAMBRIDGE", "state": "MA", "zip": "02139", "phone": "617-649-9200"}, "mail-address": {"street1": "128 SYDNEY STREET", "city": "CAMBRIDGE", "state": "MA", "zip": "02139"}}
UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549
FORM 8-K
CURRENT REPORT
Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934
Date of Report (Date of earliest event reported):
September 18, 2013
Acceleron Pharma Inc.
(Exact name of registrant as specified in its charter)
Delaware
001-36065
27-0072226
(State or other jurisdiction of
incorporation)
(Commission File Number)
(IRS Employer
Identification No.)
128 Sidney Street
Cambridge, MA
02139
(Address of principal executive offices)
(Zip Code)
(Registrant's telephone number, including area code):
(617) 649-9200
Not Applicable
(Former name or former address, if changed since last report)
Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:
o
Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)
o
Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)
o
Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))
o
Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c))
Item 3.02 Unregistered Sales of Equity Securities
On September 24, 2013 Acceleron Pharma Inc. (the "Company") issued and sold 666,667 shares of its common stock $0.001 par value per share (the "Shares") to Celgene Corporation ("Celgene") in connection with the private placement which occurred concurrently with the Company's initial public offering of its common stock (the "IPO").  The sale and issuance of the Shares was effected pursuant to a Common Stock Purchase Agreement between the Company and Celgene dated September 24, 2013.
The aggregate purchase price of the Shares was approximately $10,000,000 representing a price per share of $15.00, the price at which shares of the Company's common stock were sold to the public in the IPO.  The purchase price of the Shares was paid for by Celgene in immediately available funds.
The sale and issuance by the Company of the Shares to Celgene described under this Item 3.02 was not registered under the Securities Act.  The Shares were sold in reliance on the exemption from registration provided by Rule 506 promulgated under the Securities Act of 1933.
Item 5.02 Departure of Directors or Certain Officers; Election of Directors; Appointment of Certain Officers; Compensatory Arrangements of Certain Officers
As previously disclosed in the Company's Registration Statement on Form S-1 (File No. 333-190417) (the "Registration Statement"), on September 18, 2013 and in connection with the IPO, Carl L. Gordon resigned from the Company's board of directors. The resignation of Dr. Gordon was not caused by any disagreement with the Company.
Item 5.03. Amendments to Articles of Incorporation or Bylaws; Change in Fiscal Year.
As previously disclosed in the Registration Statement, on September 23, 2013 and in connection with the consummation of the IPO, the Company filed a restated certificate of incorporation (the "Restated Certificate") with the Secretary of State of the State of Delaware. The Restated Certificate became effective as of 12:00 p.m. on September 24. The Company's board of directors and stockholders previously approved the Restated Certificate to be filed in connection with the consummation of the IPO. The Restated Certificate restates the Company's certificate of incorporation in its entirety to, among other things: (i) increase the authorized number of shares of common stock to 175 million shares; (ii) eliminate all references to previously-existing series of preferred stock; and (iii) authorize 25 million shares of undesignated preferred stock that may be issued from time to time by the Company's board of directors in one or more series.
The foregoing description of the Restated Certificate is qualified by reference to the Restated Certificate, a copy of which is attached hereto as Exhibit 3.1 and is incorporated herein by reference.
As previously disclosed in the Registration Statement, on September 24, 2013 and in connection with the consummation of the IPO, the amended and restated by-laws of the Company (the "Amended and Restated By-Laws"), previously approved by the Company's board of directors and stockholders to become effective immediately upon the consummation of the IPO, became effective. The Amended and Restated By-Laws amend and restate the Company's by-laws 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; (ii) establish procedures relating to the presentation of stockholder proposals at stockholder meetings; (iii) establish procedures relating to the nomination of directors; (iv) provide for the indemnification of the Company's directors and officers; and (v) conform to the amended provisions of the Restated Certificate.
The foregoing description of the Amended and Restated By-Laws is qualified by reference to the Amended and Restated By-Laws, a copy of which is attached hereto as Exhibit 3.2 and is incorporated herein by reference.
Item 8.01. Other Items.
As previously disclosed in the Registration Statement, on September 24, 2013 and in connection with the consummation of the IPO, an automatic conversion (the "Conversion") of all previously outstanding shares of preferred stock into common stock, previously approved by the Company's board of directors and stockholders to become effective immediately upon the consummation of the IPO, became effective.  The Conversion was
2
effected on a mandatory basis in accordance with the Company's certificate of incorporation in effect at the time of its approval and at the applicable conversion rates determined under the certificate of incorporation in effect at such time.
Item 9.01. Financial Statements and Exhibits.
(d) Exhibits
Exhibit
No.
Description
3.1
Amended   and Restated Certificate of Incorporation of Acceleron Pharma Inc.
3.2
Amended   and Restated By-Laws of Acceleron Pharma Inc.
10.1
Common   Stock Purchase Agreement dated as of September 24, 2013
3
SIGNATURES
Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.
ACCELERON PHARMA INC.
By:
/s/   John Knopf Ph.D.
John   Knopf Ph.D.
Chief Executive Officer and President
Date: September 24, 2013
4
EXHIBIT INDEX
No.
Description
3.1
Amended   and Restated Certificate of Incorporation of Acceleron Pharma Inc.
3.2
Amended   and Restated By-Laws of Acceleron Pharma Inc.
10.1
Common   Stock Purchase Agreement dated as of September 24, 2013
5