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Preliminary Proxy Statement

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Definitive Proxy Statement

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Definitive Additional Materials

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Soliciting Material Pursuant to §240.14a‑12

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Check box if any part of the fee is offset as provided by Exchange Act Rule 0‑11(a)(2) and identify the filing for which the offsetting fee was paid previously. Identify the previous filing by registration statement number, or the Form or Schedule and the date of its filing.

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*** Exercise Your Right to Vote *** Important Notice Regarding the Availability of Proxy Materials for the Stockholder Meeting to Be Held on June 23, 2020 BLUEPRINT MEDICINES CORPORATION You are receiving this communication because you hold shares in the above named company. This is not a ballot. You cannot use this notice to vote these shares. This communication presents only an overview of the more complete proxy materials that are available to you on the Internet. You may view the proxy materials online at www.proxyvote.com or easily request a paper copy (see reverse side). We encourage you to access and review all of the important information contained in the proxy materials before voting. BLUEPRINT MEDICINES CORPORATION ATTN: TRACEY L. MCCAIN 45 SIDNEY STREET CAMBRIDGE, MA 02139 0000460146_1 R1.0.1.18 See the reverse side of this notice to obtain proxy materials and voting instructions. Meeting Information Meeting Type: Annual Meeting For holders as of: April 24, 2020 Date: June 23, 2020Time: 3:30 PM EDT Location: Meeting live via the Internet - please visit www.virtualshareholdermeeting.com/BPMC2020

Before You Vote How to Access the Proxy Materials Proxy Materials Available to VIEW or RECEIVE:  Have the information that is printed in the box marked by the arrow (located on the  by the arrow (located on the following page) in the subject line. How To Vote Please Choose One of the Following Voting Methods possession  available and follow the instructions. marked by the arrow 0000460146_2 R1.0.1.18 Vote In Person: Many stockholder meetings have attendance requirements including, but not limited to, the of an attendance ticket issued by the entity holding the meeting. Please check the meeting materials for any specia requirements for meeting attendance. At the meeting, you will need to request a ballot to vote these shares. Vote By Internet: To vote now by Internet, go to www.proxyvote.com. Have the information that is printed in the box Vote By Mail: You can vote by mail by requesting a paper copy of the materials, which will include a proxy card. l 1. Notice & Proxy Statement2. Annual Report to Stockholders How to View Online: following page) and visit: www.proxyvote.com. How to Request and Receive a PAPER or E-MAIL Copy: If you want to receive a paper or e-mail copy of these documents, you must request one. There is NO charge for requesting a copy. Please choose one of the following methods to make your request: 1) BY INTERNET:www.proxyvote.com 2) BY TELEPHONE:1-800-579-1639 3) BY E-MAIL*:sendmaterial@proxyvote.com * If requesting materials by e-mail, please send a blank e-mail with the information that is printed in the box marked Requests, instructions and other inquiries sent to this e-mail address will NOT be forwarded to your investment advisor. Please make the request as instructed above on or before June 09, 2020 to facilitate timely delivery.

The Board of Directors recommends you vote FOR the director nominees in Proposal 1. 1. Election of Directors Nominees 01 Alexis Borisy 02 Charles A. Rowland, Jr. 03 Lonnel Coats The Board of Directors recommends you vote FOR Proposals 2 and 3. 2. To approve an advisory vote on named executive officer compensation. 3. Ratification of the appointment of Ernst & Young LLP as independent registered public accounting firm for the fiscal year ending December 31, 2020. NOTE: The proxies are authorized to vote, in their discretion, upon such other business as may properly come before the meeting or any adjournment or postponement thereof. 0000460146_3 R1.0.1.18 Voting items

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