Company: SNY
Filing Date: 2025-06-17
Form Type: SC TO-T
Source: 0001193125-25-141704
Chunk: 2

Company: Sanofi
Filing Date: 2025-06-17
Form: SC TO-T
Chunk 2
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5 (as it may be amended from time to time, the “”), by and
among the Company, Parent, Aventis and Purchaser, a copy of which is attached as Exhibit (d)(1) hereto, is incorporated herein by reference with respect to Items 4 through 9 and 11 of this Schedule TO.

Item 1. Summary Term Sheet.

The information
set forth in the “Summary Term Sheet” of the Offer to Purchase is incorporated herein by reference.

Item 2. Subject Company Information.

(a) The name of the subject company and the issuer of the securities to which this Schedule TO relates is Blueprint Medicines
Corporation, a Delaware corporation. The Company’s principal executive offices are located at 45 Sidney Street, Cambridge, Massachusetts 02139. The Company’s telephone number is (617) 374-7580.

(b) This Schedule TO relates to the outstanding Shares. The Company has advised Purchaser and Parent that, as of the close of business on June 12, 2025
(the most recent practicable date) 64,733,629 Shares were issued and outstanding.

(c) The information concerning the principal market, if any, in which
the Shares are traded and certain high and low sales prices for Shares in the principal market in which the Shares are traded are set forth in Section 6 (entitled “Price Range of Shares; Dividends on the Shares”) of the Offer to
Purchase is incorporated herein by reference.

Item 3. Identity and Background of the Filing Person.

(a) – (c) This Schedule TO is filed by Purchaser, Parent and Aventis. The information set forth in Section 8 (entitled “Certain Information
Concerning Parent, Aventis, Purchaser and Certain Related Persons”) of the Offer to Purchase and Schedule I to the Offer to Purchase is incorporated herein by reference.

2

Item 4. Terms of the Transaction. (a)(1)(i) – (viii), (x), (xii), (a)(2)(i) – (iv), (vii) The information set forth in the following sections of the Offer to Purchase is incorporated herein by reference:

| • |     | the “Summary Term Sheet” |

| • |     | the “Introduction” |

| • |     | Section 1 – “Terms of the Offer” |

| • |     | Section 2 – “Acceptance for Payment and Payment for Shares” |

| •