X0306 4 2017-02-07 0 0001597264 Blueprint Medicines Corp BPMC 0001319446 Borisy Alexis C/O THIRD ROCK VENTURES, LLC, 29 NEWBURY STREET, 3RD FLOOR BOSTON MA 02116 1 0 0 0 Common Stock 2017-02-07 4 S 0 0 0.00 D 0 I See Footnote Common Stock 40842 D On February 7, 2017, Third Rock Ventures II, L.P. ("TRV II") sold 1,000,000 shares of Common Stock of the Issuer in multiple transactions at prices ranging from $35.25 to $35.45, inclusive. The reporting person undertakes to provide to the Issuer, any security holder of the Issuer, or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the ranges set forth in this footnote (1). After the above transaction, TRV II owned 4,453,753 shares of Common Stock of the Issuer. The reporting person is a partner of Third Rock Ventures GP II, L.P., which is the general partner of the TRV II. The reporting person disclaims beneficial ownership over such shares, except to the extent of his pecuniary interest therein, if any, and this report shall not be deemed an admission that he is the beneficial owner of such shares. /s/ Alexis Borisy 2017-02-09