X0306 4 2018-05-10 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 2018-05-10 4 S 0 0 0.00 D 0 I See Footnote Common Stock 2018-05-11 4 S 0 0 0.00 D 0 I See Footnote Common Stock 53249 D On May 10, 2018, Third Rock Ventures II, L.P. ("TRV II") sold (i) 210,000 shares of Common Stock of the Issuer in multiple transactions at prices ranging from $80.05 to $80.25, inclusive, at a weighted average price per share of $80.0503 and (ii) 200 shares of Common Stock of the Issuer at a price of $81.11 per share. 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 950,653 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. On May 11, 2018, TRV II sold (i) 71,778 shares of Common Stock of the Issuer in multiple transactions at prices ranging from $78.03 to $78.73, inclusive, at a weighted average price per share of $78.0398. 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 (4). After the above transaction, TRV II owned 878,875 shares of Common Stock of the Issuer. /s/ Alexis Borisy 2018-05-14