Certain Common Stock of Blueprint Medicines Corporation are subject to a Lock-Up Agreement Ending on 11-JAN-2018. These Common Stock will be under lockup for 30 days starting from 12-DEC-2017 to 11-JAN-2018. Details: The company's directors and executive officers have agreed that for a period of 60 days after the date of this prospectus supplement, subject to specified exceptions, they will not offer, sell, contract to sell, pledge, grant any option to purchase, make any short sale or otherwise dispose of any shares of common stock or securities convertible into or exchangeable or exercisable for any shares of common stock. In addition, certain stockholders who own more than 5% of the company's outstanding common stock have agreed to such restrictions for a period of 30 days after the date of this prospectus supplement. In addition, the representatives of the underwriters may, in their sole discretion, release all or some portion of the shares subject to such lock-up restrictions at any time and for any reason.