X0306 4/A 2018-05-04 2018-05-07 0 0001552451 MyoKardia, Inc. MYOK 0001413822 GIANAKAKOS ANASTASIOS C/O MYOKARDIA, INC. 333 ALLERTON AVENUE SOUTH SAN FRANCISCO CA 94080 1 1 0 0 President and CEO Common Stock 2018-05-04 4 M 0 3000 1.5068 A 77968 D Common Stock 2018-05-04 4 S 0 1700 47.2735 D 76268 D Common Stock 2018-05-04 4 S 0 1300 47.95 D 74968 D Stock Option (Right to Buy) 1.5068 2018-05-04 4 M 0 3000 0.00 D 2025-06-03 Common Stock 3000 123235 D This transaction was effected pursuant to a Rule 10b5-1 trading plan adopted by the Reporting Person. The Form 4 filed by the Reporting Person on May 7, 2018 incorrectly reported that the Reporting Person exercised stock options to purchase 2,981 shares of Common Stock and sold 3,000 shares of Common Stock in a same day sale. This Form 4/A hereby amends such entry to state that the Reporting Person exercised 3,000 shares of Common Stock underlying the options and sold 3,000 shares of Common Stock in a same day sale. Represents the weighted average sale price of the shares sold ranging from $46.80 to $47.70 per share. The Reporting Person will provide, upon request by the Commission staff, the Issuer, or a security holder of the Issuer, full information regarding the number of shares sold at each separate price. Represents the weighted average sale price of the shares sold ranging from $47.85 to $48.10 per share. The Reporting Person will provide, upon request by the Commission staff, the Issuer, or a security holder of the Issuer, full information regarding the number of shares sold at each separate price. 25% of the shares subject to the option vested and became exercisable on June 4, 2016, and the remaining shares subject to this option vest and become exercisable in monthly installments through June 4, 2019, subject to the Reporting Person's continuous service to the Issuer. /s/ Jacob Bauer, as Attorney-in-Fact 2020-03-04