Exhibit (e)(15)

May 8, 2020

Denelle Waynick

[***]

[***]

Dear Denelle:

We are pleased to offer you the position of General Counsel and Corporate Secretary with MyoKardia, Inc. Your compensation will be $18,750, semi-monthly, which is equal to $450,000 annualized, payable in accordance with the Company’s standard payroll schedule. This position will report directly to me. This is a full-time position. While you render services to the Company, you will not engage in any other employment, consulting or other business activity (whether full-time or part-time) that would create a conflict of interest with the Company. By signing this letter, you confirm to the Company that you have no contractual commitments or other legal obligations that would prohibit you from performing your duties for the Company.

Cash Compensation: This salary will be subject to adjustment pursuant to the Company’s employee compensation policies in effect from time to time. In addition, the company has a performance-based variable cash bonus program. Subject to an acceptable level of corporate performance, the Board of Directors may approve payment of performance bonuses after the first of next year. If bonuses are paid, your target percentage will be 40% of your salary as the basis for calculating your bonus. Your actual bonus will depend on your own and the company’s performance for the year just completed. Bonuses will be pro-rated for partial years of service and only if you are hired prior to October 1 of the current year.

As part of your offer, we are pleased to offer you a sign-on bonus of $175,000. This bonus will be paid as 75% within forty-five (45) days of your hire date and 25% upon final relocation. This sign-on bonus is taxable, and all regular payroll taxes will be withheld. In the event that you leave MyoKardia within 12 months of your hire date, you will be responsible for reimbursing the company for the entire net bonus amount.

Employee Benefits: As a regular employee of the Company, you will be eligible to participate in a number of Company-sponsored benefits, including 401(k) Retirement and Investment Plan and also in ESPP (Employee Stock Purchase Plan) during scheduled enrollment periods. In addition, you will be entitled to 20 days of paid time off in accordance with the Company’s policy. You can also review additional benefits information in the attached MyoKardia Employee Benefits Information Guide 2020.

As part of your offer, you will have the additional benefits of flexibility to commute back to Atlanta, Georgia every two to three weeks. MyoKardia will pay for air and hotel for up to six months.

Stock Options: Subject to the approval of the Company’s Compensation Committee, you will be granted an option to purchase 80,000 shares of the Company’s Common Stock. The exercise price per share will be equal to the closing price of the Company’s Common Stock as reported on NASDAQ as of the first trading day of the month following the later of (a) your date of hire or (b) the date of approval by the Company’s Compensation Committee.

Driven by the Heart

333 Allerton Avenue, South San Francisco, CA 94080 / +1 650 351 4705 / myokardia.com

Denelle Waynick

May 8, 2020

Page 2

The options will be subject to the terms and conditions applicable to options granted under the Company’s 2015 Stock Option and Incentive Plan (the “Plan”), as described in the Plan and the applicable stock option agreement. You will vest in 25% of the option shares after 12 months of continuous employment, and the balance will vest in equal monthly installments over the next 36 months of continuous employment, as described in the applicable stock option agreement.

Restricted Stock Units. Subject to the approval of the Company’s Compensation Committee, you will be granted Restricted Stock Units (“RSUs”) for 25,000 shares of the Company’s Common Stock under the Plan, effective as of the first trading day of the month following the later of (a) your date of hire or (b) the date of approval by the Company’s Compensation Committee (such date, the “Grant Date”). You will vest in 25% of the shares underlying the RSUs after 12 months of continuous employment from the Grant Date, and the balance will vest in equal annual installments over the next three (3) years of your continuous employment, as described in the applicable RSU award agreement.

Relocation: In order to assist you with your move, MyoKardia will also include relocation benefits as follows:

Relocation Dimensions is available to assist you with the coordination of your move, temporary house and home finding assistance.

Please note you will have twelve (12) months to fully utilize your relocation benefits. If you voluntarily terminate your employment with MyoKardia or are removed for cause with twelve (12) months of utilizing the relocation benefits, you will be responsible for reimbursing the Company 100% of your total relocation expenses.

In addition to the relocation benefits, the Company will support your commute to/from your current residence by reimbursing up to 2 round trip flights and hotel expenses for each month up to six months consistent with the Company’s travel policy.

Employee Confidentiality and Assignment Agreement: You will be required, as a condition of your employment with the Company, to sign the Company’s standard Employee Confidentiality and Assignment Agreement, a copy of which is attached.

Background Check: The Company may conduct a background or reference check (or both). If so, then you agree to cooperate fully in those procedures, and this offer is subject to the Company’s approving the outcome of those checks, in the discretion of the Company.

Employment Relationship: Employment with the Company is for no specific period of time. Your employment with the Company will be “at will,” meaning that either you or the Company may terminate your employment at any time and for any reason, with or without cause. Any contrary representations that may have been made to you are superseded by this letter agreement. This is the full and complete agreement between you and the Company on this term.

Driven by the Heart

333 Allerton Avenue, South San Francisco, CA 94080 / +1 650 351 4705 / myokardia.com

Denelle Waynick

May 8, 2020

Page 3

Although your job duties, title, reporting relationship, compensation and benefits, as well as the Company’s personnel policies and procedures, may change from time to time, the “at will” nature of your employment may only be changed in an express written agreement signed by you and a duly authorized officer of the Company (other than you).

Change in Control Benefits: As a Vice President or above, you will be eligible for the benefits available to members of the Company’s senior management team pursuant to the terms and conditions of the Company’s Change in Control Policy (as the same may be amended from time to time), a copy of which will be made available to you upon request.

Taxes: All forms of compensation referred to in this letter agreement are subject to reduction to reflect applicable withholding and payroll taxes and other deductions required by law. You agree that the Company does not have a duty to design its compensation policies in a manner that minimizes your tax liabilities, and you will not make any claim against the Company or its Board of Directors related to tax liabilities arising from your compensation.

Interpretation, Amendment and Enforcement: This letter agreement, the Employee Confidentiality and Assignment Agreement and Exhibit A constitute the complete agreement between you and the Company, contain all of the terms of your employment with the Company and supersede any prior agreements, representations or understandings (whether written, oral or implied) between you and the Company. This letter agreement may not be amended or modified, except by an express written agreement signed by both you and a duly authorized officer of the Company. The terms of this letter agreement and the resolution of any disputes as to the meaning, effect, performance or validity of this letter agreement or arising out of, related to, or in any way connected with, this letter agreement, your employment with the Company or any other relationship between you and the Company will be governed by California law, excluding laws relating to conflicts or choice of law.

We look forward to working with you, and hope that you will accept our offer to join the Company. You may indicate your agreement with these terms and accept this offer by signing and dating both the enclosed duplicate original of this letter agreement and the enclosed Proprietary Information and Inventions Agreement and return these documents to the Human Resources Department to confirm your acceptance no later than May 11, 2020, as this offer, if not accepted, will expire at the close of business on May 11, 2020. As required by law, your employment with the Company is contingent upon your providing legal proof of your identity and authorization to work in the United States. We would like your official start date to be on or before June 22, 2020.

Driven by the Heart

333 Allerton Avenue, South San Francisco, CA 94080 / +1 650 351 4705 / myokardia.com

Denelle Waynick

May 8, 2020

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If you have any questions, please do not hesitate to contact me at 650-741-7796.

ACKNOWLEDGMENT AND ACCEPTANCE OF THE TERMS STATED ABOVE:

Attachment

Employee Confidentiality and Assignment Agreement

Exhibit A—Prior Inventions

Exhibit B—California Labor Code (reference)

Sign-On Bonus Repayment Form

Relocation Recapture Agreement

Driven by the Heart

333 Allerton Avenue, South San Francisco, CA 94080 / +1 650 351 4705 / myokardia.com