Addendum to Executive Letter Agreement Between Beam Therapeutics, Inc. and Executive

Notwithstanding anything to the contrary contained in this Agreement, to the extent that any of the payments and benefits provided for under this Agreement or any other agreement or arrangement between the Company and you ( collectively, the "Payments") (i) constitute a "parachute payment" within the meaning of Section 280G of the Code and (ii) but for this paragraph, would be subject to the excise tax imposed by Section 4999 of the Code, then the Payments shall be reduced to the extent necessary so that no portion of such Payments retained by you shall be subject to excise tax under Section 4999 of the Code; provided, however, such reduction shall only occur if after taking into account the applicable federal, state and local income taxes and the excise tax imposed by Section 4999, such reduction results in your receipt on an after-tax basis, of the greatest amount of benefits under this Agreement, notwithstanding that all or some portion of such benefits may be taxable under Section 4999 of the Code. In the event of a determination that such reduction is to take place, reduction shall occur in the following order: first, reduction of cash payments, which shall occur in reverse chronological order such that the cash payment owed on the latest date following the occurrence of the event triggering such excise tax will be the first cash payment to be reduced; second, cancellation of accelerated vesting of equity awards, which shall occur in the reverse order of the date of grant for such stock awards (i.e., the vesting of the most recently granted stock awards will be reduced first); and third, reduction of employee benefits, which shall occur in reverse chronological order such that the benefit owed on the latest date following the occurrence of the event triggering such excise tax will be the first benefit to be reduced. If two or more equity awards are granted on the same date, each award will be reduced on a prorata basis.

Unless you and the Company otherwise agree in writing, any determination required under this Section shall be made in writing by a nationally recognized independent public accounting or consulting firm (the "Accountants"), whose determination shall be conclusive and binding upon you and the Company for all purposes. For purposes of making the calculations required by this Section, the Accountants may make reasonable assumptions and approximations concerning applicable taxes and may rely in reasonable, good faith interpretations concerning the application of Sections 280G and 4999 of the Code. The Company and you shall furnish to the Accountants such information and documents as the Accountants may reasonably request in order to make a determination under this Section. The Company shall bear all costs the Accountants may reasonably incur in connection with any calculations contemplated by this Section.