Side Letter dated June 11, 2020 to the Second Amended and Restated Strategic Neurology Drug Discovery and Development Collaboration, Option and License Agreement by and between the Registrant and Biogen MA Inc. dated October 17, 2018 (portions of the exhibit have been omitted because they are both (i) not material and (ii) would be competitively harmful if publicly disclosed)

Reference is hereby made to that certain Second Amended and Restated Strategic Neurology Drug Discovery and Development Collaboration, Option and License Agreement by and between Ionis Pharmaceuticals, Inc. (“Ionis”) and Biogen MA Inc. (“Biogen”) dated October 17, 2018 (the “Neurology II Agreement”), as supplemented and/or amended to date. Biogen and Ionis each may be referred to herein as a “Party” or collectively as the “Parties”. Any capitalized terms not defined herein will have the meaning set forth in the Neurology II Agreement, as applicable.

The Parties acknowledge and agree that, notwithstanding anything to the contrary in the Neurology II Agreement:

If you accept the terms and conditions set forth in this letter agreement, please so indicate by executing a copy of this letter agreement and returning it to Biogen. This letter agreement may be executed in counterparts, each of which will be deemed an original, notwithstanding variations in format or file designation which may result from electronic transmission, store and printing of copies of this letter agreement from separate computers or printers. Facsimile signatures and signatures transmitted via electronic mail in PDF format will be treated as original signatures.