Conflict of interest: TOC has been a consultant to Avexis/Novartis, Biogen, Cytokinetics, Genentech, Ionis, and Scholar Rock. He is/has been a site principal or co–principal investigator for the Biogen EMBRACE, NURTURE, and DEVOTE clinical trials, the Avexis/Novartis STR1VE and STRONG clinical trials, and individual clinical trials with Catabasis, Catalyst, Cytokinetics, Santherra, and Sarepta. CJS has been a consultant to Avexis, Novartis, Ionis Pharmaceuticals, Biogen, PTC Therapeutics, Roche, Genentech, Cytokinetics, Sarepta, Nura Bio, and Atalanta. CJS received a research grant from Ionis Pharmaceuticals in 2016 ($75,000) and currently receives grant support from Roche. CJS is a coholder of 2 pending patent applications (BIOL0274USA and BIOL0293WO) with Ionis Pharmaceuticals on antisense oligonucleotides targeting SMN-AS1. CJS receives royalties from Elsevier for the book Spinal Muscular Atrophy: Disease Mechanisms and Therapy (editors, CJ Sumner, S Paushkin, CP Ko; Elsevier, 2017).