A study of sevasemten (EDG-5506) in Becker muscular dystrophy (known as CANYON) and pivotal cohort (known as GRAND CANYON). The EDG-5506-201 CANYON study was expanded to include an additional 120 adult participants in a cohort called GRAND CANYON, that is a multicenter, randomized, double-blind, placebo-controlled study to evaluate the safety and efficacy of sevasemten in adults with Becker. CANYON and GRAND CANYON are fully enrolled.