LYNX is a placebo-controlled trial to assess the effect of different dose levels of sevasemten over 12 weeks on safety, pharmacokinetics and biomarkers of muscle damage. Study participants include boys with Duchenne on stable corticosteroids as well as a single cohort of boys not currently treated with corticosteroids. After the initial 12 weeks, LYNX participants will then continue on open-label sevasemten for an additional 21 months to gain further insights into safety and functional measures.