Study Summary: This study is testing a drug called sevasemten (EDG-5506) for adults with Becker muscular dystrophy, a genetic muscle disorder. The study is in two parts: CANYON (already full) and GRAND CANYON (still enrolling). A "randomized" study means participants are randomly assigned to different groups, like flipping a coin. "Double-blind" means neither the participants nor the researchers know who is getting the real drug or a "placebo," which is a harmless pill that looks like the real drug. Study Duration: GRAND CANYON study includes a 4-week screening, an 18-month treatment, and a 4-week follow-up. Eligibility: Adults aged 18-50 with a specific muscle condition can join, but they must be able to walk a certain distance within a set time and meet other health criteria. Key Points:

• The study lasts about 19 months.
• Participants are randomly assigned to receive either the real drug or a placebo.
• There are strict health criteria for joining the study.