This study is for kids with Duchenne Muscular Dystrophy (DMD), a condition that weakens muscles over time. It checks the safety and effects of a new medicine called sevasemten (EDG-5506). **Part A** lasts 12 weeks and involves some children taking sevasemten and others a placebo (a sugar pill) without knowing which one they get. **Part B** lasts 92 weeks where everyone gets sevasemten. Kids aged 4-9 years can join if they have a specific mutation in the DMD gene. They need to be able to get up from lying down and climb stairs quickly.

• The study lasts about 2 years with multiple visits.
• Kids will take sevasemten once a day.
• Participants may not have used certain drugs recently.

Some kids can’t join if they have serious health issues or have taken other experimental drugs recently. This study helps understand if sevasemten can help kids with DMD by reducing muscle damage. It's important to talk with a doctor to see if this study is a good fit for your child.