Duchenne Muscular Dystrophy (DMD) is a muscle-wasting disease that mainly affects boys. This study tests a new drug called DYNE-251 to see if it's safe and if it helps muscles in kids with a special type of DMD. The drug is given through an IV (a needle into a vein). The study has three parts: a 24-week period with some kids getting the drug and others a fake treatment (placebo), another 24-week period where everyone gets the real drug, and a 96-week period to check long-term effects.

• Participants must be boys aged 4 to 16 with a specific DMD mutation.
• The study lasts over two years, with different phases.
• The drug is given through an IV, and muscle biopsies are required.

This study involves checking muscle protein levels and ensuring the drug is safe. Boys need to meet certain health requirements, like having stable heart function and being on stable medication. They should not have had certain other treatments recently. There are also rules about not having major surgeries or needing daytime breathing help. Always talk to your doctor before joining a study to ensure it's right for you.