**Study of SGT-003 Gene Therapy in Duchenne Muscular Dystrophy** This study is testing a new treatment called **SGT-003** for Duchenne Muscular Dystrophy (DMD), a condition that weakens muscles. The study includes children in five different age groups, from birth to 18 years. Participants will receive a one-time medicine through an IV (intravenous) drip and will be monitored for 5 years to check for safety and effectiveness. **Key points:** - **Study Length:** Participants will be in the study for 5 years for follow-up. - **Visits Needed:** Participants will receive one IV infusion of the treatment. - **Eligibility:** Participants must meet age and health criteria, and cannot have had certain previous treatments. Participants need to have a confirmed diagnosis of DMD and meet specific health criteria. They must not have been treated with certain drugs recently. This study aims to find out if SGT-003 is safe and works well. Each group will be studied to make sure it is safe before moving to the next. Participants cannot have certain genetic mutations or previous gene therapy.