Duchenne Muscular Dystrophy (DMD) is a genetic disorder that affects muscles, making them weak. This study is exploring a new treatment called SGT-003 gene therapy, which is given through a single intravenous (IV) infusion, meaning it's delivered directly into the bloodstream. The study will have five groups, or cohorts, based on age: 0-4, 4-7, 7-12, 10-18, and 12-18 years old. The goal is to learn about the safety (is it safe?), tolerability (how well do people handle it?), and efficacy (does it work?) of SGT-003. Participants will be followed for five years to see long-term effects.

• Participants will need to visit the study center for tests and evaluations over five years.
• Participants must meet certain health criteria, like no recent gene therapy or specific medications.
• All participants will receive the SGT-003 treatment.