This is a controlled, open-label, single-ascending dose study to evaluate the safety, tolerability and efficacy of SGT-001 in adolescents and children with Duchenne muscular dystrophy (DMD). Participants will receive a single intravenous (IV) infusion of SGT-001 and will be followed for approximately 5 years. The protocol was amended to drop the control arm after 4 participants were dosed. Participants currently enrolling are assigned to active treatment. Control participants enrolled under original protocol will continue through the study per the original protocol.