This is a controlled, open-label, single-ascending dose study to evaluate the safety and tolerability 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.

Inclusion Criteria:

• Established clinical diagnosis of DMD and documented dystrophin gene mutation predictive of DMD phenotype
• Confirmed absence of dystrophin as determined by muscle biopsy (ambulatory participants)
• Anti-AAV9 antibodies below protocol-specified thresholds
• Stable cardiac and pulmonary function
• Adolescents: non-ambulatory by protocol-specified criteria
• Children: ambulatory by protocol-specified criteria
• Stable daily dose (or equivalent) of oral corticosteroids ≥ 12 weeks

Exclusion Criteria:

• Prior or ongoing medical condition or physical examination, ECG or laboratory findings that could adversely affect participant safety, compromise completion of treatment and follow-up, or impair assessment of study results
• Abnormal liver function
• Abnormal renal function
• Clinically significant coagulation abnormalities
• Impaired cardiovascular function based on cardiac MRI or ECHO
• Impaired respiratory function based on FVC % predicted or need for daytime ventilatory support
• Significant spinal deformity or presence of spinal rods
• Body mass index ≥ 95th percentile for age
• Exposure to another investigational drug within 3 months or 5 half-lives prior to screening
• Exposure to drugs affecting dystrophin or utrophin expression within 6 months prior to screening

Additional inclusion/exclusion criteria may apply.