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.