This is a multicenter, open-label, non-randomized study to investigate the safety, tolerability, and efficacy of a single intravenous (IV) infusion of SGT-003 in participants with Duchenne muscular dystrophy. There will be 5 cohorts in this study. Cohort 1 will include participants 4 to \< 7 years of age. Cohort 2 will include participants 7 to \< 12 years of age. Cohort 3 will include participants 0 to \< 4 years of age. Cohort 4 will include participants 12 to \< 18 years of age. Cohort 5 will include participants 10 to \< 18 years of age. Initiation of participant enrollment in Cohorts 4 and 5 will be subject to the accrual of safety and efficacy data from Cohorts 1-3. All participants will receive SGT-003 and will be enrolled in the study for 5 total years for long-term follow up.