A Study of SGT-003 Gene Therapy in Duchenne Muscular Dystrophy (INSPIRE DUCHENNE)

Inclusion Criteria: * Cohort 1: 4 to \<7 years of age * Cohort 2: 7 to \<12 years of age * Cohort 3: 0 to \< 4 years of age * Cohort 4: 12 to \< 18 years of age * Cohort 5: 10 to \< 18 years of age * Participant ambulatory status at the time of Screening Part A or Rescreening, as defined by the ability to complete a 10-meter walk/run test in \< 30 seconds: * Cohorts 1, 2, and 4: Ambulatory * Cohort 3: Either ambulatory or non-ambulatory * Cohort 5: Non-ambulatory, but having been previously ambulatory by history * Established clinical diagnosis of DMD and documented dystrophin gene mutation predictive of DMD phenotype confirmed by Sponsor genetic testing. In cases where a genotype may be predictive of residual dystrophin production and/or a clear clinical diagnosis of DMD cannot be made (e.g., due to age), evaluation of dystrophin levels in baseline muscle biopsies may be required to determine eligibility under this criterion. * Negative for AAV antibodies. * Steroid regimen: * Cohorts 1, 2, 4, and 5: A stable daily oral steroid regimen of at least 0.5 mg/kg/day of prednisone or 0.75 mg/kg/day of deflazacort for ≥12 weeks prior to Screening Part A or Rescreening, allowing for weight-based modifications consistent with clinical practice. * Cohort 3: N/A * Meet 10-meter walk/run time criteria * Meet time to rise from supine criteria * Cohort 5: Meet Performance of Upper Limb (PUL) 2.0 criteria * Participant has body weight: ≤ 90 kg Exclusion Criteria: * Treatment with dystrophin modifying drugs within 3 months prior to screening. * Current or prior treatment with an approved or investigational gene transfer drug. * Exposure to certain approved or investigational drugs within 3 months prior to screening or 5 half-lives since last administration, whichever is longer. * Established clinical diagnosis of DMD that is associated with any deletion mutation invariant or variant predicted to not express exons 1 to 11 or, exons 42 to 45, or exons 57 to 69, inclusive, in the DMD gene as documented by a genetic report and confirmed by Sponsor genetic testing. Other inclusion or exclusion criteria apply.