• Cohort 1: 4 to \<7 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