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.

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.