Inclusion Criteria:

   - For Cohorts 1-7: Has a definitive diagnosis of DMD based on documented clinical
   findings and prior genetic testing.

   - Cohort 1: Is ambulatory, and ≥4 to <8 years of age at the time of Screening.

   - Cohort 2: Is ambulatory, and ≥8 to <18 years of age at the time of Screening.

   - Cohort 3: Non-ambulatory per protocol specified criteria at the time of Screening.

   - Cohort 4: Is ambulatory and ≥3 to <4 years of age at the time of Screening.

   - Cohort 5a: Is ambulatory and ≥4 to <9 years of age.

   - Cohort 5b: Non-ambulatory per protocol specified criteria at the time of Screening.

   - Cohort 6: Is ambulatory, and ≥2 to <3 years of age at the time of Screening.

   - Cohort 7: Non-ambulatory per protocol-specified criteria at the time of Screening.

   - Ability to cooperate with motor assessment testing.

   - Cohorts 1, 2, 3, 5, and 7 only: Stable dose equivalent of oral glucocorticoids for at
   least 12 weeks before screening and the dose is expected to remain constant (except
   for modifications to accommodate changes in weight) throughout the first year of the
   study.

   - Cohorts 4 and 6: Do not yet require use of chronic steroids for treatment of their
   DMD, in the opinion of the Investigator, and are not receiving steroids at the time of
   Screening.

   - rAAVrh74 antibody titers are not elevated as per protocol-specified requirements.

   - Genetic mutation inclusion criteria vary by cohort.

Exclusion Criteria:

   - Has a concomitant illness, autoimmune disease, chronic drug treatment, and/or
   cognitive delay/impairment that in the opinion of the Investigator creates unnecessary
   risks for gene transfer.

   - Exposure to gene therapy, investigational medication, or any treatment designed to
   increase dystrophin expression within protocol-specified time limits.

   - Abnormality in protocol-specified diagnostic evaluations or laboratory tests.

Other inclusion/exclusion criteria apply.