Inclusion Criteria: * Definitive diagnosis of DMD based on documented clinical findings and prior genetic testing. * Cohort 1 only: Non-ambulatory per protocol-specified criteria. * Cohort 2 only: Ambulatory per protocol-specified criteria and ≥8 to \<18 years of age at the time of Screening. * Ability to cooperate with motor assessment testing. * Stable daily dose of oral corticosteroids for at least 12 weeks prior to Screening, and the dose is expected to remain constant throughout the study (except for modifications to accommodate changes in weight). * Recombinant Adeno-Associated Virus Serotype rh74 (rAAVrh74) antibody titers are not elevated as per protocol-specified requirements. * A pathogenic frameshift mutation or premature stop codon in the DMD gene, except for any deletion mutations in exon 8 and/or 9. Exclusion Criteria: * 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. * Presence of any other clinically significant illness, medical condition, or requirement for chronic drug treatment that in the opinion of the Investigator creates unnecessary risk for gene transfer. Other inclusion or exclusion criteria could apply.