AFFINITY DUCHENNE: RGX-202 Gene Therapy in Participants With Duchenne Muscular Dystrophy (DMD)

Part 1 - Key Inclusion Criteria: * The participant's legal guardian(s) is (are) willing and able to provide written, signed informed consent prior to any study-related procedures; and, where applicable, the minor participant has provided written or verbal assent according to local requirements. * Is a male at least 4 years of age and less than 12 years of age at consent or 1 to \<4 years of age at the time of dosing and ≥ 10 kg at the time of screening. * Must meet any of the following criteria: * DMD gene mutation in exons 18 and above, and a clinical picture consistent with typical DMD with the exception of a participant (Cohort 1b) with DMD gene mutation in exons 12-17. * Participant is able to walk 100 meters independently without assistive devices. Cohort 2c participant must be able to walk 10 meters independently without assistive devices. Cohort 1b participant must be able to walk with or without assistive devices. * Participant is able to complete the TTSTAND per protocol-specific criteria. * Participant has been on a stable dose of systemic glucocorticoids according to the standard of care for at least 12 weeks. Cohort 2c participants must be consistently on or off a stable dose of systemic glucocorticoids according to the standard of care for at least 12 weeks. * Clinical laboratory test results, including hepatic and renal function, are within the normal range during screening, or if abnormal, are not clinically significant, in the opinion of the invest…