In Part 1, participants will be stratified by age at randomization (≥4 to <6 years or ≥6 to <8 years) and North Star Ambulatory Assessment total score (NSAA ≤22 points or >22 points) at screening and randomized (1:1) to receive a single intravenous dose of intended commercial process delandistrogene moxeparvovec material (1.33x10¹⁴ vg/kg by linear standard qPCR) or placebo. Participants will be evaluated at Week 52. In Part 2 (52-week follow-up period), participants randomized to placebo in Part 1 will receive delandistrogene moxeparvovec and participants randomized to delandistrogene moxeparvovec in Part 1 will receive placebo.

The primary endpoint is change from baseline to Week 52 in NSAA total score (Part 1). Secondary endpoints include safety; SRP-9001 dystrophin protein production at Week 12 by western blot and change from baseline to Week 52 in: key timed function tests, stride velocity 95^th centile measured by a wearable device, and Patient-Reported Outcomes Measurement Information Score^®.