The study will assess the efficacy of PF-06939926 gene therapy on ambulatory function while also monitoring its safety. Approximately 99 boys with DMD will be enrolled and randomly assigned to one of two groups: approximately two thirds will be in Cohort 1 and receive gene therapy at the start of the study; approximately one third will be in Cohort 2 and receive placebo at the start of the study and receive gene therapy after one year, as long as it remains safe to do so. The treatment (PF-06939926 gene therapy or placebo) will be given as an intravenous infusion lasting up to 2 hours.

The study includes boys who are at least 4 years old and less than 8 years old (including 7 year olds up until their 8th birthday). All boys will need to be on a daily dose of glucocorticoids (prednisone, prednisolone, or deflazacort) for at least 3 months prior to enrolling and to stay on daily glucocorticoids for the first 2 years of the study. All boys will need to be negative for neutralizing antibodies against AAV9, as measured by the test done for the study as part of screening.

The primary outcome of the study will be assessed at 52 weeks. All participants will be followed in the study for 15 years after treatment with gene therapy. Participants who received fordadistrogene movaparvovec in Pfizer studies C3391001 and C3391008 or are currently enrolled in Pfizer study C3391011 will be allowed to roll over into the long-term safety follow-up period of this study and will be considered Cohort 3.

The study medication, all medical tests associated with the study, and the visits to the study sites are free of charge. Participants will also be supported for travel costs associated with study visits.