This is an open-label, multi-center study to evaluate the efficacy and safety of eteplirsen in patients with genotypically confirmed Duchenne muscular dystrophy (DMD) with genetic deletions amenable to exon 51 skipping (treated group), with a concurrent control arm of DMD patients not amenable to exon 51 skipping (untreated group). Following primary efficacy endpoints, dosing will continue to week 144 to evaluate the long term effects of eteplirsen.