Duchenne muscular dystrophy (DMD) is the most frequent inherited fatal childhood disease. Antisense oligonucleotide (AON)-induced exon skipping is a promising therapeutic strategy for DMD that is currently being explored in clinical trials. Magnetic resonance imaging (MRI) and ultrasound imaging methods are sensitive to key processes in dystrophic muscle such as edema and fat infiltration and therefore could serve as a biomarker of disease progression and therapeutic response. Our objective is to explore the potential of these imaging biomarkers for GSK2402968 (AON) effects in ambulatory boys with DMD. The primary objective is to assess longitudinal changes in skeletal muscle structural MRI measures reflecting fat and edema in the lower extremities in ambulatory boys with DMD receiving GSK2402968 or placebo