Primary Outcome Measure: MRI changes in skeletal muscle percent fat in the lower extremities using T1w GRE Dixon method at 24 weeks from baseline in the parent study in ambulatory boys with DMD receiving GSK2402968 or placebo. Secondary outcome measures: Differences in the following outcome measures between healthy boys and ambulatory boys with DMD at baseline; and changes in these measures over time in the parent study at 12 weeks, 24 weeks, and 48 weeks from baseline in ambulatory boys with DMD receiving GSK2402968 or placebo: 1. Skeletal muscle MRI: relative muscle fat/water quantified by T1w GRE Dixon imaging method in skeletal muscles; Muscle edema assessed by T2 imaging; Muscle fat/water content and edema additionally quantified by IDEAL-CPMG method; and 2. Cardiac MRI: Cardiac function (ejection fraction/ LV function) assessed by SSFP Cine MRI and manual planimetry of LV volumes and mass at end systole and end diastole; Myocardial fat content assessed by Multiecho Dixon Fat /Water Separation method; Myocardial edema assessed by T2 quantification; Myocardial T1 assessed by MOLLI (modified Look-Locker Inversion recovery).