This prospective study of skeletal muscle, cardiac, and diaphragm imaging at the NIH will be offered to all subjects participating in a phase 2, double blind, exploratory parallel-group, placebo-controlled clinical study in ambulatory subjects with DMD resulting from a mutation that can be corrected by exon 51 skipping induced by GSK2402968 (parent study; DMD114876). Subjects will travel with a family member to the NIH for MRI and ultrasound assessments during the screening phase of the parent study or up to 3 weeks after randomization and additionally at the following time points in the parent study: at 12 weeks ( 3 weeks), and 24 weeks ( 3 weeks) during the blinded treatment period; and finally, after completion of 24 week post-treatment phase (at 48 weeks 4 weeks). If not randomized, the subjects will have a one-time evaluation during the screening phase of the parent study. Pilot data also will be obtained from healthy boys (matched for the age-range) for comparisons to allow exploration of MRI and ultrasound measures specific to pathology in the ambulatory boys with DMD. Subjects will not be treated with GSK2402968 or any other experimental drug at the NIH. There are no follow-up or termination procedures for this study.