We will utilize the Cooperative International Neuromuscular Research group (CINRG) network to collect cohorts of Becker muscular dystrophy (BMD) patients with in-frame deletions in the dystrophin gene. We will collect clinical data across multiple body systems and correlate these findings to the high-resolution deletion break-point mapping performed from the tissue samples. We will investigate the observed variability to deepen our understanding of molecular mechanisms relevant to the optimization of exon skipping therapeutic approaches.