This is a multi-center natural history study that will be conducted at participating centers in the Cooperative International Neuromuscular Research Group (CINRG). Following a baseline evaluation, participants will have three follow-up visits over a three-year period. The investigators will characterize the Becker muscular dystrophy phenotype, and correlate specific abnormal dystrophin proteins with the range of clinical outcomes.

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.

Inclusion Criteria:

• Male
• Age 4 or older
• Diagnosis of BMD with an in-frame deletion in the dystrophin gene, where the boundaries of the mutations are confirmed.

Exclusion Criteria:

• Investigator assessment of inability to comply with protocol

BMD participants over 4 years of age with in-frame deletions in the dystrophin gene.