DMD and nervous system disorder: While we have primarily focused on the role of PEs in neurodevelopment and neurological disease, PE usage is also implicated in dystrophinopathies, which are characterized by progressive degeneration of skeletal muscle due to genetic lesions within the dystrophin encoding gene, DMD. DMD is one of the largest genes within the human genome, containing 79 protein-coding exons and 42 putative PEs.