C9orf72 and amyotrophic lateral sclerosis: Finally, products of the C9orf72 hexanucleotide repeat expansion (HRE), G4C2 RNA foci and dipeptide repeat (DPR) proteins have been identified in skeletal muscles of animal models of C9-ALS [88,89] and in the skeletal muscles of ALS patients with C9orf72 mutations [90,91].