C9 and amyotrophic lateral sclerosis: As a result of the expansion, three pathogenic mechanisms have been proposed as the underlying cause of C9-ALS/FTD: (1) loss of function due to G4C2 repeat expansion leading to downregulation of C9orf72 protein expression; (2) toxic gain of function by recruitment of other RNA-binding proteins into G4C2 RNA foci; and (3) the non-ATG initiated RAN translation of RNA repeats, which results in the production of toxic dipeptide protein repeat (DPRs) [3–5].