C9orf72 and amyotrophic lateral sclerosis: In contrast, three non-mutually exclusive mechanisms have been proposed as a potential cause of C9orf72-linked ALS: loss-of-function of the C9orf72 protein, sequestration of essential RBPs by foci containing G4C2-containing RNA, and toxicity induced by one or more dipeptide repeat (DPR) proteins9,10.