C9orf72 and frontotemporal dementia: Three non-exclusive pathogenic mechanisms have been implicated in C9ALS/FTD: (1) RNA-mediated toxicity4; (2) repeat-associated non-ATG translation of dipeptide proteins5 produced by both sense and antisense strands of the pathogenic allele, which induce neuronal dysfunction by interacting with key cellular proteins; and (3) loss of endogenous C9orf72 protein function due to reduced C9orf72 transcription resulting from the GGGGCC repeat expansion (C9orf72 haploinsufficiency)6.