C9orf72 and amyotrophic lateral sclerosis: C9orf72 hexanucleotide repeat expansion (GGGGCC) is the most typical genetic origin of ALS and FTD (C9ALS/FTD) 10-12 and has been suggested to cause neurodegeneration through three potential mechanisms: (1) the accumulation of sense (GGGGCC) and/or antisense (CCCCGG) repeat RNA 10, 13-15, (2) the accumulation of dipeptide repeat (DPR) proteins translated from repeat RNA 14-19, and (3) haploinsufficiency of the C9orf72 protein 10, 20-22.