C9orf72 and amyotrophic lateral sclerosis: While most ALS cases lack an identifiable genetic cause, the expansion of the G4C2 hexanucleotide repeats (GGGGCC) within the first intron of the chromosome 9 open reading frame 72 (C9orf72) gene contributes to approximately 25%–40% of familial cases and 5% of sporadic ALS/FTD cases (Balendra and Isaacs, 2018; Boillée et al., 2006).