C9orf72 and amyotrophic lateral sclerosis: Expansion of a GGGGCC hexanucleotide repeat (hereafter G4C2) within the first intron of the C9orf72 gene is the most frequent genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) (DeJesus-Hernandez et al., 2011; Renton et al., 2011).