C9orf72 and amyotrophic lateral sclerosis: The GGGGCC (G4C2) hexanucleotide repeat expansion (HRE) in the non-coding region of the chromosome 9 open reading frame 72 (C9orf72) gene is considered the most prevalent genetic abnormality associated with the spectrum disease of amyotrophic lateral sclerosis and frontotemporal dementia (ALS/FTD) to date (DeJesus-Hernandez et al., 2011; Renton et al., 2011b).