C9orf72 and amyotrophic lateral sclerosis: The presence of hexanucleotide GGGGCC (G4C2) repeat expansions in the C9orf72 gene represents the most common genetic cause of Frontotemporal Dementia (FTD) and Amyotrophic Lateral Sclerosis (ALS), two devastating neurodegenerative disorders commonly designated as c9FTD/ALS (Barker et al., 2017).