C9orf72 and amyotrophic lateral sclerosis: The most common genetic mutation linked to ALS is the hexanucleotide repeat expansion (HRE) GGGGCC (G4C2) present in the first intronic region of the C9orf72 gene, seen in 40% of fALS cases (DeJesus-Hernandez et al., 2011; Renton et al., 2014).