C9orf72 and amyotrophic lateral sclerosis: The most common genetic cause of ALS stems from an expansion mutation in C9orf72 (chromosome 9 open reading frame 72), characterized by a hexanucleotide repeat (HRE) expansion of GGGGCC in the first intron of the gene (DeJesus-Hernandez et al., 2011; Renton et al., 2011).