C9orf72 and amyotrophic lateral sclerosis: A hexanucleotide GGGGCC repeat expansion in the first intron of the C9orf72 gene is the most frequent genetic cause of FTD and ALS (collectively C9ALS/FTD), resulting in both loss- and gain-of-function effects (Balendra and Isaacs, 2018; DeJesus-Hernandez et al., 2011; Renton et al., 2011).