C9orf72 and amyotrophic lateral sclerosis: The first gene identified as a cause of ALS was SOD1 in 1993 (Rosen et al., 1993) and together with TARDBP (Sreedharan et al., 2008), FUS (Kwiatkowski et al., 2009), and chromosome 9 open reading frame 72 (C9orf72) (DeJesus-Hernandez et al., 2011; Renton et al., 2011), represent the most common ALS-related genes, covering up to 70% of familial and 10% of sporadic cases (Riva et al., 2016; Chia et al., 2018).