The most common genetic cause of familial ALS is a massive expansion of the GGGGCC hexanucleotide repeat region in the first intron of C9orf72 (DeJesus-Hernandez et al. 2011; Renton et al. 2011; Gijselinck et al. 2012). Here, C9orf72 is linked to amyotrophic lateral sclerosis.