C9orf72 and frontotemporal dementia: Hexanucleotide repeat expansions (GGGGCC) within the first intron of the chromosome 9 open reading frame 72 (C9orf72) gene are the most common cause of FALS and FTD (∼30–50% of FALS, ∼10% of SALS 25% of familial FTD and ∼5% of apparently sporadic ALS and FTD) (DeJesus-Hernandez et al., 2011b; Renton et al., 2011; Majounie et al., 2012; Devenney et al., 2014) (Supplementary Table 1 and Figure 2), in both Europe and North America (DeJesus-Hernandez et al., 2011b; Renton et al., 2011).