C9orf72 and frontotemporal dementia: In 2011, the pathogenic G4C2 hexanucleotide repeat expansion (HRE) in intron 1 of chromosome 9 open reading frame 72 (C9orf72) (Figure 1) was found to be the most common cause of familial ALS and FTD (DeJesus-Hernandez et al., 2011; Renton et al., 2011) with ∼40% fALS and 25% fFTD carrying the C9orf72 repeat expansion (Majounie et al., 2012).