C9orf72 and frontotemporal dementia: The most common genetic cause of ALS/FTD is an intronic hexanucleotide repeat expansion (HRE) of GGGGCC (G4C2) within the first intron of C9orf72, referred to as C9ALS/FTD (DeJesus-Hernandez et al, 2011; Renton et al, 2011).