The identification of the hexanucleotide GGGGCC repeat expansion (G4C2exp) in the first intron of C9orf72 as a genetic cause of ALS and FTD in 2011 was a breakthrough discovery that linked ALS and FTD to a larger group of repeat expansion disorders [108,109,110,111]. This evidence concerns the gene C9orf72 and frontotemporal dementia.