C9orf72 and frontotemporal dementia: A (G4C2)n hexanucleotide repeat expansion (HRE) in the C9orf72 gene accounts for 30–50% of FALS, as well as 5–10% of SALS cases [2], and represents the most common genetic defect in ALS and in frontotemporal dementia (FTD) [3, 4].