C9orf72 and frontotemporal dementia: The hexanucleotide repeat, r(G4C2)exp, harbored in intron 1 of chromosome 9 open reading frame 72 (C9orf72) mRNA, causes genetically defined amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) (c9ALS/FTD) (25,26).