C9orf72 and frontotemporal dementia: A total of 25% of familial FTD [36,44,50,51,52] and 6% of sporadic cases [53] are associated with a pathological expansion of the hexanucleotide GGGGCC (>30) in the first intron/promoter of C9orf72 [36,44,50,51,52,53].