C9orf72 and frontotemporal dementia: In the majority of familial FTD cases, hexanucleotide expansions in the C9orf72 gene (20–30%), mapped on chromosome 9, autosomal dominant mutations on chromosome 17 involving the granulin (GRN) gene (5–25%) and microtubule-associated protein tau (MAPT) gene (5–20%) are identified [21,22].