C9orf72 and frontotemporal dementia: Associations with loci on chromosome 17 and 9q and subsequent causative genes for both diseases were identified (TARDBP gene, FUS gene), leading to the pivotal discovery of the chromosome 9 open reading frame 72 (C9orf72) hexanucleotide GGGGCC repeat expansion in 2011 (DeJesus-Hernandez et al., 2011; Renton et al., 2011), which was rapidly recognized as the major cause of both familial FTD and familial ALS, and the most frequent mutation associated with FTD-ALS cases.