C9orf72 and frontotemporal dementia: In 2011, an ALS-FTD causative mutation was identified in the form of a GGGGCC hexanucleotide repeat expansion (from a typical 5–10 repeats to hundreds or more) in an intron of the gene Chromosome 9 open reading frame 72 (C9orf72; DeJesus-Hernandez et al., 2011; Renton et al., 2011).