Expansions of a GGGGCC hexanucleotide repeat in the first intron and/or promoter region of C9orf72 are the most frequent known monogenic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) in populations of European descent (DeJesus-Hernandez et al., 2011, Gijselinck et al., 2012, Majounie et al., 2012, Renton et al., 2011). This evidence concerns the gene C9orf72 and frontotemporal dementia.