The G4C2-repeat expansion in C9orf72 is the most common known cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) (DeJesus-Hernandez et al., 2011; Renton et al., 2011; Gijselinck et al., 2012) in Caucasians. Here, C9orf72 is linked to frontotemporal dementia.