The most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) is a hexanucleotide (GGGGCC) repeat expansion in the first intron of C9orf72 (DeJesus-Hernandez et al., 2011; Renton et al., 2011). Here, C9orf72 is linked to frontotemporal dementia.