Hexanucleotide-repeat GGGGCC expansions (30-4,400) in the first intron of the C9ORF72 gene are the most commonly identified genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) (DeJesus-Hernandez et al., 2011; Renton et al., 2011). The gene discussed is C9orf72; the disease is frontotemporal dementia.