Genetic evidence suggests that both diseases form part of a clinical spectrum, most prominently underpinned by a large GGGGCC (G4C2) expansion in intron 1 of chromosome 9 open reading frame 72 (C9orf72) (DeJesus-Hernandez et al., 2011; Renton et al., 2011) that is the most common cause of ALS and FTD (C9ALS/FTD) (Stepto et al., 2014; Rohrer et al., 2015). Here, C9orf72 is linked to frontotemporal dementia.