Chiò et al. demonstrated that C9orf72 repeat expansions have a primary role in increasing the risk of cognitive impairment in patients with ALS, and only in the presence of the C9orf72 repeat expansion, the APOE ε2 allele, to a lesser extent, also increases the risk of FTD in this subpopulation [53,54▪▪]. Here, C9orf72 is linked to frontotemporal dementia.