ALS and/or FTD caused by mutations in C9orf72 (C9-ALS/FTD) is inherited in an autosomal dominant manner, suggesting that the HRE causes disease through gain-of-function or haploinsufficiency (DeJesus-Hernandez et al., 2011; Ling et al., 2013). Here, C9 is linked to frontotemporal dementia.