C9 and frontotemporal dementia: Three major pathomechanisms are thought to be involved in the pathogenesis of C9orf72-linked ALS/FTD (C9-ALS/FTD): first, expansion of the G4C2 repeats results in decreased expression of the C9orf72 gene, leading to its haploinsufficiency (Boivin et al., 2020; DeJesus-Hernandez et al., 2011; Gijselinck et al., 2012; Shi et al., 2018; Waite et al., 2014; Zhu et al., 2020).