C9orf72 and frontotemporal dementia: Taken the above challenges together with the role of C9orf72 in signaling pathways previously implicated in ALS/FTD [64,65], C9orf72 haploinsufficiency combined with gain-of-function mechanisms and/or mutations in other modifier genes (for instance, Ataxin-2 Q30x, as discussed above) are possible mechanisms underlying the ALS/FTD pathogenesis.