Familial hexanucleotide GGGGCC repeat expansion in the C9orf72 gene accounts for a third of familial amyotrophic lateral sclerosis and a quarter of frontotemporal dementia (FTD) cases, leading to its haploinsufficiency and inducing the formation of RNA-rich nuclear aggregates [160]. This evidence concerns the gene C9orf72 and frontotemporal dementia.