C9orf72 and amyotrophic lateral sclerosis: Importantly, the involvement of R-loops in expansion diseases is not limited to trinucleotide repeats, since R-loops associated with expanded hexanucleotide GGGGCC repeats in C9orf72 contribute to the molecular event leading to amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) [78].