C9orf72 and amyotrophic lateral sclerosis: As an example of the role in a human genetic disease, G-quadruplex structures of GGGGCC (G4C2) repeats form 800 to >4000 large hexanucleotide repeat expansions (HRE) in the C9orf72 gene implicated in the pathogenesis of C9orf72 amyotrophic lateral sclerosis and frontotemporal dementia (C9ALS/FTD) [44,45].