C9orf72 and frontotemporal dementia: Methylation at the C5 position of cytosine within (GGGGCC)8•(GGCCCC)8 repeats—associated with two neurodegenerative diseases amyotrophic lateral sclerosis (ALS) and fronto-temporal dementia (FTD)—was reported to influence stability of G4 structures in the promoter as well as coding region of the C9orf72 gene [38].