Recently, it was reported that an hexanucleotide GGGGCC (G4C2) repeat of the C9orf72 gene in human, which is the most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), adopts an antiparallel four-layer chair-type G-quadruplex with or without C•C/C+•C base pair in the three lateral CC loops (62–64). The gene discussed is C9orf72; the disease is frontotemporal dementia.