C9orf72 and amyotrophic lateral sclerosis: Multivalent intermolecular base pairings are also created by GC repeats that are involved in many repeat expansion diseases such as CAG in Huntington disease and spinocerebellar ataxias and a hexanucleotide expansion GGGGCC (G4C2) in the chromosome 9 open reading frame 72 (C9orf72) gene in amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD).