C9orf72 and fragile X syndrome: Many of these repeat sequences have been shown to form non-canonical DNA structures such as hairpins [e.g., spinocerebellar ataxia type 2, (CAG)n]58, G-quadruplexes [e.g., C9orf72 amyotrophic lateral sclerosis-frontotemporal dementia, (GGGGCC)•(GGCCCC)]59, triplexes [e.g., Friedreich’s ataxia, (GAA•TTC)]60, Z-DNA [e.g., skeletal displasias, (GAC)n]61, and slipped DNA junctions [e.g., myotonic dystrophy type 1, (CTG)•(CAG) and fragile X syndrome, (CGG)•(CCG)]62.