Fragile X syndrome (FXS), and its related diseases-fragile X-associated immature ovarian insufficiency (FXPOI) and fragile X-associated tremor/ataxia syndrome (FXTAS)-are caused by the expansion of a CGG repeat tract located in the 5′-UTR of fragile X mental retardation 1 gene (FMR1) [104]. The gene discussed is FMR1; the disease is fragile X syndrome.