The fragile X mental retardation 1 (FMR1) gene, discovered in 1991, encodes the fragile X mental retardation protein (FMRP), which is absent and leads to FXS when a triplet repeat expansion (CGG)—located in the 5′ untranslated region of the first exon of the FMR1gene—expands to more than 200 copies and the promoter region becomes hypermethylated (full mutation or FM). The gene discussed is FMR1; the disease is fragile X syndrome.