Ninety-nine percent of all FXS cases are caused by aberrant hypermethylation and hyperexpansion of a trinucleotide (CGG) repeat in the 5′ untranslated region of the FMR1 gene (Pieretti et al., 1991; Sutcliffe et al., 1992) located on chromosome Xq27.3 (Fu et al., 1991; Verkerk et al., 1991). The gene discussed is FMR1; the disease is fragile X syndrome.