FXS is a single gene disorder in which a triplet repeat (CGG) expansion mutation [2] inactivates the FMR1 (fragile X mental retardation 1) gene, resulting in loss or significant reduction of expression of the FMR1 gene product, FMRP (fragile X mental retardation protein) [3, 4]. The gene discussed is FMR1; the disease is fragile X syndrome.