More than 95% of fragile X syndrome is caused by reduced or absent fragile X intellectual disability protein 1 (FMRP) synthesis due to dynamic mutation expansion of the CGG triplet repeat in the 5'UTR and abnormal methylation of the <i>FMR1</i> (fragile X messenger ribonucleoprotein 1) gene [OMIM 309550]. The gene discussed is FMR1; the disease is fragile X syndrome.