FXS results from a trinucleotide expansion of a CGG repeat in the 5′ untranslated region of the fragile X mental retardation 1 (FMR1) gene on the X chromosome, that leads to atypical gene methylation and transcriptional silencing with the consequent reduction in the synthesis of the gene product (i.e., fragile X mental retardation protein or FMRP). This evidence concerns the gene FMR1 and fragile X syndrome.