FMR1 and fragile X syndrome: FXS results from a trinucleotide repeat (CGG) expansion mutation of >200 repeats (full mutation) in the promoter of FMR1 (fragile X mental retardation 1) gene (3) which leads to transcriptional silencing of FMR1 and loss or significant reduction of expression of the gene product, FMRP (fragile X mental retardation protein) (4).