FXS results from a mutation in a single gene on the X chromosome, FMR1. In the majority of cases, a >200 copy trinucleotide (CGG) repeat expansion in the 5′-UTR of the FMR1 gene (59) is associated with transcriptional silencing of the FMR1 promoter and loss of expression of fragile X mental retardation protein (FMRP) (60). The gene discussed is FMR1; the disease is fragile X syndrome.