MECP2 and Rett syndrome: For example, Fragile X syndrome with the FMR1 gene residing in the X chromosome and preventing the synthesis of the RNA binding protein fragile X mental retardation protein (FMRP), or Rett syndrome, which affects girls where there is a mutation of the MECP2 gene, thus interfering with the normal methylation of cytosine phosphate guanine (CpG) islands and the activation/deactivation of gene expression.