In most cases, FXS is caused by a trinucleotide repeat expansion mutation in the 5′-UTR of the FMR1 gene [10] that is associated with transcriptional silencing of the FMR1 promoter and loss of expression of fragile X messenger ribonucleoprotein (FMRP) [11]. The gene discussed is FMR1; the disease is fragile X syndrome.