FXS is mainly caused by an expansion of trinucleotide repeats (CGG) to over 200 repeats in the 5′ untranslated region of the fragile X messenger ribonucleoprotein 1 (FMR1) gene which leads to transcriptional silencing of the gene with a subsequent reduction or absence of FMR1 (also known as FMRP, fragile X mental retardation protein) [4, 5]. This evidence concerns the gene FMR1 and fragile X syndrome.