FXS results from an epigenetic silencing of the responsible gene FMR1 caused by an abnormal expansion of a CGG repeat sequence in the 5′ untranslated region, leading to hypermethylation, transcriptional silencing, and a loss of expression of the protein product fragile X mental retardation protein (FMRP) [19,20,21]. This evidence concerns the gene FMR1 and fragile X syndrome.