The most common cause of FXS is the expansion of CGG repeats in the 5′ untranslated region of the FMR1 gene resulting in hypermethylation and silencing of the gene, although other loss-of-function mutations in the FMR1 coding regions have also been identified in rare cases of FXS [3,4,5,6]. Here, FMR1 is linked to fragile X syndrome.