FXS occurs as a result of Fmr1 gene hypermethylation due to an unstable CGG triplet repeat expansion (> 200 repeats) in the 5′ untranslated region of the Fmr1 gene located on the X chromosome, which leads to gene methylation, inactivation, and resultant loss of fragile X mental retardation protein expression (FMRP; [147]). Here, FMR1 is linked to fragile X syndrome.