FXS is mainly caused by a massive CGG trinucleotide repeat expansion (usually more than 200 repeats) within the 5′ untranslated region (UTR) of the fragile X mental retardation 1 gene (FMR1), which results in abnormal DNA methylation of both a nearby CpG island and the repeat itself; as a result, the transcription of FMR1 is silenced [2]–[6]. Here, FMR1 is linked to fragile X syndrome.