FXS occurs when FMR1 is silenced by methylation or inactivation due to an abnormal expansion of a CGG trinucleotide (>200 repeats and called Full mutation, FM), located in the untranslated sequence at 5′, before the FMR1 gene's first exon (Oberle et al., 1991; Verkerk et al., 1991; Yu et al., 1991). The gene discussed is FMR1; the disease is fragile X syndrome.