FMR1 and fragile X syndrome: The most common cause of FXS is the expansion of CGG trinucleotide repeats within the 5′ untranslated region of the FMR1 gene, which triggers hypermethylation of the CpG island in the promoter region and subsequent silencing of the FMR1 gene, resulting in the loss of FMRP expression (Fu et al., 1991).