FMR1 and fragile X syndrome: One such example is Fragile X syndrome (FXS), which has been linked to an expansion of the trinucleotide CGG repeat in the 5′ untranslated region (UTR) of the FMR1 gene located on the long arm of Chromosome X. When such an expansion occurs, the 5′ UTR and promoter of the gene become hypermethylated (Jin 2000; Alisch et al. 2013).