FMR1 and fragile X syndrome: For example, fragile X syndrome (FXS) has been attributed to the absence of fragile X mental retardation 1 (FMR1) gene expression due to a long CGG tri-nucleotide repeat in the FMR1 gene 5′-UTR adjacent to the promoter that results in the FMR1 gene’s epigenetic silencing (Colak et al., 2014).