FMR1 and fragile X syndrome: FXS is characterized by the absence of expression of the fragile X mental retardation 1 (FMR1) gene due to CGG trinucleotide repeat expansion in the 5′ untranslated region of FMR1. This aberrant expression leads to hypermethylation and gene silencing, leading to the decrease or absence of FMRP (fragile X mental retardation protein) (46, 63).