The etiology of FXS lies in an abnormal trinucleotide repeat (CGG) expansion in the 5′UTR of the fragile X mental retardation 1 (FMR1) gene, which leads to its hypermethylation, transcriptional silencing, and subsequent lack of the fragile X mental retardation protein (FMRP)5. Here, FMR1 is linked to fragile X syndrome.