FXS, which can cause intellectual disability and has a suggested linked to autism spectrum disorders, is triggered by the transcriptional repression of fragile X mental retardation gene 1 (FMR1) due to the hypermethylation of the CGG repeat motif when pathologically expanded (> 200 repeats) in the 5′-UTR of the gene. Here, FMR1 is linked to fragile X syndrome.