The fragile X syndrome (FXS, MIM #300624), the most common cause of inherited mental retardation, is due to the amplification (> 200 repeats) of a sequence of CGG triplets in the 5' UTR of the FMR1 gene, followed by methylation of cytosines, including those of the promoter upstream [1]. Here, FMR1 is linked to fragile X syndrome.