The fragile site associated with FXS is located at the Fragile X messenger ribonucleoprotein 1 (FMR1) gene, which contains a large CGG repeat in the 5’ UTR of affected individuals (>200 repeats) (Fu et al., 1991; Heitz et al., 1991; Oberlé et al., 1991; Verkerk et al., 1991; Yu et al., 1991). Here, FMR1 is linked to fragile X syndrome.