Since the identification of FMR1 as the X-linked gene affected in FXS (Warren et al., 1987; Oberlé et al., 1991; Verkerk et al., 1991; Vincent et al., 1991), significant research efforts have focused on FMRP, the protein encoded by FMR1 (Ashley et al., 1993). Here, FMR1 is linked to fragile X syndrome.