FMR1 and fragile X syndrome: The fundamental difference between commonly used FXS model mice (Fmr1−/y mice8) and patients is that mice never have any FMRP, beginning with their very first moment of existence, whereas in patients, the gene is active at least until the 10th week of gestation (Devys et al., 1992; Sutcliffe et al., 1992; Suzumori et al., 1993; Iida et al., 1994; Willemsen et al., 2002).