Interestingly, in fragile X mental retardation 1 knock out mice (Fmr1 KO), a well-accepted model of FXS (Bear et al., 2004; Bassell and Warren, 2008; Graziano et al., 2008) in whichFMRP is no longer expressed, we found that synaptic FXR2 distribution is disrupted while FXR1 remains unperturbed. This evidence concerns the gene FXR2 and fragile X syndrome.