In the case of fragile X syndrome, which is a hereditary form of mental retardation caused by functional absence of Fragile X mental retardation protein (FMRP), Schütt et al. reported that mRNA of SAPAP1, 2 and 3 were part of the targets of the FMRP and increased postsynaptic SAPAP2 levels was observed in the hippocampus of FMRP knock-out mice [29], The increased SAPAP2 level may contribute to the aberrant synaptic function in patients with Fragile X syndrome. This evidence concerns the gene DLGAP2 and fragile X syndrome.