FMR1 and fragile X syndrome: However, MEF2 fails to eliminate functional or structural excitatory synapses in the hippocampus of Fmr1 KO mice, demonstrating FMRP functions downstream of MEF2-induced synapse elimination, consistent with the excess dendritic spines seen in Fmr1 KO mice and patients with FXS (Pfeiffer et al., 2010) FMRP and MEF2 cooperatively regulate the expression of protocadherin-10 (Pcdh10), a member of the cadherin superfamily of calcium-dependent cell adhesion molecules encoded by the PCDH10 gene.