MMP9 and fragile X syndrome: Matrix metalloproteinase 9 (MMP-9), a member of the MMPs family, is a dendritic protein whose expression is tightly regulated by FMRP in neurons13. Specifically, upon synaptic stimulation of the glutamate receptor (mGluR), FMRP dissociates from the MMP-9 mRNA complex allowing MMP-9 translation to occur14. In FXS, the absence of FMRP leads to excessive synthesis of MMP-9.