FMR1 and fragile X syndrome: The genetic disruption of MMP-9 in the Fmr1 KO mouse rescued key aspects of Fmr1 abnormalities, including abnormal mGluR5-dependent LTD and dendritic spine abnormalities [150], providing evidence that MMP-9 is necessary to the development of FXS-associated defects in the Fmr1 KO mouse.