MMP9 and fragile X syndrome: Additionally, genetic reduction (e.g., haploinsufficiency) of MMP-9 [256] as well as MMP-2/9 inhibitor treatment [257] restores developmental PNN impairments in Fmr1 knockout mice, a model of Fragile X Syndrome (FXS); interestingly, MMP-2/9 inhibitor treatment also enhances WT PNN formation in the developing auditory cortex [257].