MMP9 and fragile X syndrome: Minocycline-based inhibition of MMP9 [330] or Mmp9 deficiency [331] rescues the immature dendritic spine phenotype in fragile X mouse hippocampal neurons in vitro, and pharmacological blockade [257] or genetic reduction [256] of MMP-9 restores in vivo cortical PNN density in FXS mice, as discussed earlier.