MMP9 and fragile X syndrome: Perineuronal nets (PNNs) are a specialized extracellular matrix structure in the cortex, which increases the excitability of PV cells (parvalbumin-expressing interneurons) and increases inhibitory signaling pathways; PNNs can be degraded by the activity of MMP-9, causing abnormal auditory system responses in Fragile X Syndrome (FXS) [79].