MMP9 and fragile X syndrome: KO: knock-out; FXS: fragile X syndrome; STEP: striatal-enriched protein tyrosine phosphatase; APP: amyloid precursor protein; MMP9: matrix metalloproteinase 9; AMPA: α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid; FMRP: fragile X mental retardation 1 protein; BDNF: brain-derived neurotrophic factor; TBS: Townes–Brocks syndrome; LTP: long-term potentiation; GABA: gamma-aminobutyric acid; NMDA: N-methyl-D-aspartate, ERK: extracellular signal-regulated kinase