MMP9 and psychotic disorder: To test whether MMP‐9 levels influence performance in a behavioral model of the positive symptoms of schizophrenia, we used Mmp‐9 heterozygous mice with twofold lower levels of MMP‐9 in the brain (Fig EV2) and exposed them to psychosis‐related locomotor hyperactivity induced by NMDA receptor antagonist MK‐801 (van den Buuse, 2010).