Serine protease inhibitor that inhibits plasminogen activators and plasmin but not thrombin (PubMed:9442076, PubMed:26329378, PubMed:19265707, PubMed:19285087, PubMed:11880376). May be involved in the formation or reorganization of synaptic connections as well as for synaptic plasticity in the adult nervous system. May protect neurons from cell damage by tissue-type plasminogen activator (Probable). Monomer. Has a tendency to form large polymers already at 41 and 45 degrees Celsius (in vitro). Detected in brain cortex and hippocampus pyramidal neurons (at protein level) (PubMed:17040209). Predominantly expressed in the brain (PubMed:9070919). The disease is caused by variants affecting the gene represented in this entry. Belongs to the serpin family.