Serine protease inhibitor that inhibits plasminogen activators and plasmin but not thrombin (PubMed:9364046, PubMed:11557034). 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). Detected in neurons in embryonic brain cortex (at protein level) (PubMed:17040209). During embryonic development mostly expressed in CNS (PubMed:9364046). In adult expressed in brain and much less in spinal cord, heart, kidney and testis (PubMed:9364046). Belongs to the serpin family.