Serine protease inhibitor that inhibits plasminogen activators and plasmin but not thrombin (PubMed:9442076). 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 embryonic ocular vitreous fluid (at protein level) (PubMed:9442076). In the embryo present in retina, brain, cerebellum and spinal cord (PubMed:8670795). In adult, predominantly expressed in the brain (PubMed:8670795). Belongs to the serpin family.