Protein C Inhibitor (PCI), a member of the serine protease inhibitor (serpin) superfamily, is abundant in plasma and other body fluids. Protein C Inhibitor inhibits a number of "arginine specific" serine proteases of the coagulation and fibrinolytic systems (including thrombin, trypsin, and activated Protein C), as well as plasma and tissue kallikreins and the sperm protease acrosin. This inhibition is regulated by glycosaminoglycans, such as heparin and heparan sulphate, and several polyanions. As well as its roles in the regulation of hemostasis and fertilisation, Protein C Inhibitor is involved in cell inflammation, proliferation, apoptosis, tumour cell migration, invasion, and metastasis. Recent studies have demonstrated an association between Protein C Inhibitor and multiple sclerosis lesions.