Protein Z-dependent protease inhibitor （ZPI） is a protein circulating in the blood which inhibits factors Xa and XIa of the coagulation cascade. Inhibits activity of the coagulation protease factor Xa in the presence of PROZ, calcium and phospholipids. Also inhibits factor XIa in the absence of cofactors. Heparin acts as an important cofactor, producing 20 to 100-fold accelerations of SERPINA10 reactions with factor Xa and factor XIa. It is a member of the class of the serine protease inhibitors （serpins）. Its name implies that it requires protein Z, another circulating protein, to function properly, but this only applies to its inhibition of factor X. It is about 72 kDa heavy and 444 amino acids large. It is produced by the liver.