Serpin B12 is a member of the serpin family. Serpins are the largest and most diverse family of serine protease inhibitors. Most serpins are secreted and attain physiologic concentrations in the blood and extracellular fluids. Serpin B12 is expressed in many tissues, including brain, bone marrow, lymph node, heart, lung, liver, pancreas, testis, ovary, and intestine. Serpins are involved in a number of fundamental biological processes such as blood coagulation, complement activation, fibrinolysis, angiogenesis, inflammation and tumor suppression and are expressed in a cell-specific manner. SerpinB12 inhibits trypsin and plasmin, but not thrombin, coagulation factor Xa, or urokinase-type plasminogen activator.