Antithrombin-III (AT-III), also known as serpin C1, is the most important serine protease inhibitor in plasma that regulates the blood coagulation cascade. AT-III inhibits serine proteases of the intrinsic coagulation pathway such as thrombin and factors IXa, Xa and XIa. Its inhibitory activity is greatly enhanced in the presence of heparin. It also inhibits matriptase-3/TMPRSS7; this inhibition seems to be lost in the majority of carcinoma cells.This entry corresponds to the antithrombin serpin domain.