Plasminogen activator inhibitor-1 is the principal inhibitor of tissue plasminogen activator（tPA) and urokinase（uPA)， the activators of plasminogenand hence fibrinolysis（the physiological breakdown of blood clots). It is a serine protease inhibitor（serpin) protein（SERPINE1). The other PAI， plasminogen activator inhibitor-2（PAI-2) is secreted by the placentaand only present in significant amounts during pregnancy. In addition， protease nexinacts as an inhibitor of tPA and urokinase. PAI-1， however， is the main inhibitor of the plasminogen activators.
