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. SERPINE1， however， is the main inhibitor of the plasminogen activators.
