A group of drugs, also known as thrombolytics, with the ability to break down the protein FIBRIN, the prime constituent of blood clots (see THROMBUS; THROMBOSIS). They are used to disperse blood clots that have formed in the vessels of the circulatory system. The group includes STREPTOKINASE, alteplase and reteplase. The drugs work by activating PLASMINOGEN to form PLASMIN which degrades FIBRIN and breaks up the blood clot (see COAGULATION).