ADAMTS13 and thrombotic thrombocytopenic purpura: Sometimes, the initial mechanism is unknown, as in the case of thrombotic thrombocytopenic purpura (TTP), in which the metalloenzyme ADAMTS13, a protease that transforms von Willebrand factor (FVW), which has a powerful pro-coagulant activity, into low-molecular-weight polymers (with low pro-coagulant activity), decreases its function, and promotes the formation of platelet aggregates.