ADAMTS13 and thrombotic thrombocytopenic purpura: Thrombotic thrombocytopenic purpura (TTP) is due to a genetic or acquired severe deficiency of the von Willebrand factor-cleaving serine protease ADAMTS13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13), allowing unrestrained adhesion of the von Willebrand factor multimers to platelets and microthrombi formation, resulting in thrombocytopenia, hemolytic anemia, and tissue ischemia, mostly in the kidney and brain.