Thrombotic thrombocytopenic purpura (TTP) is a disease that develops when a disintegrin-like and metalloproteinase with thrombospondin type l motif 13 (ADAMTS13) activity decreases to < 10% of that in normal plasma, causing platelet thrombosis in microvessels throughout the body. The gene discussed is ADAMTS13; the disease is thrombotic thrombocytopenic purpura.