Thrombotic thrombocytopenic purpura (TTP) is associated with low levels of von Willebrand factor cleaving protease, ADAMTS-13.1 ADAMTS-13 deficiency is due to gene mutations in the hereditary form of the disease, and to anti-ADAMTS-13 autoantibodies in the acquired form.2 It is characterized by the development of microthrombi in small blood vessels leading to consumptive thrombocytopenia and microangiopathic hemolytic anemia. The gene discussed is ADAMTS13; the disease is thrombotic thrombocytopenic purpura.