Patients with a deficiency of the VWF present with bleeding, whereas severe deficiency (<10% of normal levels) of ADAMTS-13 causes thrombotic thrombocytopenic purpura.9 Severe deficiency in ADAMTS-13 activity is caused by the presence of inhibitory antibodies against ADAMTS-13 or gene polymorphisms. The gene discussed is ADAMTS13; the disease is thrombotic thrombocytopenic purpura.