Ultralarge multimers are proteolytically cleaved by ADAMTS-13 to yield the distribution of multimers that is observed in plasma and in purified preparations of VWF, including VWF/FVIII products used to treat von Willebrand disease and hemophilia A. Congenital or autoimmune-mediated deficiency of ADAMTS-13 results in the appearance of ultralarge VWF multimers and produces the disorder thrombocytopenic thrombotic purpura (14, 15). The gene discussed is ADAMTS13; the disease is thrombotic thrombocytopenic purpura.