The prophylactic treatment of hemophilia involves the replacement of factor VIII or IX in affected persons. A new approach involves the depletion of the clot inhibitor antithrombin. Hemophilia is an X-linked bleeding disorder resulting from a deficiency of coagulation factor VIII or IX (causing hemophilia A or B, respectively) and is characterized by spontaneous and traumatic bleeding into the muscles, joints, and body cavities. Treatment is expensive, with an annual cost of several hundred thousand dollars for an average adult, and the complications of bleeding often result in greater severity of disease, poor quality of life, a requirement for orthopedic surgery, and a shortened life span. To prevent bleeding and its complications, the missing coagulation factor is replaced several times weekly by means of an intravenous infusion, . . .