Hemophilia A is a bleeding disorder caused by a deficiency of factor VIII, a protein essential for blood clotting. Injection of recombinant or plasma-derived factor VIII is an effective prophylactic treatment. Factor replacement therapy has drawbacks, however, as it requires multiple injections each week and can induce antibodies that inhibit factor VIII. Shima et al.clinically tested an alternative therapy: an engineered bispecific antibody called emicizumab that mimics the critical function of factor VIII, which is to bridge together clotting factors IXa and X. Nineteen patients with severe hemophilia A were injected with emicizumab once a week for 12 weeks. Most patients showed reduced bleeding rates without adverse effects.