To date, two main approaches have been used: the first is the increase in defective thrombin formation through the inhibition of the naturally occurring anticoagulants, namely antithrombin, activated protein C, and tissue factor (TF) pathway inhibitor (TFPI) to rebalance the hemostatic system, an approach that can be applied to both hemophilia A and B; the second is a novel FVIII-activity mimicking agent, Mim8, which has been developed for patients with hemophilia A (Figure 1). The gene discussed is PROC; the disease is hemophilia A.