Under normal circumstances, the coagulation system is balanced in favour of anticoagulation. Thrombin is the key effector enzyme of the clotting cascade. Antagonists of vitamin K inhibit a vitamin-K-dependent post-translational modification of several coagulation proteins, which is required for these proteins to attain a phospholipid-binding conformation. Heparin stimulates the activity of antithrombin, a serine-protease inhibitor. Analysis of knock-out mice has shown the relative importance of the coagulation factors in vivo. Gene therapy may soon be a therapeutic option for inherited deficiencies of factors VIII and IX.