When a person is injured, blood clotting is essential. However, once the danger has passed, it is equally essential to stop the clotting response in order to prevent thrombosis, or the obstruction of blood flow by clots. A protein called antithrombin is responsible for stopping coagulation, but about one in two thousand people have a hereditary deficiency in antithrombin that puts them at much higher risk of life-threatening blood clots.