Protein S deficiency is a disorder that causes abnormal blood clotting. When someone bleeds, the blood begins a complicated series of rapid chemical reactions involving proteins called blood coagulation factors to stop the bleeding. Other proteins in the blood, such as protein S, usually regulate these chemical reactions to prevent excessive clotting. When protein S is missing (deficient), clotting may not be regulatednormally and affected individuals have an increased risk of forming a blood clot called a thrombosis. People at risk to haveprotein S deficiency are those with an individual or family history of multiple blood clots in the veins. Treatment may include taking medication known as blood thinners to decrease the chance of developing a blood clot. The disease is protein S deficiency (MONDO_0002304). Also known as: Protein S deficiency, Protein S deficiency disease.