Mondo Description 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 havean increased risk of forming a blood clot called a thrombosis. People at risk to haveprotein S deficiencyare 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.