Protein S deficiency is a disorder that increases the risk of developing blood clots. People with protein S deficiency are at risk of developing a type of clot called a deep vein thrombosis (DVT) that occurs in the deep veins of the arms or legs. A DVT can travel through the bloodstream and lodge in the lungs, causing a life-threatening clot called a pulmonary embolism (PE). Both DVTs and PEs are a type of venous thromboembolism, a general term for blood clots that form in a vein. Factors such as age, surgery, immobility, or pregnancy can increase the risk of clotting in people with protein S deficiency. In addition, people who have protein S deficiency and another inherited blood clotting disorder may have a higher risk of developing blood clots. Venous thromboembolism occurs in approximately fifty to sixty percent of people with protein S deficiency.