Protein S deficiency is a rare genetic disorder of blood coagulation that is caused by a variation in the PROS1 gene. This variation is inherited in an autosomal dominant manner. Affected individuals are at an increased risk of developing blood clots in the legs (deep venous thrombosis), which can break off and travel to the lungs, which is termed pulmonary embolism. Affected individuals are deficient in protein S, which is a specialized blood protein. Specifically, protein S is involved in inhibiting coagulation. This means that it helps to prevent the blood from clotting too much. Affected individuals who inherit one abnormal protein S gene are at risk for developing blood clots. Although very rare, there is a severe form that is present at birth (congenital) due to the presence of 2 abnormal protein S genes that can potentially cause widespread small clots in the body and life-threatening complications in infancy. Occasionally protein S deficiency may be acquired as a result of acquired conditions such as kidney disease (i.e., nephrotic syndrome), pregnancy, or the use of oral contraceptives.
