Protein S (PTS) is a Vitamin-K dependant factor which serves as a cofactor for activated Protein C in the degradation of Factors V and VIII. It circulates in blood plasma in both free and bound (to C4b binding protein) forms. Only the free portion has activity. Inherited PTS deficiency can account for recurrent thrombo-embolic episodes in young adults. Most cases are heterozygous. Acquired PTS deficiency can occur with Vitamin K deficiency, Oestroprogesterone therapy, oral anticoagulation therapy, liver disease, DIC, the third trimester of pregnancy, and in the early post-operative phase.