Protein S (PS) is a vitamin K-dependent glycoprotein produced in the liver and circulates at levels of 20 ug/ml in plasma. PS is a single chain molecule of 69kD that acts as a cofactor in the proteolytic inactivation of factors VIIIa and Va by activated Protein C (APC). Approximately 60% of PS in plasma circulates as a complex with C4-binding protein (C4BP). Only the uncomplexed or