Protein S is a vitamin K-dependent plasma glycoprotein synthesized in the liver and it functions as a cofactor to Protein C in the inactivation of Factors Va and VIIIa.

In the circulation, Protein S exists in two forms: a free form and a complex form bound to complement protein C4b. Only the free form has cofactor activity.

The gene (PROS1) is located on the third chromosome (3p11.1-q11.2).

Protein S deficiency is a rare blood disorder which can lead to an increased risk of thrombosis.

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