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In 1979, researchers in Seattle, Wash, first discovered protein S and arbitrarily named it after the city of its discovery. Protein S is a vitamin K–dependent anticoagulant protein. Its major function is as a cofactor to facilitate the action of activated protein C (APC) on its substrates, activated factor V (FVa) and activated factor VIII (FVIIIa). Protein S deficiencies are associated with thrombosis.
Protein S deficiency may be hereditary or acquired, the latter is usually due to hepatic diseases or a vitamin K deficiency. Protein S deficiency usually manifests clinically as venous thromboembolism (VTE). The association of protein S deficiency with arterial thrombosis appears coincidental or weak at best. Arterial thrombosis is not evident with other hereditary anticoagulant abnormalities (eg, protein C or antithrombin III deficiency, factor V Leiden gene mutation). Protein S deficiency manifests as an autosomal dominant trait; manifestations of thrombosis are observed in both heterozygous and homozygous genetic deficiencies of protein S.
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