Decreased amounts of certain proteins (protein C, protein S, and antithrombin III).

This is because the level of protein C and protein S are also dependent on vitamin K activity.

Various mechanisms, such as deficiency of protein S and tissue factor pathway inhibitor, are said to be responsible.

Numerous anti-clotting proteins (antithrombin, protein C, protein S, and others) exist in a natural balance with clotting factors.

Deficiencies in antithrombin, protein C, and protein S are rare but strong, or moderately strong, risk factors.

For populations in China, Japan, and Thailand, deficiences in protein S, protein C, and antithrombin predominate.

In humans, protein S is encoded by the PROS1 gene.

In 1979, researchers in Seattle, Washington first discovered protein S and arbitrarily named it after the city of its discovery.

Only free protein S has activated protein C cofactor activity.

Binding of activated protein C to protein S leads to a modest increase in its activity.