PROS1 and thrombophilia: Inherited thrombophilia is caused by a variety of genetic abnormalities in anticoagulant factors such as antithrombin (AT), protein C (PC), and protein S (PS), or coagulation factors such as prothrombin and factor V. Genetic abnormalities in anticoagulant factors (such as deficiencies of antithrombin, protein C and protein S) are found in < 1% of the population and often present with unusual clinical episodes and localization of venous thrombosis.