In addition to these common variants, rare but clinically important deficiencies of natural anticoagulants—including antithrombin (SERPINC1), protein C (PROC), and protein S (PROS1)—are well-established causes of hereditary thrombophilia, conferring a markedly elevated risk of venous thromboembolism [18,19,20,21]. This evidence concerns the gene PROS1 and venous thromboembolism.