PROC and chronic thromboembolic pulmonary hypertension: Although some studies have suggested that variations in SERPINC1, a gene encoding antithrombin (AT), may increase the risk of PE and CTEPH due to impaired anticoagulant activity, classical hereditary thrombophilia risk factors (i.e., causative mutations in SERPINC1, PROC, PROS1 and presence of factor V Leiden and prothrombin 20210A) seemed not to be independent risk factors of CTEPH [16,17].