F5 and Venous thrombosis: Prothrombin (PT) (G20210A) [2], methylenetetrahydrofolate reductase (MTHFR) (C677T) [1], and Factor V Leiden (G1691A) [3] are well-recognized genetic risk factors for venous thrombosis, while their role in patients with arterial thrombosis remains to be clarified.