Factor V Leiden thrombophilia is characterized by venous thromboembolism (VTE) manifesting most commonly in adults as deep vein thrombosis (DVT) in the legs or pulmonary embolism. Thrombosis in unusual locations is less common. Factors that predispose to VTE in factor V Leiden thrombophilia include: the number of factor V Leiden variant alleles (homozygotes have a much greater thrombotic risk); family history of VTE; presence of coexisting genetic thrombophilic disorders; acquired thrombophilic disorders (e.g., antiphospholipid antibody syndrome, paroxysmal nocturnal hemoglobinuria, myeloproliferative disorders); and circumstantial risk factors (e.g., pregnancy, malignancy, central venous catheters, travel, combined oral contraceptive use and other combined contraceptives, oral hormone replacement therapy [HRT], obesity, leg injury, and advancing age).