Five major inherited disorders are considered risk factors for recurrent venous thrombosis: protein C, protein S, or antithrombin III deficiency, hyperhomocystinemia, and activated protein C (APC) resistance. In 1993, Dahlbäck1 noted that the addition of APC to plasma obtained from a patient with recurrent thrombosis, without any previously recognized thrombotic disorder, did not prolong the activated partial thromboplastin time as occurred when APC was added to plasma from healthy individuals. The term APC resistance was used to describe this patient. Most patients with APC resistance have a mutant factor V molecule (Leiden mutation) that resists proteolysis by APC when activated to factor Va. This resistance is the result of a specific point mutation (replacement of arginine 506 by glutamine) in the gene coding for coagulation factor V. Not all patients with APC resistance will experience thrombosis. Additional genetic defects or exogenous factors may be necessary to induce thrombosis in
Depaire-Duclos F, Gris JC, Dandurand M, Guillot B. Thrombotic Klinefelter Syndrome Associated With Factor V Leiden Mutation. Arch Dermatol. 1997;133(8):1051–1052. doi:10.1001/archderm.1997.03890440135027
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