The herpes simplex virus (HSV) infection is initiated by VP16, a viral transcription factor that activates the viral immediate-early (IE) genes.The anticoagulant action of heparin is dependent on plasma components termed heparin cofactors. The first of these to be well characterized was antithrombin III. Heparin Cofactor II is antigenically distinct from AT III. Heparin Cofactor II is normal in patients with AT III deficiency and is low in patients with disseminated intravascular coagulation. It is expressed predominantly in liver. Its function is: thrombin inhibitor activated by the glycosaminoglycans, heparin or dermatan sulfate. In the presence of the latter, Heparin Cofactor II becomes the predominant thrombin inhibitor in place of antithrombin III (AT III). Also inhibits chymotrypsin, but in a glycosaminoglycan independent manner and peptides at the N terminal of Heparin Cofactor II have chemotactic activity for both monocytes and neutrophils. The N terminal acidic repeat region mediates, in part, the glycosaminoglycan accelerated thrombin inhibition. Defects in SERPIND1 are the cause of Heparin Cofactor II deficiency. Heparin Cofactor II deficiency is an important risk factor for hereditary thrombophilia, a multifactorial trait characterized by recurrent thrombosis and abnormal platelet aggregation in response to various agents. Heparin Cofactor II deficiency is inherited as an autosomal dominant disorder, in which affected individuals are prone to develop serious spontaneous thrombosis.