SERPINC1 and Thromboembolism: Antithrombin (AT), a serine protease inhibitor, is the most important anticoagulant in the blood circulation system, accounting for 50% to 70% of the total anticoagulant activity in plasma.[7] It is the most known effective endogenous anticoagulant and is key to maintaining the balance between coagulation and anticoagulation in vivo and preventing thromboembolism.[8,9] There are 4 subtypes of antithrombin, of which only antithrombin III (and possibly thrombin I) is clinically significant.