Heparin cofactor II (HCII) inhibits thrombin, the final protease of the coagulation cascade. HCII is allosterically activated by binding to cell surface glycosaminoglycans (GAGs). The specificity of HCII for thrombin is conferred by a highly acidic hirudin-like N-terminal tail, which becomes available after GAG binding for interaction with the anion-binding exosite I of thrombin. HCII belongs to clade D of the serpin superfamily; it is also known as Serpin D1.