SERPIND1 (Heparin cofactor 2) is a serine endopeptidase inhibitor (SERPIN) that acts as a pseudosubstrate for activated thrombin, forming a stable complex which has the effect of inactivating thrombin protease activity (Church et al. 1985), although with slower kinetics than SERPINC1 (antithrombin-III). The presence of the glycosaminoglycans heparin or dermatan sulphate increases thrombin inactivation 1000-fold (Van Deerlin & Tollefsen 199) by facilitating the interaction between the active site of thrombin and the reactive site of SERPIND1. Thrombin specificity is conferred by a 90-residue N-terminal extension that contains two acidic motifs containing sulphated Tyr residues, resembling the C-terminus of hirudin (Tollefsen et al. 1997). SERPIND1 also inhibits chymotrypsin and neutrophil cathepsin G, but in a glycosaminoglycan independent manner (Church et al. 1985). In contrast to SERPINC1 deficiency, SERPIND1 deficiency is not associated with venous thrombosis (Corral et al. 2004).