Abstract: An isolated polynucleotide which encodes human prothrombin is modified to encode amino acid substitutions at positions 155, 271 and 284 of the prothrombin polypeptide. A second polynucleotide is further modified to encode an additional amino acid substitution at residue 320.The polypeptide encoded by the first polynucleotide is resistant to cleavage by thrombin and factor Xa, exhibits greatly reduced procoagulant activity towards fibrinogen and unchanged anticoagulant activity towards protein C. The polypeptide can be cleaved between amino acid positions 320 and 321 to produce an active serine protease which can activate protein C, thereby inhibiting coagulation and stimulating activated protein C fibrinolytic activity.The polypeptide encoded by the second polynucleotide cannot be cleaved, mimics inactive prothrombin and thereby acts as a reversible inhibitor of coagulation by competing with Factor Xa and Factor Va for interaction at the phospholipid surface.