Human antithrombin III (ATIII) which has progressive inhibitory activity and is stimulated by heparin is advantageously obtained by expression after cotransfection of mammalian cells whose dihydrofolate reductase (DHFR) activity is deficient (dhfr-) with a) vectors which contain the ATIII cDNA and b) vectors which harbour the DHFR gene, especially after gene amplification with methotrexate.
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