Hemophilia B patients suffer from an inherited blood-clotting defect andrequire regular administration of blood-clotting factor IX replacement therapy.Recombinant human factor IX produced in cultured CHO cells is nearly identicalto natural, plasma-derived factor IX and is widely used in clinical practice.Development of a biosimilar recombinant human factor IX for medicalapplications requires the generation of a clonal cell line with the highestspecific productivity possible and a high level of specific procoagulantactivity of the secreted factor IX. The gene discussed is F9; the disease is hemophilia B.