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  1. The activated form of factor IX (F.IXa), a vitamin K-dependent protein made exclusively in the liver, is an enzyme required for amplification in the series of reactions leading to blood clot formation. Factor IX deficiency (hemophilia B or Christmas disease) results in a bleeding disorder, phenotypically identical to the more common Hemophilia A, which results from a deficiency of factor VIII.

  2. Clinically, the disease exists in severe (F.IX levels <1 percent normal), moderate (1 to 5 percent), and mild (6 to 30 percent) forms. The major symptom of the disease is spontaneous bleeding into joints and soft tissues. Bleeding at others sites is less common but can be life or limb threatening; for example, intracranial or compartmental syndrome bleeding.

  3. A detailed and extensive database of mutations resulting in hemophilia B that includes more than 1918 patient entries as of mid 2000 ( provides information about all known mutations.

  4. Treatment options include replacement of the missing protein with purified or recombinant F.IX protein and most recently gene therapy. Complications of treatment include thrombogenicity, blood-borne viral diseases, and the development of an inhibitory F.IX antibody. The presence of an anti-F.IX inhibitory antibody is clinically problematic and treated acutely by delivery of proteins (recombinant Factor VIIa) that bypass the requirement for F.IX. In addition, initiation of immune tolerance regimens (daily injection of high doses of F.IX) is designed to eradicate the inhibitory antibody.

  5. Murine and canine models of hemophilia B facilitate testing of novel therapies such as gene therapy.

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