The majority of cases of human hemophilia B are the result of missense mutations in the coagulation factor IX gene and defective circulating factor IX is detectable in most patients. The available mouse factor IX knockout models of hemophilia B (FIXKO mouse) reproduce the bleeding phenotype of human hemophilia B, but because the models produce no factor IX they fail to reproduce the dominant human phenotype. We have created a human factor IX mouse model of hemophilia B (R333Q-hFIX mouse) by homologous recombination in embryonic stem cells. The mouse expresses no mouse factor IX, but instead expresses a missense mutant human factor IX from the mouse FIX promoter. Mutant human factor IX mRNA transcript and circulating human factor IX are detectable throughout development, but factor IX activity is less than 1% and the mouse exhibits the hemophilic phenotype. When R333Q-hFIX mice were challenged by intramuscular injection of adeno-associated virus expressing human factor IX, factor IX expression without the development of antibodies was observed. In contrast, given the same treatment, FIXKO mice consistently develop antibodies. Our R333Q-hFIX mice strain will complement the FIXKO mice for studying factor IX circulating kinetics and gene therapy. (Blood. 2004;104:1733-1739)