We have generated a mouse where the clotting factor IX (FIX) gene has been disrupted by homologous recombination. The FIX nullizygous (-/-) mouse was devoid of factor IX antigen in plasma. Consistent with the bleeding disorder, the factor IX coagulant activities for wild-type (+/+), heterozygous (+/-), and homozygous (-/-) mice were 92%, 53%, and <5%, respectively, in activated partial thromboplastin time assays. Plasma factor IX activity in the deficient mice (-/-) was restored by introducing wild-type murine FIX gene via adenoviral vectors. Thus, these factor IX-deficient mice provide a useful animal model for gene therapy studies of hemophilia B.