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Hemophilia B, a deficiency of functional factor IX (FIX), has been extensively explored as a model for gene transfer. Two U.S. Food and Drug Administration-approved clinical studies for hemophilia B have been undertaken, both using adeno-associated viral vectors (AAV). AAV vectors have tropism for liver, muscle, central nervous system, and the respiratory tract; both skeletal muscle and liver have been used as target tissues in the hemophilia B studies. In both studies, proof of principle was first established in the hemophilia B dog model, with long-term expression of canine FIX at therapeutic levels achieved before clinical studies were initiated. In the AAV-FIX muscle trial, vector was introduced into skeletal muscle of the upper and lower extremities of eight human patients by direct intramuscular injection. Muscle biopsies taken 2 to 10 months postinjection demonstrated gene transfer and expression (by Southern blot and immunofluorescence, respectively) in all patients, but circulating FIX levels were generally not >1%, and escalation of dose to levels that proved therapeutic in animals was thwarted by feasibility issues regarding the number of injections required. Nevertheless, the study demonstrated that parenteral injection of AAV-FIX was safe at the doses tested, and could result in long-term expression of the transgene. Moreover, the general characteristics of transduction of human muscle were similar to those observed in other animal models. The safety and efficacy data established in the first trial formed the basis for a second trial in which AAV-FIX is administered systemically to target the liver. The liver study is currently ongoing, with six patients enrolled to date.