Using techniques to target the liver, enhance gene expression, and avoid eliciting an immune response, the authors administered an adenovirus-associated virus containing the gene for factor IX (FIX) in six patients with hemophilia B. The result was either reduction or elimination of the need for FIX prophylaxis. Hemophilia B is an X-linked bleeding disorder that results from a defect in the gene encoding coagulation factor IX (FIX), a serine protease that is critical for blood clotting. Persons with severe hemophilia B have functional FIX levels that are less than 1% of normal values and have frequent bleeding episodes, which are associated with crippling arthropathy and early death. 1 , 2 Current treatment involves frequent intravenous injections of FIX protein concentrate (i.e., two to three times a week). However, this treatment is prophylactic rather than curative, is extremely expensive, and is associated with inhibitor formation. Somatic gene therapy for hemophilia . . .