hFIX expression following gene therapy. Human factor IX in FIXKO (n = 5) and R333Q-hFIX (n = 6) mice before and after intramuscular rAAV2-CBA-hFIX gene therapy, as detected by immunoassay with monoclonal antibody A-1 (specific for the factor IX Thr148 polymorphism). The Thr148 factor IX is incorporated in the gene therapy vector but is not present in the genome of either strain of mice. Data represent the mean ± SD.