For hemophilia B, AAV-mediated delivery to the liver of less than one milligram of DNA encoding human coagulation factor IX (FIX), a serine protease constituent of the coagulation cascade, achieved continuous therapeutic levels of FIX for more than two years (and counting) [11], corresponding to in vivo production of an estimated 200 milligrams of the therapeutic transgene product FIX per year per patient. The gene discussed is F9; the disease is hemophilia B.