Gene transfer often has multiple and unpredictable effects on cells. The high hope of genetic medicine for 30 years has been to develop a way of using recombinant DNA techniques to treat patients through the genes involved in their diseases. As Richard Roblin, scientific director of the Council on Bioethics of the President of the United States, put it in 1979: “There is something aesthetically compelling about cutting to the heart of the problem, by treating the disease at the molecular level, where it originates.“ Since 1990, this vision has generated a modest industry of bench research and animal studies, culminating in almost 1000 clinical trials in humans around the world, for a wide variety of diseases. In the past few years, however, the field has learned that in genetic medicine, as in war, the “surgical strike“ is rarely as clean and effective as theory implies it should be.