The death last fall of a 17-year-old patient at the University of Pennsylvania Medical Center shocked the medical research community. The young man, who suffered from a rare genetic defect that stopped his body from metabolizing ammonia, was undergoing an experimental form of treatment called gene therapy. In theory at least, it should be possible to treat a number of devastating diseases by patching in bits of DNA to repair defective or missing genes.

In practice, gene therapy remains difficult and the methods crude. One problem is in sneaking the new genetic material into the cell and getting it incorporated into existing DNA without the individual's immune system going berserk. Most approaches rely on nature's masterful Trojan Horse, the virus. In the case of the patient at Penn, researchers injected very high doses of a modified form of the common cold virus directly into his liver. Once inside, the viral particles were supposed to insert replacement DNA into tissue cells so his liver could start to process ammonia normally. Four days later the patient was dead from multiple organ failure, and speculation centered on the possibility that the immense and sudden injection of the virus caused an overwhelming immune response.