Jesse Gelsinger was an energetic eighteen-year-old from Tucson, Arizona, who loved motorcycles and professional wrestling. He let little stand in his way in spite of having been born with a rare disease that kept his liver from breaking down ammonia, a chemical that is poisonous if it builds up in the body. Jesse had a fairly mild form of this illness, called ornithine transcarbamy- lase deficiency, and drugs and a strict diet kept it under control. Still, when he heard that James Wilson and others at the University of Pennsylvania's Institute for Human Gene Therapy were doing phase I tests of a possible gene therapy for the disease, he was eager to sign up. "He said, 'Hey, this may be good for me, and I'll be helping newborn infants,'"30 Jesse's father, handyman Paul Gelsinger, remembered later.

On September 13, 1999, researchers injected trillions of adenoviruses...