When people in the future think of gene therapy, they probably will not think of someone like Ashi DeSilva, a child stricken with a rare inherited disease. Instead, they may picture a person like Floyd Stokes. Genes did not cause Stokes's health problem, at least not in as direct a way as they caused Ashi's. Still, genes may cure him.

Feeding Starved Hearts

Stokes is a peanut farmer in Texas. In the late 1990s, like almost 12.5 million other people in the United States, he had a heart that was starved for oxygen. Fatty deposits had blocked most of his coronary arteries, which carry the blood that nourishes his heart. As a result, he had had several heart attacks.

He felt exhausted and suffered chest pain, or angina, every time he did the slightest exercise. He could hardly even walk around his house.

Doctors commonly...