By the year 2000 Ashanthi DeSilva had grown from a frail, serious-faced four-year-old into a smiling, active girl on the edge of adolescence. She had not had a gene treatment in seven years, though she had received eleven between September 1990 and August 1992. She still took PEG-ADA shots, but the dose was a mere quarter of the amount a child her age with the disease would need without gene therapy. Although only about a fourth of Ashi's immune cells produced ADA, this apparently was enough to give her body a basically nor- mal defense system. She went to school, played basket- ball, and in general led an ordinary life. She was still, as French Anderson had described her in a 1995 article, "a healthy, vibrant [girl] who loves life and does every- thing."12 Cindy Cutshall has done equally well.

Unfortunately, in spite of researchers' continuing efforts, few...