Medicine: Giant Step for Gene Therapy

She weighed around 35 lbs., came down with an occasional cold or ear infection, and appeared to be a healthy four-year-old. But a dark cloud hung . over her future. She suffered from ADA deficiency, the rare, incurable and deadly genetic disease that shuts down the immune system -- a disorder similar to the one that in 1984 finally claimed the life of David, the famous but unfortunate "bubble boy." Only the weekly injection of a newly developed drug seemed to stand between the little girl and the same fate.

Last week, on the 10th floor of the massive Clinical Center...