Gene therapy researchers are approaching a key milestone: They appear to have controlled an inherited blood disorder called β-thalassemia that's more common than any disease treated so far with gene therapy. A young man who received new genes to repair blood cells no longer needs regular transfusions and, 2 years later, seems healthy. Describing this success last week to a U.S. review panel in Bethesda, Maryland, a researcher added a caveat: The inserted gene may have turned on growth signals, raising the potential for cancer.