Medicine. Gene therapy--new challenges ahead.

The successful use of retroviral gene transfer to treat 10 patients with X-linked severe combined immunodeficiency (SCID-X1) has been welcomed as evidence of the therapeutic potential of gene therapy. However, as Williams and Baum suggest in their Perspective, the discovery that 2 of the 10 patients developed leukemia within 3 years of gene therapy (Hacein-Bey-Abina et al.) reinforces the need to develop even more specific gene therapy interventions.