Donald B. Kohn, MD, of the University of California, Los Angeles, discusses gene therapy for primary immunodeficiencies. Defective genes can be overcome by providing the missing gene product or by providing a new permanent source of the normal gene by hematopoietic stem cell transplant. Autologous transplantation of corrected hematopoietic stem cells for ADA-deficiency SCID has been shown to be safe and effective for most patients. Dr Kohn notes that approaches to direct gene correction are in development that may broaden indications.