Gene therapy is a novel approach under investigation for the treatment of genetic diseases, cancer and AIDS. Hematopoietic stem cells would be the target cell for correction of hemoglobinopathies, immune deficiencies and lysosomal storage diseases. Retroviral vectors derived from murine leukemia viruses have been used most extensively for gene delivery, but are limited in their capacity to transduce pluripotent human hematopoietic stem cells. In a trial of gene transfer for adenosine deaminase (ADA)-deficient severe combined immunodeficiency (SCID), three neonates were treated with infusion of autologous umbilical cord blood CC34+ cells. Up to 3 years later, a low number of leukocytes are still being produced containing the inserted ADA gene, with evidence of selective accumulation of transduced T lymphocytes. Further successful applications of gene therapy will require development of more efficient methods of gene transfer into stem cells.