The ability to transfer new genetic information into murine and human hematopoietic cells with retrovirus vectors offers a powerful approach to identifying the molecular mechanisms governing stem cell development. The newly integrated provirus also serves as a clonal marker for elucidating the lineage relationships of the cells in the stem cell hierarchy. High efficiency gene transfer into human cells opens the way to developing new therapy for treating genetic diseases by introducing functional genes into deficient cells. Present transplantation technology would only permit gene therapy with bone marrow cells. However, as gene transfer and transplantation technologies improve it may be possible to consider treating diseases that affect other organs and tissues. I would like to acknowledge the support of Alan Bernstein and R. Phillips in whose labs the murine work was carried out during a post-doctoral fellowship. I also thank Sharon Kerbel for excellent assistance in the preparation of this manuscript.