The 104D2 monoclonal antibody specifically binds to human CD117, the receptor for stem cell factor (SCF). It selectively recognizes NIH- 3T3 cells transfected with human c-kit, the gene that codes for SCF-R. The 104D2 antibody does not block the epitope that binds SCF. In the bone marrow of humans and mice, SCF is expressed primarily on hematopoietic progenitor cells. Lack of functional SCF or deficient SCF-R caused by mutations in the Sl and W loci, respectively, can result in severe anemia and a decrease in the number of primitive progenitor cells in mice. Human hematopoietic progenitor cells can be recognized by their surface expression of CD34. This cell population constitutes a small subset (1% to 5%) of bone marrow cells. CD34+ cells contain a small subpopulation of primitive/non-committed progenitors, with the remaining fraction being cells committed to the various hematopoietic lineages. SCF alone induces extensive proliferation of erythroid-committed progenitor cells (CD34lo CD71hi CD64-). On primitive (CD34hi CD38lo CD50+) and granulo-monocytic (CD34+ CD64+) progenitor cells, SCF synergistically enhances the effects of other cytokines, the strongest of which are on the primitive progenitor cells. In addition, SCF promotes survival of primitive progenitors in the absence of proliferation. The receptor is highly expressed at similar levels on all of the three mentioned CD34+ cell subsets, whereas B-lymphoid committed progenitor cells (CD34+ CD19+) express low levels of SCF-R. Among CD34- bone marrow cells, only a small number of cells (mostly erythroid) express the receptor.