HEMATOPOIESIS IS A life-long process responsible for replenishing both hematopoietic progenitor cells and mature blood cells from a pool of pluripotent, long-term reconstituting stem cells. 1 The daily turnover in a normal adult of approximately 1012 blood cells is tightly regulated, involving, in part, a complex interaction between soluble and membranebound stimulatory and inhibitory cytokines and their corresponding receptors. 2-4 The molecular cloning of these hematopoietic growth factors (HGFs) and their receptors has been instrumental in delineating the pathways that lead from a single hematopoietic stem cell to the various terminally differentiated cells in the hematopoietic system.