The daily production of hundreds of billions of blood cells through the process of hematopoiesis is a remarkable feat of human physiology. Transport of oxygen to tissues, blood clotting, antibody- and cellular-mediated immunity, bone remodeling, and a host of other functions in the body are dependent on a properly functioning hematopoietic system. As a consequence, many pathological conditions are attributable to blood cell abnormalities, and a fair number of these are now clinically treatable as a direct result of hematopoietic research. Proliferation of hematopoietic stem cells, and their...