The hematopoietic system comprises a hierarchy of cell types with differing capacities for proliferation and differentiation (Figure 2-1). Although such a schema shows a sequential series of cellular compartments, in reality the system is a continuum of cell types with progressive restriction of self-renewal, proliferation, and differentiation potential. The most primitive stem cells are identified by their capacity to repopulate all lymphoid (T lymphocyte, B lymphocyte, natural killer), myeloid (granulocytic, monocytic), erythroid, and megakaryocytic cell lineages upon transplantation into lethally irradiated or immunocompromised hosts. Repopulating stem cells cycle slowly and exhibit potential for self-renewal, thereby maintaining the stem . . . [Full Text of this Article]