Currently there is significant interest inin vitro (ex vivo) expansion of human hematopoietic stem and progenitor cells in hematology and oncology. Transplantation of umbilical cord blood stem cells promises to be a new and exciting mode of bone marrow/stem cell transplantation since it is inexpensive and appears to be associated with milder graft vs. host disease (GVHD) than transplantation of adult cells. A potential problem of cord blood transplantation is the limited number of stem cells that may be harvested. The majority of cases of cord blood transplantation, therefore, have been attempted in children. If it were possible to expand the population of stem cells in umbilical cord blood and transplant them to adult recipients, it would significantly enlarge the scope of this form of transplantation. Already many preclinical studies have shown that it is possible to expand the population of total cells and progenitors in culture in the presence of combinations of cytokines. Whether or not it is possible to expand the population of hematopoietic stem cells which have long‐term reconstitution capabilities remains to be clarified (1–3). The cytokines used in the majority of attempts ofin vitroexpansion were selected from a group of early‐acting cytokines such as steel factor (SF, kit ligand), flt3 ligand (FL), interleukin (IL)‐3, IL‐6, IL‐11, granulocyte/ macrophage‐colony stimulating factor (GM‐CSF), and granulocyte‐CSF (G‐CSF). The rationale for combining these cytokines has been explained in a recent review (4). Almost all protocols forin vitroexpansion include IL‐3 and many contain IL‐1 because of their kno
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