UMBILICAL CORD BLOOD STEM/PROGENITOR CELLS of choice in selected high-risk or recurrent hematologic malignancies, marrow failure syndromes, severe congenital im- It has been shown in early studies by Broxmeyer et al7 munodeficiency states, and selected metabolic disorders. 1 and others that both term and preterm umbilical cord blood Recently, the use of matched related allogeneic peripheral contains a significantly higher number of early and commitblood stem cells as a source of transplantable stem cells has ted progenitor cells when compared with adult peripheral been reported by a variety of groups. 2-4 Initial studies have blood. The number of colony-forming unit–granulocytesuggested that the risk of graft rejection and the risk of macrophage (CFU-GM) is greatly increased in umbilical developing severe acute graft-versus-host disease (GVHD) cord blood obtained from term neonates compared with peare similar when using matched related allogeneic peripheral ripheral blood obtained from adults (Fig 1A). 8-14 The CFU-blood stem cells as compared with matched related alloge- GM proliferative rate, as assayed by thymidine suicide studneic bone marrow (BM) stem cells. However, the major ies, is also significantly higher in term umbilical cord blood limitation of using HLA-matched related sibling donors in(Fig 1B). 15 The number of circulating colony-forming unit BM transplantation (BMT) has been that only 30% to 40% granulocyte, erythroid, monocyte, megakaryocyte (CFU-of potential recipients in need of such therapy have an HLA- GEMM) also appears to be significantly increased in term matched related family donor. 1 The recent use of either re- umbilical cord blood (Fig 1A) 9 and the CFU-GEMM proliflated or unrelated donor umbilical cord blood stem cells for erative rate is also significantly higher in term umbilical cord allogeneic stem cell transplantation has been secondary to a blood compared with that of adult peripheral blood (Fig number of factors, most important of which have been (1) 1B). 10 Lastly, committed megakaryocytic progenitor cells as the attempt to reduce transplant-related complications and identified by circulating colony-forming unit megakaryocyte (2) augmentation of the donor pool. Gluckman et al5 first(CFU-Meg) are also enriched in term umbilical cord blood reported the successful use of HLA-matched sibling umbili- compared with adult peripheral blood but to a much less cal cord blood stem cells to reconstitute a child with severe degree than CFU-GM and CFU-GEMM (Fig 1A). 11 Fanconi anemia. Since 1988, there have been an estimated 500 related and unrelated donor umbilical cord blood transplants.