tially lethal condition known as graft versus host disease. Although autologous transplants, in which material from a person is implanted into the same person (for example, when a cancer patient stockpiles his own blood in advance of chemotherapy or irradiation) solve the problem of immune system rejection, the inability to purify the material leads to the risk that diseased or cancerous cells in the transplant will later be reintroduced to the patient along with the stem cells.

In contrast, transplants of highly purified and concentrated populations of HSCs in mice have been shown to greatly reduce the incidence of graft versus host disease (Shizuru et al., 1996; Uchida et al., 1998). Purified and concentrated populations of autologous HSCs transplanted in breast cancer patients after chemotherapy have been shown to engraft more swiftly and with fewer complications (Negrin et al., 2000). Transplants of concentrated HSCs also have been shown to repopulate the blood more readily, reducing the period during which an individual is vulnerable to infection.

There is also evidence that transplants derived from umbilical cord blood are less likely to provoke graft versus host disease, possibly because the cells in cord blood are immature and less reactive immunologically (Laughlin, 2001). The quantity of HSCs present in cord blood and its attached placenta is small, and transplants from cord blood take longer to graft, but for children, whose smaller bodies require fewer HSCs, cord blood transplants are valuable, especially when there is no related sibling to donate HSCs (Gluckman et al, 2001). Banks of frozen umbilical cord and placenta blood (drawn out of the umbilical vein of the cord) are an important source of HSCs because the histocompatibility markers on the cells in these tissues can be identified and catalogued in advance of the need for a transplant.

Irving Weissman, who presented research findings on HSC transplantations at the workshop, has explored ways to improve the identification and purification of HSCs by looking for proteins on the surface of the stem cells that can be closely associated only with HSCs. Finding the specific profile of proteins that identifies HSCs, particularly those called long-term HSCs, is important, because these cells are believed to hold



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