The transplantation of bone marrow to restore a healthy blood system in leukemia patients is limited by the unavailability of hematopoietic stem cells in quantity and purity that are crucial for successful transplantation. Because of their relative rarity (one in every 10,000 bone marrow cells) and the difficulty of separating them from other components of the blood, bone marrow transplants are generally impure. All cells of the body express on their surface a set of molecules called histocompatibility antigens. If a patient receives a transplant of hematopoietic stem cells from a donor that has histocompatibility antigens different from his own, the patient's body will recognize and react to the cells as foreign. To increase the likelihood that histocompatibility antigens will match, it is preferred that donor be a related sibling of the transplant recipient.
          Even if their histocompatibility antigens do match, however, hematopoietic stem cell transplants can be contaminated by T-cells from the donor's immune system. This contamination can cause the recipient's body to reject the transplant or can produce an immune reaction in which the T-cells of the transplant attack the tissues of the recipient's body, leading to a potentially lethal condition known as graft-versus-host disease. To solve the problem of immune rejection, autologous transplantations should be performed, but the inability to purify the material leads to the risk that diseased or cancerous cells in the transplant will be reintroduced to the patient along with the stem cells. Obtaining purified hematopoietic stem cells is a major challenge, and purification in a clinical setting is expensive and difficult.