Each year in the United States, approximately 120,000 people develop a cancer of the white blood cells such as leukemia or lymphoma. After destruction of tumor cells with chemotherapy and radiation therapy, many of these patients need to undergo transplantation of stem cells from a healthy person to get their immune system working properly again. Although ingrafting of foreign stem cells is advantageous in that new cells can attack any leftover tumor cells, this treatment has the disadvantage of possibly triggering graft-versus-host disease (GVHD), a disorder in which the new immune system cells attack healthy tissues. GVHD can cause significant illness and death, and it affects 30% to 50% of patients who receive stem cells from matched sibling donors. The incidence of GVHD increases to 65% to 70% for transplantation from unmatched donors. Gastrointestinal tract damage, which may cause severe diarrhea; liver damage; and skin pain and irritation are common symptoms of GVHD.