Early studies of allogeneic stem cell transplants found an interesting effect in which transplanted immune cells had a powerful response to tumor cells. This response was the result of differences in minor mHA antigens between the donor and recipient. As a result, now leukemia patients can limit the toxic chemotherapy used to control their disease. Another therapy, use of donor lymphocytes for prevention of relapse, has been shown to be as effective as an anti-tumor agent. This is especially helpful if the original transplant was conditioned to remove donor T-cells.