Transplants with stem cells collected from the blood of an unrelated donor (allo-HSCT) are being used more commonly for many blood cancers which are not curable with more conventional methods of chemotherapy. Although allo-HSCT has great potential, there are still high risks due to infections, graft-versus-host disease (GVHD), where the donor's cells attack the recipient's tissues as foreign, and due to toxic effects of the chemotherapy drugs given to prepare (or condition) the recipient's bone marrow for transplant.