Allogeneic bone marrow transplantation (allo-BMT) is a widely-used treatment for leukemias and lymphomas. For many children with these malignancies, allo-BMT represents the only treatment with the potential for cure. Graft-versus-host disease (GVHD), however, afflicts between 30-80% of patients following transplant and remains a major cause of death. The primary cause of GVHD is known to be linked to donor T cells, which can recognize host cells as foreign and attack indiscriminately, often targeting both cancer cells and normal tissues.