While the donor cells in a bone marrow transplant can help cure the patient’s blood cancer, they can also cause graft-versus-host disease — in which donor T cells, a specialized immune cell in the blood, attack the patient’s healthy cells. Before this study, there was no finite T cell population identified as the cause of GVHD. Now, researchers have identified the specific type of T cells that cause GVHD in immunodeficient mice. To further confirm their findings, the researchers directly investigated human patient samples.