A technique being tested in stem cell transplants from imperfectly matched donors has revealed an unforeseen response that can suppress graft-versus-host disease. The previously unrecognized specificity of regulatory T cells helps explain why the patients treated with the new strategy — known as “co-stimulatory blockade” — have shown a low level of GVHD. The findings suggest that the technique might prove valuable in solid organ transplants, as well as in treating autoimmune disease.