Megan Sykes, head of the bone marrow transplantation section of the Massachusetts Transplantation Biology Research Center and professor of surgery at Harvard Medical School, recently described how infusing kidney transplant recipients with bone marrow from their donors immediately after the transplant surgery induced a state of mixed chimerism, a blending of donor and recipient immune systems. All of the patients Sykes described had developed kidney failure as a result of multiple myeloma, a cancer of the bone marrow. Traditionally, such patients had no good treatment options. They were not eligible for kidney transplantation because of their cancer, and the kidney failure made them unable to tolerate the toxic aspects of standard bone marrow transplantation, which has been used for some myeloma patients. For many years Sykes and her colleagues at MGH — along with collaborators at BioTransplant Inc. of Charlestown, Mass. — have been studying mixed chimerism and its application for both treatment of blood-cell cancers and for inducing tolerance, a state in which an organ recipient’s immune system no longer recognizes the donor’s tissues as foreign.