Allogeneic bone marrow transplantation (BMT) became feasible in the 1960s after elucidation of the Human Leukocyte Antigen (HLA) complex. Since then, the therapy has evolved into an effective therapy for many hematologic disorders(7). Otherwise incurable malignancies are frequently cured by this approach, with the likelihood of cure ranging from 10% to 85%, depending on the disease and disease-status. The treatment strategy incorporates very large doses of chemotherapy and often radiation to eliminate malignant cells and to immunosuppress the recipient enough to allow engraftment of donor cells. Donor cells give rise to hematopoiesis within two to three weeks, rescuing the patient from the effects of high dose therapy. In the ideal circumstance, immune recovery and recipient-specific tolerance occurs over the following 6-18 months, and the patient is cured of his or her underlying hematologic disorder, off of immunosuppression, with a functionally intact donor-derived immune system. However, complications of the process are common and include potentially permanent or fatal organ damage from the effects of high dose chemotherapy, infection, hemorrhage, and, in particular, graft-versus-host-disease. A realistic estimate of transplant-related mortality, including deaths due to acute and chronic graft-versus-host diseases, in the standard HLA-matched sibling setting is as high as 25%. The risk of treatment related mortality thus limits the success of the approach even in younger patients and certainly precludes its use in older patients. Thus, particularly for older patients, new strategies in transplantation are needed.