Fig. 1.

In the Leventhal trial (2), an intensive conditioning regimen was used, including total body irradiation (TBI), cyclophosphamide (Cy), fludarabine, and postoperative immunosuppression with mycophenolate mofetil (MMF) and tacrolimus. This regimen, together with bone marrow stem cells and FCRx, achieved full allogeneic chimerism (top), with replacement of essentially all host hematopoietic cells with those of the donor (blue). The conditioning used by Kawai et al. (8) consisted of cyclophosphamide, thymic irradiation (TI), antibody to CD2, and postoperative calcineurin inhibitor therapy (peritransplant rituximab was later added). This regimen resulted in transient mixed-chimerism, with both host (orange) and donor (blue) hematopoietic cells coexisting.