This P01 was initiated in 1973 as a comprehensive effort to study the biology of blood and marrow transplantation (BMT). Since then, BMT has proven to be effective therapy, and even the treatment of choice, for a variety of malignant and nonmalignant diseases that affect the lymphohematopoietic system. A number of issues, including conditioning regimen toxicities, infections, graft-versus-host disease (GVHD), immunodeficiency, and inability to completely eradicate tumor, have limited the success of BMT over the years. Major advances, particularly in the area of supportive care, have gradually decreased the magnitude of many of these problems. However, two inter-related issues, GVHD and tumor recurrence, remain as major obstacles to successful allogeneic (allo) BMT. Translational studies funded by this P01 over the past decade have found that high-dose cyclophosphamide (Cy) early after BMT effectively generates bidirectional tolerance, even in partially HLA mismatched donor-recipient pairs in animals studies and clinically. Importantly, our clinical data demonstrating that haploidentical related donor BMT is safe and effective, producing results similar to that seen with HLA-matched alloBMT, was confirmed by a recent BMT CTN clinical trial. Additionally, although the importance of the cancer stem cell (CSC) concept has been a matter of debate because of limited data demonstrating clinical relevance, work over this P01's last funding period suggest that CSCs in a variety of hematologic malignancies are in fact the cells responsible for relapse. Accordingly, the new proposal will build on continuing work from the past funding period primarily in the areas of modulating GVHD with high-dose post-BMT Cy and targeting CSCs. Because of the effects of GVHD on relapse, the studies into modulating GVHD and targeting CSCs are highly interdependent with most Projects studying both areas. There are 5 interactive Projects and 3 Cores. The specific hypotheses to be tested in this proposal are: 1) high-dose post-transplant Cy's ability to modulate GVHD allows safe and effective mismatched BMT, making AlloBMT a feasible alternative for severe non-malignant hematologic diseases and allowing donor selection based on genetic factors other than HLA, 2)improved immunologic recovery after high-dose post-transplant Cy allows early and effective utilization of immunologic antitumor approaches, and 3) targeting CSC can decrease relapse after BMT.