His research involved the immunology of transplantation or its complications, particularly in studying the immunopathophysiology of GVHD. He has served as principal investigator or co-investigator on over 100 clinical trials including multiple initiatives sponsored by numerous national transplant organizations including SWOG, CIBMTR, ISCT, NMDP and BMT CTN. Within the BMT CTN, he serves on the Steering committee, chaired the Regimen Related Toxicity Committee, was a member of the GVHD Committee and served as the principal investigator for the BMT CTN on the first multicenter, stem cell transplant trial for patients with advanced chronic lymphocytic leukemia (BMT CTN 0804).