- Because no one knows which of the study options is best, participants will be "randomized" into one of the two possible groups for GVHD prophylaxis: 1) a sirolimus-containing regimen (tacrolimus, sirolimus and methotrexate) or 2) a sirolimus-free regimen (tacrolimus and methotrexate or cyclosporine and mycophenolate mofetil).