A comparative trial where all patients will receive daily doses of tacrolimus (TAC) until day +60 when tapering will begin, in the absence of graft-versus-host disease (GVHD), and discontinued by day +180. In addition patients will be randomized to methotrexate (MTX) or mycophenolate mofetil (MMF) and again, in the absence of GVHD, a tapering schedule will begin on day +240 and be completed on day +360. Doses will be adjusted to maintain blood levels.