Determine the relapse-free, donor lymphocyte infusion (DLI)-free survival in patients receiving the investigational regimen.This is a randomized phase II clinical trial, comparing two different dosing schedules of mycophyenolate mofetil for graft versus host disease (GVHD) prevention following allogeneic stem cell transplantation. Risk for relapse, GVHD and non-relapse mortality will be assessed. Adaptive randomization between two study arms will be performed based on T cell counts at day 60.