Trial of 2 Step ATG for Acute GVHD Prevention Post Myeloablative Allogeneic Stem Cell Transplant

On Day -7, subjects will be admitted to the hospital and receive a dose of prednisone at 1 mg/kg (ATG premedication). Subjects will receive a steroid injection 3 hours before every ATG infusion. On day -6, subjects will receive a small dose of ATG as an IV infusion. ATG will be repeated on days -5,-4 and -1. Routine transplant chemotherapy agent fludarabine will be given on days -6 to -2 as daily IV infusions. Busulfan, a routine transplant chemotherapy will be given on days -5 to -2 as IV infusion. Subjects with lymphoblastic leukemia will receive an alternative regimen of cyclophosphamide, a routine chemotherapy on days -6 and -5, followed by total body radiation on days -3 to -1. Tacrolimus (standard immune suppression agent) starts on day -3 as continuous IV infusion and switched to oral after engraftment. Methotrexate a standard immune suppression medication which is given IV on day +1,+3,+6, and +11 post-transplant. Blood draws on days -4,-1,+3,+7,+14 to measure ATG levels.