I. Compare the post-transplant 2-year event-free survival of pediatric patients with intermediate-risk or high-risk acute lymphoblastic leukemia (ALL) in second complete remission or high risk ALL in first remission undergoing allogeneic hematopoietic stem cell transplantation treated with graft-versus-host disease (GVHD) prophylaxis comprising tacrolimus and methotrexate with or without sirolimus.