A Study of Withdrawal of Immunosuppression and Donor Lymphocyte Infusions Following Allogeneic Transplant for Pediatric Hematologic Malignancies

Inclusion Criteria: * Age 6 months - 25 years. * Diagnoses of acute leukemia (AML, ALL, biphenotypic leukemia), pre-leukemic syndromes (monosomy 7 or other bone marrow clonal malformations), JMML, myelodysplastic syndromes or CML. * Undergoing an allogeneic transplant as standard care. * Performance status: Karnofsky/Lansky\>60%. * Availability of pre-transplant recipient's DNA and donor's DNA for chimerism testing. This could be DNA or material from which DNA could be extracted. Frozen blood would be preferred. For some patients, post transplant specimens that are not infiltrated with donor cells may be used. * Bone marrow or PBMTC as stem cell source.HLA matching: donor and recipient should be matched at a minimum of 7/8 antigens (A,B,C and DrB1) for bone marrow and PBMTC transplants. * No history of ≥grade III acute GVHD. Exclusion Criteria: * Treatment on other experimental protocols, if withdrawal of immunosuppression interferes with procedures of follow-up on the primary study. * Leukemia relapse defined as \> 5% blasts on bone marrow exam or \>1% leukemia cells by immunoflow MRD, or presence of extramedullary leukemia. * History of acute GVHD ≥ stage III or with any degree of active acute or cGVHD. * On steroids for any reason. * Any condition that compromises compliance with the objectives and procedures of this protocol, as judged by the principal investigator. * Cells for DLI cannot be obtained from the donor.