Dose-Expansion Study of Low Dose Post-Transplant Cyclophosphamide/Tacrolimus/Ruxolitinib for Graft-versus-Host Disease (GVHD) Prophylaxis in Myeloablative Allogeneic Peripheral Blood Stem Cell Transplantation

Inclusion Criteria: * Age 18.0 years or older at the time of enrollment * Patients undergoing allogeneic hematopoietic cell transplantation for one of the following indications: * Acute leukemia with no circulating blasts and with less than 5% blasts in the bone marrow * Myelodysplasia/chronic myelomonocytic leukemia with no circulating blasts and with less than 10% blasts in the bone marrow (higher blast percentage allowed in MDS due to lack of differences in outcomes with \<5% versus 5-10% blasts in this disease). * Planned myeloablative (MAC) conditioning regimen (see eligible regimens in Section 9.2) * Patients must have a related or unrelated peripheral blood stem cell donor as follows: * Sibling donor must be a 6/6 match for HLA-A and -B at intermediate (or higher) resolution, and -DRB1 at high resolution using DNA-based typing and must be willing to donate peripheral blood stem cells and meet institutional criteria for donation. * Unrelated donor must be an 8/8 match at HLA-A, -B, -C and -DRB1 at high resolution using DNA-based typing. Unrelated donor must be willing to donate peripheral blood stem cells and meet National Marrow Donor Program (NMDP) criteria for donation. \* Donor selection must comply with 21 CFR 1271 * Cardiac function: Left ventricular ejection fraction at least 45% * Estimated creatinine clearance greater than 60 ml/min (C-G formula) * Pulmonary function: DLCO (diffusing capacity of lung for carbon monoxide) corrected for hemoglo…