Key Inclusion Criteria:

        Recipients must meet all of the following criteria:

          1. Patients must diagnosed with one of the following histopathologically confirmed
             diseases, for which a myeloablative hematopoietic stem cell transplant (HCT) is
             planned:

               -  acute myeloid, lymphoid or mixed phenotype leukemia

               -  high or very high risk myelodysplastic syndromes

          2. Patients with active acute leukemia (i.e. not in morphologic complete response) must
             have bone marrow infiltration by leukemic blasts of <= 10%,

          3. Patients must be matched to a related or unrelated donor

          4. Estimated glomerular filtration rate (eGFR) > 50 mL/minute

          5. Cardiac ejection fraction at rest ≥ 45% or shortening fraction of ≥ 27% by
             echocardiogram or radionuclide scan (MUGA)

          6. Diffusing capacity of the lung for carbon monoxide (DLCO) (adjusted for hemoglobin) ≥
             50%

          7. Total bilirubin < 2 times upper limit of normal (ULN) (patients with Gilbert's
             syndrome may be included where hemolysis has been excluded) and ALT/AST < 3 times ULN

        Key Exclusion Criteria:

        Recipients meeting any of the following exclusion criteria will not be eligible:

          1. History of prior allogeneic HCT

          2. Currently receiving corticosteroids or other immunosuppressive therapy. Topical
             corticosteroids or oral systemic corticosteroid doses less than or equal to 10 mg/day
             are allowed.

          3. Pre-planned donor lymphocyte infusion (DLI)

          4. Planned pharmaceutical in vivo or ex vivo T cell depletion

          5. Positive for anti-donor HLA antibodies against an allele in the selected donor

          6. Karnofsky performance score < 70%

          7. Hematopoietic cell transplantation-specific Comorbidity Index (HCT-CI) > 4

          8. Uncontrolled bacterial, viral or fungal infections (currently taking antimicrobial
             therapy and with progression or no clinical improvement) at time of enrollment

          9. Seropositive for HIV-1 or -2 antibody, HTLV-1 or -2 antibody, Hepatitis B sAg, or
             Hepatitis C antibody

         10. Any uncontrolled autoimmune disease requiring active immunosuppressive treatment

         11. Concurrent malignancies or active disease within 1 year, except non-melanoma skin
             cancers that have been curatively resected

         12. Women who are pregnant or breastfeeding