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 that is not in CR/CRi [Ages
      18-65]

      - myelodysplastic syndromes with > 10% to < 20% bone marrow blast burden [Ages
      18-65]

      - acute myeloid, lymphoid or mixed phenotype leukemia that is in CR/CRi but is
      categorized as DRI very high risk [Ages 18-65]

      - Myelofibrosis [Ages 18-65]

      - Blastic Plasmacytoid Dendritic Cell Neoplasm [Ages 18-65]

      - acute myeloid, lymphoid, mixed phenotype leukemia or myelodysplastic syndromes
      (therapy-related/secondary MDS or eligible for alloHCT per 2017 International
      Expert Panel) that are in CR and intermediate to high risk per the DRI in
      patients aged 66 to 75 [Ages 66-75]

      - chronic myeloid leukemia that is in chronic phase with history of blast crisis or
      accelerated phase and/or is resistant to or intolerant of >1 first- or
      second-generation TKI [Ages 18-65]

   2. Patients must be matched to a 8/8 HLA-matched related or unrelated donor

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

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

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

   6. 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