Inclusion Criteria:

          -  MDS, myelodysplastic syndrome/myeloproliferative neoplasia overlap disorders
             (including chronic myelomonocytic leukemia [CMML], and MDS/myeloproliferative neoplasm
             [MPN] unclassifiable syndromes)

          -  AML, other than acute promyelocytic leukemia (APL), in first or second remission or
             with minimal residual disease

          -  With Karnofsky index or Lansky Play-Performance scale > 70% on pre-transplant
             evaluation

          -  Able to give informed consent (if > 18 years), or with a legal guardian capable of
             giving informed consent (if < 18 years)

          -  Patients with previous autologous or allogeneic HCT are allowed to enroll

          -  DONOR: Human leukocyte antigen (HLA)-identical related donors or

          -  DONOR: Unrelated donors matched for HLA-A, B, C, DRB1, and DQB1 as defined by high
             resolution deoxyribonucleic acid (DNA) typing; mismatch for one HLA allele is allowed

          -  DONOR: Donors able to undergo peripheral blood stem cell collection or bone marrow
             harvest

          -  DONOR: Donors in good general health, with a Karnofsky or Lansky play performance
             score > 90%

          -  DONOR: Donors able to give informed consent (if > 18 years), or with a legal guardian
             capable of giving informed consent (if < 18 years)

        Exclusion Criteria:

          -  Receiving umbilical cord blood

          -  With impaired cardiac function as evidenced by ejection fraction < 35% (or, if unable
             to obtain ejection fraction, shortening fraction of < 26%) or cardiac insufficiency
             requiring treatment or symptomatic coronary artery disease; patients with a shortening
             fraction < 26% may be enrolled if approved by a cardiologist

          -  With impaired pulmonary function as evidenced by partial pressure of oxygen (pO2) < 70
             mm Hg and carbon monoxide diffusing capability test (DLCO) < 70% of predicted or pO2 <
             80 mm Hg and DLCO < 60% of predicted; (or, for pediatric patients unable to perform
             pulmonary function tests, then oxygen (O2) saturation < 92% on room air), or receiving
             supplementary continuous oxygen

          -  With impaired renal function as evidenced by creatinine-clearance < 50% for age,
             weight, height or serum creatinine > 2 x upper limit of normal or dialysis-dependent

          -  With hepatic dysfunction as evidenced by total bilirubin > 2.0 x upper limit of normal
             or evidence of synthetic dysfunction or severe cirrhosis

          -  With hepatic dysfunction as evidenced by aspartate aminotransferase (AST) > 2.0 x
             upper limit of normal or evidence of synthetic dysfunction or severe cirrhosis

          -  With active infectious disease requiring deferral of conditioning, as recommended by
             an infectious disease specialist

          -  With human immunodeficiency virus (HIV)-positivity or active infectious hepatitis

          -  With central nervous system (CNS) leukemic involvement not clearing with intrathecal
             chemotherapy, cranial irradiation or both prior to initiating conditioning (day -6)

          -  Patients with active non-hematological malignancies (except non-melanoma skin cancers)
             or those with non-hematological malignancies who have been rendered with no evidence
             of disease, but have a greater than 20% chance of having disease recurrence within 5
             years; this exclusion does not apply to patients with non-hematologic malignancies
             that do not require therapy

          -  With life expectancy severely limited by diseases other than malignancy

          -  Women who are pregnant or lactating

          -  With known hypersensitivity to treosulfan or fludarabine (fludarabine phosphate)

          -  Receiving another experimental drug within 4 weeks before initiation of conditioning
             (day -6)

          -  Unable to give informed consent (if > 18 years) or with a legal guardian (if < 18
             years) unable to give informed consent

          -  DONOR: Individuals deemed unable to undergo marrow harvesting or PBSC mobilization and
             leukapheresis

          -  DONOR: Individuals who are HIV-positive

          -  DONOR: Individuals with active infectious hepatitis

          -  DONOR: Females with a positive pregnancy test

          -  DONOR: Persons unable to give informed consent (if > 18 years) or with a legal
             guardian (if < 18 years) unable to give informed consent