Inclusion Criteria:

Donor Inclusion Criteria:

   - Donor medical suitability and eligibility will be determined following Institution or
   NMDP/Be The Match standards

   - Age 18-65 years old at the time of signing informed consent

   - 8/8 (HLA- A, B, C, and DRB1) HLA-matched sibling or volunteer unrelated donor

   - Fulfill Institution or NMDP/Be The Match criteria to serve as a mobilized blood cell
   donor

   - Serum creatinine < 1.5 x institution upper limit of normal (ULN) or estimated
   creatinine clearance (CRCL) > 50 mL/min using the Modification of Diet in Renal
   Disease Study (MDRD) equation or similar method

Recipient Inclusion Criteria:

   - At least 18 years old at the time of signing informed consent

   - Has an available 8/8 (HLA- A, B, C, and DRB1) HLA-matched sibling or volunteer
   unrelated donor willing to donate peripheral blood stem cells (PBSC) for transplant

   - Fulfill additional individual Transplant Center Criteria for transplant beyond NMDP/Be
   The Match criteria

   - One of the following diagnoses:

      - Acute myelogenous leukemia (AML) in 1st remission or beyond with ≤ 5% marrow
      blasts and no circulating blasts. Documentation of bone marrow assessment will be
      accepted within 45 days prior to the date of consent.

      - Acute lymphoblastic leukemia (ALL) in 1st remission or beyond with ≤ 5% marrow
      blasts and no circulating blasts. Documentation of bone marrow assessment will be
      accepted within 45 days prior to the date of consent.

      - Patients with myelodysplasia (MDS) 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% or 5-10% blasts in MDS). Documentation of
      bone marrow assessment will be accepted within 45 days prior to the date of
      consent.

   - Cardiac function: Left ventricular ejection fraction at least 45% based on most recent
   echocardiogram or MUGA results obtained via standard of care

   - Estimated creatinine clearance acceptable per local institutional guidelines

   - Pulmonary function: diffusing capacity of the lungs for carbon monoxide (DLCO)
   corrected for hemoglobin at least 50% and forced expiratory volume in first second
   (FEV1) predicted at least 50% based on most recent DLCO results obtained via standard
   of care

   - Liver function acceptable per local institutional guidelines

   - Karnofsky performance status (KPS) of 70% or greater

   - Hematopoietic Cell Transplantation-Comorbidity Index (HCT-CI) score of 4 or less

Exclusion Criteria:

Donor Exclusion Criteria:

   - Donor unwilling or unable to give informed consent, or unable to comply with the
   protocol including required follow-up and testing

   - Donor already enrolled on another investigational agent study

   - Pregnant or breastfeeding females, sexually active female and male donors not willing
   or able to use adequate contraception, or males who do not agree to refrain from
   donating sperm, from the time of consent through 3 months after treatment with
   MGTA-145 + plerixafor

Recipient Exclusion Criteria:

   - Subject unwilling or unable to give informed consent, or unable to comply with the
   protocol including required follow-up and testing

   - Subject whose donor does not meet the eligibility criteria and is a screen fail

   - Subjects with a prior allogeneic transplant

   - Subjects with active, uncontrolled infection at the time of the transplant preparative
   regimen

   - Pregnant or breastfeeding females, sexually active female or male subjects not willing
   or able to use adequate contraception, or males who do not agree to refrain from
   donating sperm, from the time of consent through 3 months after PBSC infusion

   - Subjects with clinical evidence of active Central Nervous System (CNS) tumor
   involvement as evidenced by documented disease on examination of spinal fluid or MRI
   within 45 days of start of conditioning

   - A condition, which, in the opinion of the clinical investigator, would interfere with
   the evaluation of primary and secondary endpoints

   - Planned treatment with a new investigational agent from the time of transplant through
   30 days post-transplant