Children and adolescents ages 1 to 21 years of age who are undergoing allogeneic
      hematopoietic cell transplantation for a myeloid malignancy (AML, MDS, JMML, MPAL) will be
      eligible. There are no restrictions on donor type, conditioning, stem cell source, of GVHD
      prophylaxis approach.

      All participants will be treated on a single arm, and will initially receive 2 cycles of
      standard post-transplant azacitidine at a dose of 32mg/m2/dose IV/subcutnaeous for 5 days, in
      28 day cycles. This is considered standard of care.

      After tolerance of 2 cycles of azacitidine has been established, patients will be assigned to
      receive vorinostat orally at different dose levels, depending on the stage of the study. The
      dose level assignments will be conducted on a standard 3+3 design, whereby dose-escalation is
      peformed if previous patients tolerated the dose without dose-limiting toxicities, and
      dose-reduction is performed if dose-limiting toxicities are seen. The starting dose will be
      100mg/m2/dose on days 1-7 and 15-21 of each 28 day cycles. This will be in addition to
      receiving azacitidine at the fixed dose above. In order to start each cycle, participants
      will be required to meet specific clinical parameters to ensure safety.

      The dose of vorinostat between patients will be escalated or de-escalated until criteria for
      finding the maximum tolerated dose (MTD) is reached, and this will complete the study.
      Participants will continue to receive the prescribed dose of vorinostat for up to 7 cycles (9
      total cycles of azacitidine).

      Participants are followed for dose-limiting toxicities primarily during the first two course
      of combined therapy (cycles 3 and 4), but are continued to be tracked until the completion of
      all potential combined treatment (1 year or 7 combined cycles, whichever is earlier).

      Principal aims:

      1. To evaluate the maximum tolerated dose (MTD) of vorinostat used in combination with
      low-dose azacitidine after allogeneic hematopoietic cell transplantation (alloHCT) for
      childhood myeloid malignancies.

      Secondary aims:

        1. To describe the dose-limiting toxicities (DLT) of the vorinostat used in combination
           with low-dose azacitidine.

        2. To describe rates of relapse, transplant related mortality, graft-versus-host disease,
           and overall survival.

        3. To describe the effect of epigenetic modification on lymphocyte reconstitution in the
           post-alloHCT setting.