Inclusion Criteria:

          1. Has AML or ALL and failed any prior induction therapy regimen or have relapsed after
             prior therapy

          2. Has Eastern Cooperative Oncology Group (ECOG) performance status 0-2

          3. Has adequate renal and hepatic function

          4. Had at least 14 days for prior treatment to clear the body before initiation of
             DS-3201b administration (except for hydroxyurea that needs only 2 days for clearance)

          5. Able to provide written informed consent, comply with protocol visits and procedures,
             be able to take oral medication, and not have any active infection or comorbidity that
             would interfere with therapy.

          6. Agrees to use an adequate method of contraception during the study and until 3 months
             after the last treatment.

          7. Is willing to provide bone marrow biopsies and comply with protocol-defined
             evaluations

          8. Has a life expectancy of at least 3 months

        Exclusion Criteria:

          1. Has presence of central nervous system (CNS) involvement of leukemia or a history of
             CNS leukemia

          2. Has a second concurrent active primary malignancy such as solid tumor or lymphoma
             under active treatment

          3. Has refractory nausea and vomiting, malabsorption, biliary shunt, significant bowel
             resection, graft versus- host disease (GVHD) significantly affecting gut motility or
             absorption, or any other condition that would preclude adequate absorption of DS-
             3201b in the opinion of the treating physician and/or principal investigator (PI)

          4. Has an uncontrolled infection requiring intravenous antibiotics, antivirals, or
             antifungals, known human immunodeficiency virus infection, or tested positive for
             active hepatitis B or C infection

          5. Has a concomitant medical condition that would increase the risk of toxicity, in the
             opinion of the Investigator or Sponsor

          6. Has unresolved toxicities from previous anticancer therapy

          7. Has received hematopoietic stem cell transplantation (HSCT) within 60 days of the
             first dose of DS-3201b

          8. Has received concomitant treatment with a strong inhibitor or inducer of cytochrome
             P450 (CYP)3A4/5 within 7 days of first receipt of DS-3201b

          9. Has consumed herbs/fruits that may have an influence on pharmacokinetics (PK) of
             DS-3201b from 3 days (14 days for St. John's wort) prior to the start of the study and
             throughout the entire study

         10. Had major surgery within 4 weeks before study drug treatment

         11. Has prolonged corrected QT interval by Fridericia's method (QTcF) at rest, where the
             mean QTcF interval is > 450 milliseconds (ms) based on triplicate electrocardiograms
             (ECGs)

         12. Is pregnant or breastfeeding

         13. Has substance abuse or medical, psychological, or social conditions that, in the
             opinion of the Investigator, may interfere with the subject's participation in the
             clinical study or evaluation of the clinical study results

         14. Has received prior treatment with enhancer of zeste homolog (EZH) inhibitor