Inclusion Criteria:

          -  Relapsed or refractory primary myelofibrosis (MF) or secondary MFs (post-polycythemia
             vera MF, post-essential thrombocythemia MF).

          -  Must not be a candidate for potentially curative therapy, including hematopoietic
             stem-cell transplantation.

          -  Willingness to undergo a pretreatment bone marrow biopsy and/or aspirate at
             screening/baseline, or archival sample obtained since completion of most recent
             therapy.

          -  Willingness to avoid pregnancy or fathering children.

        Exclusion Criteria:

          -  Prior receipt of any BET inhibitor.

          -  Have received allogeneic hematopoietic stem-cell transplant within 6 months of
             enrollment or have active graft-versus-host disease, or have received
             immunosuppressive therapy following allogeneic transplant within 2 weeks of Cycle 1
             Day 1.

          -  Use of any potent CYP3A4 inhibitors or inducers within 14 days or 5 half lives
             (whichever is longer) before the first dose of study drug.

          -  History of bleeding disorder or at a high risk of bleeding (eg, chronic liver disease,
             prior gastrointestinal bleed).