REFINE

Inclusion Criteria:

- Participants with documented diagnosis of intermediate-2 or high-risk primary
Myelofibrosis, Post Polycythemia Vera Myelofibrosis or Post-essential Thrombocythemia
Myelofibrosis.

- Participant must be ineligible due to age, comorbidities, or unfit for unrelated or
unmatched donor transplantation or unwilling to undergo stem cell transplantation at
time of study entry.

- Eastern Cooperative Oncology Group (ECOG) of 0, 1, or 2.

- Prior treatment must meet at least one of the following criteria:

- Prior or current treatment with ruxolitinib and no prior treatment with a
Bromodomain and Extra-Terminal motif (BET) proteins inhibitor or another Janus
Kinase 2 (JAK-2) inhibitor, and meet all of the following criteria:

- Ruxolitinib treatment must meet at least one of the following criteria:

- Ruxolitinib treatment for >=24 weeks with lack of efficacy defined as a
lack of spleen response (refractory) or a loss of spleen or symptom
response (relapsed)

- Ruxolitinib treatment for <24 weeks with documented disease progression
on spleen measurements while on ruxolitinib as defined in the protocol:

- Ruxolitinib treatment for >=28 days with intolerance defined as new red
blood cell transfusion requirement (at least 2 units/month for 2
months) while receiving a total daily ruxolitinib dose of >=30 mg but
unable to reduce dose further due to lack of efficacy.

- If receiving ruxolitinib at the time of screening, must currently be on a
stable dose >=10 mg twice daily of ruxolitinib for >=4 weeks prior to the
1st dose of navitoclax.

- Participant has at least 2 symptoms each with a score >=3 or a total score
of >=12, as measured by the Myelofibrosis Symptom Assessment Form (MFSAF)
v4.0 on at least 4 out of 7 days during screening prior to study drug
dosing; OR

- Prior treatment with a JAK-2 inhibitor and meet one of the following criteria:

- Prior treatment with a JAK-2 inhibitor for at least 12 weeks

- Prior treatment with a JAK-2 inhibitor for >=28 days complicated by either
development of red blood cell transfusion requirement (at least 2
units/month for 2 months) OR Grade >= 3 adverse events of thrombocytopenia,
anemia, hematoma and/or hemorrhage while on JAK-2 inhibitor treatment; OR

- No prior treatment with a JAK-2 or BET inhibitor:

- Participant has at least 2 symptoms each with a score >=3 or a total score
of >= 12, as measured by the MFSAF v4.0 on at least 4 out of 7 days during
screening prior to study drug dosing.

- Participant has splenomegaly as defined in the protocol.

- Participant must meet the laboratory parameters (adequate bone marrow, renal and
hepatic function) as defined in the protocol.

Exclusion Criteria:

- Splenic irradiation within 6 months prior to screening, or prior splenectomy.

- Leukemic transformation (> 10% blasts in peripheral blood or bone marrow
aspirate/biopsy).

- Participant is currently on medications that interfere with coagulation (including
warfarin) or platelet function within 3 days prior to the first dose of study drug or
during the study treatment period with the exception of low dose aspirin (up to 100
mg/day) and low-molecular-weight heparin.

- Prior therapy with a BH3 mimetic compound or stem cell transplantation.

- Participant has received strong CYP3A inhibitors (e.g., ketoconazole, clarithromycin)
or moderate CYP3A inhibitors (e.g., fluconazole) within 14 days prior to the
administration of the first dose of study drug.