Inclusion Criteria:

          1. Pre- or post-menopausal women.

          2. ER-positive, HER2-negative breast cancer that is advanced or metastatic.

          3. Progressed on prior therapy. Multiple prior lines of therapy allowed in Phase 1 and 2.
             Participants under amendment 6 must have received prior cyclin-dependent kinase
             (CDK4/6) inhibitor therapy.

          4. A recent archival tumor tissue obtained within 6 months prior to enrollment or a fresh
             tumor biopsy must be provided. A second biopsy after initiating trial therapy is not
             required.

          5. Eastern Cooperative Oncology Group (ECOG) performance status (PS) of 0 or 1.

          6. Adequate bone marrow and organ function.

          7. Participants under amendment 6 must have measurable disease at baseline as per
             Response Evaluation Criteria in Solid Tumors (RECIST) 1.1 criteria.

          8. Participants under amendment 6 must have ESR1 Y537S mutation in absence of ESR1 D538G
             mutation as per the results of a central laboratory.

        Exclusion Criteria:

          1. Participants must have at least one measurable lesion.

          2. Participant with inflammatory breast cancer.

          3. Participant has received more than one prior chemotherapy regimen for metastatic
             disease (Phase 2 only).

          4. Females of childbearing potential who are unable or unwilling to follow adequate
             contraceptive measures.