• Women or men with histologically or cytologically confirmed carcinoma of the breast

• Documented ER-positive and/or progesterone receptor-positive tumor according to American Society of Clinical Oncology/College of American Pathologists (ASCO/CAP) guidelines

• Documented HER2-negative tumor according to ASCO/CAP guidelines

• De-novo HR+ , HER2- ABC, or, alternatively, relapsed HR+ , HER2- ABC after at least 2 years of standard neoadjuvant/adjuvant endocrine therapy without disease progression during that treatment and disease-free interval of at least 1 year since the completion of that treatment

• Participants who have bilateral breast cancers which are both HR-positive and HER2-negative

• Confirmation of biomarker eligibility

• Consent to provide fresh or archival tumor tissue specimen

• Measurable disease per Response Evaluation Criteria in Solid Tumors, Version 1.1 (RECIST v1.1)

• Eastern Cooperative Oncology Group (ECOG) Performance Status of 0 or 1

• Adequate hematologic and organ function within 14 days prior to initiation of study treatment