Inclusion Criteria:

          1. Self-identified Black, African or African American women of ≥ 18 years of age with
             proven diagnosis of advanced adenocarcinoma of the breast (locoregionally recurrent or
             metastatic disease)

          2. ER-positive and/or PgR-positive tumor based on local laboratory results

          3. HER2-negative breast cancer based on local laboratory results (test to be used as per
             local practice)

          4. Patients must be appropriate candidates for letrozole or fulvestrant therapy

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

          6. Adequate bone marrow function:

               -  Absolute Neutrophil Count (ANC) ≥ 1,000/mm3 (1.0 x 109/L);

               -  Platelets ≥100,000/mm3 (100 x 109/L);

               -  Hemoglobin ≥9 g/dL (90 g/L).

        Exclusion Criteria:

          1. Current use of food or drugs known to be potent inhibitors or inducers of CYP3A4

          2. Active uncontrolled or symptomatic brain metastases. Previously treated and clinically
             stable, as per Investigator's judgment, brain metastases are permitted.

          3. Previous CDK4/6 inhibitor

             -