For more information regarding Bristol-Myers Squibb Clinical Trial participation, please
        visit www.BMSStudyConnect.com.

        Inclusion Criteria:

          -  Participants must have untreated, unilateral, histologically confirmed ER+, HER2-
             invasive breast cancer with primary tumor ≥2 cm in largest diameter (cT1-3) in one
             dimension by clinical or radiographic exam, for whom neoadjuvant endocrine monotherapy
             deemed to be a suitable therapy.

          -  Participants must be deemed eligible for surgery and must agree to undergo surgery
             after completion of neoadjuvant therapy and agree to provide tumor tissue at baseline,
             on-treatment, and at surgery.

          -  Women must have documented proof that they are not of childbearing potential.

          -  ECOG <=1.

        Exclusion Criteria:

          -  Participants who may have had any treatment, including radiotherapy, chemotherapy,
             and/or targeted therapy administered for the currently diagnosed breast cancer prior
             to enrollment or upfront chemotherapy is clinically judged appropriate as optimal
             neoadjuvant treatment.

          -  Participants who have a history of or active, known or suspected autoimmune disease,
             or other syndrome that requires systemic steroids above physiological replacement dose
             or autoimmune agents for the past 2 years.

          -  Prior treatment with either ET or CDK4/6 inhibitors for Breast Cancer or an anti-PD-1,
             anti-PD-L1, anti-PD-L2, or anti-CTLA-4 antibody, or any other antibody or drug
             specifically targeting T-cell co-stimulation or checkpoint pathways, or history of
             allergy, or hypersensitivity to study drug components

          -  Prior Malignancy active within the previous 3 years as well as or participants with
             serious or uncontrolled medical disorders.

          -  Personal history of any of the following conditions: syncope of either unexplained or
             cardiovascular etiology, ventricular arrhythmia (including but not limited to
             ventricular tachycardia and ventricular fibrillation), long or short QT syndrome,
             Brugada syndrome, or known history of corrected QT prolongation, Torsade de Pointes,
             or sudden cardiac arrest.

        Other inclusion/exclusion criteria apply.