Inclusion Criteria:

          -  Signed informed consent prior to study specific procedures.

          -  Age ≥18 years (or per national guidelines).

          -  Pre- and postmenopausal women or men with Stage II (Stage IIA limited to max. 1000
             patients) or Stage III early invasive breast cancer

          -  Patients with multicentric and/or multifocal and/or bilateral early invasive breast
             cancer are eligible if all histopathologically examined tumors meet pathologic
             criteria for ER+ and/or PR+ and HER2-.

          -  Patients must have histologically confirmed ER+ and/or PR+, HER2-, early invasive
             breast cancer.

          -  Patients must have undergone adequate (definitive) breast surgery for the current
             malignancy.

        FFPE tumor tissue block must be confirmed to be received at the central sample repository
        prior to randomization.

          -  ECOG performance status 0-1.

          -  Patients must be able and willing to swallow and retain oral medication.

          -  Serum or urine pregnancy test must be negative in premenopausal women within 14 days
             of randomization, or in women with amenorrhea of less than 12 months at time of
             randomization.

          -  Patients who received neo/adjuvant therapy must be after last dose of chemotherapy
             and/or biologic therapy and must have sufficient resolution of side effects.

          -  Patients who received breast/axilla/post-mastectomy chest wall radiotherapy must be
             after last dose of radiotherapy and must have sufficient resolution of side effects.

          -  Patients must have sufficient resolution of any surgical side effects (no active wound
             healing complications).

        -Patients must either be initiating or have already started adjuvant hormonal treatment. -

          -  Patients who already received neo/adjuvant endocrine therapy are eligible as long as
             they are enrolled within 12 months of initial histological diagnosis and after
             completing no more than 6 months of adjuvant endocrine therapy.

          -  Absolute neutrophil count ≥ 1,500/µL

          -  Platelets ≥ 100,000/ mm3

          -  Hemoglobin ≥ 10g/dL

          -  Total serum bilirubin ≤ ULN; or total bilirubin ≤ 3.0 × ULN with direct bilirubin
             within normal range in patients with documented Gilbert's Syndrome.

          -  Aspartate amino transferase (AST or SGOT) and alanine amino transferase (ALT or SGPT)
             ≤ 1.5 × institutional ULN.

          -  Serum creatinine below the upper limit of the institutional normal range (ULN) or
             creatinine clearance ≥ 60 mL/min/1.73 m2 for patients with serum creatinine levels
             above institutional ULN.

        Exclusion Criteria:

          -  Concurrent therapy with other Investigational Products.

          -  Prior therapy with any CDK inhibitor.

          -  Patients with Stage I or IV breast cancer are not eligible.

          -  History of allergic reactions attributed to compounds of chemical or biologic
             composition similar to palbociclib.

          -  Patients receiving any medications or substances that are potent inhibitors or
             inducers of

          -  CYP3A isoenzymes within 7 days of randomization.

          -  Uncontrolled intercurrent illness that would limit compliance with study requirements.

          -  Pregnant women, or women of childbearing potential without a negative pregnancy test
             within 14 days prior to randomization.

          -  Patients with a history of any malignancy are ineligible

          -  Patients who previously received endocrine therapy within 5 years prior to diagnosis
             of the current malignancy.

          -  Patients on antiretroviral therapy.

          -  Patients with clinically significant history of any chronic liver disease.

          -  Patients receiving concurrent exogenous hormone therapy (topical vaginal estrogen
             therapy is allowable).