INCLUSION CRITERIA:

          -  Clinical stage operable stage I, II, or III invasive mammary carcinoma, which is
             estrogen receptor or progesterone receptor positive by immunohistochemistry and HER2
             negative by Herceptest (0 or 1+) or not amplified by in situ hybridization as per
             routine clinical testing.

          -  Have post-menopausal status, as defined by any of the following: Subjects at least 55
             years of age OR Subjects under 55 years of age and amenorrheic for at least 12 months
             OR follicule stimulating hormone (FSH) values ≥ 40 IU/L and estradiol levels ≤ 40
             pg/mL (140 pmol/L) or in postmenopausal ranges per local or institutional reference
             ranges.

          -  Breast tumor ≥1cm in diameter by either physical exam or ultrasound and suitable for
             pre and post-treatment tissue sampling.

          -  Meet either of 2 following criteria, for which neoadjuvant endocrine therapy for 2
             weeks is deemed suitable: 1) disease that is planned for surgery as initial therapy,
             in which 2 weeks of neoadjuvant endocrine therapy is deemed suitable, 2) Disease for
             which neoadjuvant systemic therapy (either chemotherapy or endocrine therapy) may be
             planned, in which 2 weeks of neoadjuvant endocrine therapy prior to start of systemic
             therapy is deemed suitable.

          -  At least 18 years of age

          -  Performance status ECOG ≤ 2

          -  Have adequate organ function (ANC ≥1,500/mcL, Platelets ≥100,000/mcL, Hemoglobin ≥8
             g/dL, Total bilirubin ≤1.5 × upper limit of normal, ALT and AST ≤3 × upper limit of
             normal, Creatinine clearance >30 mL/minute

          -  The patient is able to swallow oral medications

          -  Patients with a prior history of contralateral breast cancer are eligible if they have
             no evidence of recurrence of their initial primary breast cancer.

          -  Women may have been taking tamoxifen or raloxifene as a preventive agent prior to
             study entry but must have discontinued the drug for at least 28 days prior to study
             enrollment.

          -  Subjects have ended hormone replacement therapy at least 7 days prior to receiving the
             first dose of randomized therapy.

          -  Ability to understand and the willingness to sign a written informed consent.

        EXCLUSION CRITERIA:

          -  Active metastatic breast cancer, inflammatory breast cancer, or locally recurrent
             breast cancer.

          -  The patient has serious and/or uncontrolled preexisting medical condition(s) that, in
             the judgment of the investigator, would preclude participation in this study (for
             example, interstitial lung disease, severe dyspnea at rest or requiring oxygen
             therapy, severe renal impairment [e.g. estimated creatinine clearance <30ml/min],
             history of major surgical resection involving the stomach or small bowel, or a
             preexisting chronic condition resulting in baseline grade 2 or higher diarrhea).

          -  Females who are pregnant, lactating, or premenopausal.

          -  Severe uncontrolled malabsorption condition or disease (i.e. grade 2 or higher
             diarrhea, severe malnutrition, short gut syndrome).

          -  Dementia, altered mental status, or any psychiatric condition that would prohibit the
             understanding or rendering of informed consent.

          -  Chemotherapy, radiotherapy, or any other cancer therapy for current diagnosis of
             breast cancer.

          -  Subjects may not have received or be receiving any other investigational agents for
             the treatment of the cancer under study.

          -  History of allergic reactions attributed to compounds of similar chemical or biologic
             composition to abemaciclib or other agents used in study.

          -  Uncontrolled intercurrent illness including, but not limited to ongoing or active
             infection, symptomatic congestive heart failure, unstable angina pectoris, cardiac
             arrhythmia, or psychiatric illness/social situations that, in the opinion of the
             investigator, would limit compliance with study requirements.