Inclusion Criteria:

          -  Signed written informed consent

          -  Diagnosis of estrogen positive breast cancer, estrogen receptor-positive and
             HER2-negative by American Society of Clinical Oncology (ASCO)/College of American
             Pathologists (CAP) criteria

          -  Eastern Cooperative Oncology Group (ECOG) performance status 0-1

          -  Postmenopausal defined by: a. Age >= 55 years and 1 year or more of amenorrhea b. Age
             < 55 years and 1 year or more of amenorrhea with luteinizing hormone (LH) and/or
             follicle stimulating hormone (FSH) levels in the postmenopausal range c. Age < 55 with
             prior hysterectomy but intact ovaries with LH and/or FSH levels in the postmenopausal
             range d. Chemotherapy or medically induced ovarian suppression with 1 year or more of
             amenorrhea and with LH and/or FSH levels in the postmenopausal range e. Status after
             bilateral oophorectomy (>= 28 days prior to first study treatment)

          -  Absolute neutrophil count (ANC) >= 1500 cells/ul

          -  Platelet count >= 100,000/ul

          -  Serum creatinine concentration < 1.5 x upper limit of normal (ULN)

          -  Bilirubin level < 1.5 x ULN

          -  Aspartate aminotransferase (AST) or alanine aminotransferase (ALT) < 3 x ULN

          -  Alkaline phosphatase =< 2.5 ULN

          -  Metastatic cohorts (Phase I): Diagnosis of stage IV estrogen positive breast cancer,
             estrogen receptor-positive and HER2-negative by ASCO/CAP criteria

          -  Metastatic cohorts (Phase I): Must be a candidate for treatment with CDK4/6 inhibitor
             and hormonal therapy with an aromatase inhibitor as standard of care

          -  Metastatic cohorts (Phase I): No prior exposure to CDK 4/6 inhibitors

          -  Neoadjuvant cohorts (Phase II): Diagnosis of stage I-III estrogen positive breast
             cancer, estrogen receptor-positive and HER2-negative by ASCO/CAP criteria. If stage I,
             clinical tumor size must be >= 1.5 cm

          -  Neoadjuvant cohorts (Phase II): Baseline tumor Ki67 > 5%

          -  Neoadjuvant cohorts (Phase II): Surgical candidate and appropriate for pre-operative
             endocrine therapy

        Exclusion Criteria:

          -  Prior exposure to CDK 4/6 inhibitor therapy

          -  History of retinal disease or active visual disturbances (normal baseline
             study-specified retinal exam required)

          -  Acute illness, including infections requiring medical therapy, known bleeding
             diathesis or need for anticoagulation

          -  Treatment with any of the following medications within 4 weeks before the baseline
             diagnostic biopsy is taken: a. Oral estrogens, including hormone replacement therapy
             (but prior depot estrogen use not allowed). b. Investigational agents (or 5
             half-lives, whichever is longer)

          -  Required concomitant use of any drug that is a strong CYP3A inhibitor or inducer

          -  Psychological, familial, sociological or geographical conditions that do not permit
             compliance with the study protocol

          -  Life expectancy of less than 6 months

          -  Pregnancy, lactation or planning to be pregnant.

          -  Neo-adjuvant cohorts (Phase II): Prior therapy for breast cancer (medical, surgical or
             radiation therapy)

          -  Neo-adjuvant cohorts (Phase II): Clinical T4 disease

          -  Neo-adjuvant cohorts (Phase II): Inoperable or metastatic breast cancer based on
             standard evaluation