Examination of Breast Cancer Cells of Pre-menopausal and Post-menopausal Women Before and After Exposure to Tamoxifen or Fulvestrant.

Inclusion Criteria:

* Written informed consent prior to beginning specific protocol procedures, including expected cooperation of the patients for the study treatment regimen and follow-up, must be obtained and documented according to the local regulatory requirements
* Adult women greater than 18 years old
* Eastern Cooperative Oncology Group (ECOG) performance status (PS) 0-2
* New diagnosis of invasive cyclin D1 +, ER+, PR +/-, Her2- breast cancer

  * Cyclin D1 positive as defined as a total immunohistochemical score of 5 or greater
  * Hormone receptor positive as defined as ≥ 10% positive stained cells
  * HER2-normal (IHC score 0-1 or FISH negative \[in-situ hybridization (ISH) ratio \<= 2.0 status\])
* Tumor size at least 5 mm with planned primary surgery at Mount Sinai
* A negative urine dipstick pregnancy test

Exclusion Criteria:

* Estrogen receptor negative invasive breast carcinoma as defined as less than 10% stained cells
* Prior antiestrogen therapy
* Tumor size less than 5 mm
* Prior diagnosis of thrombosis or known hypercoagulable state
* Known history of bleeding diathesis
* Known liver disease
* Prior treatment with neoadjuvant therapy
* Inflammatory breast cancer defined as clinically significant erythema of the breast and/or documented dermal lymphatic invasion (not direct skin invasion by tumor or peau d'orange without erythema).
* Current severe or uncontrolled systemic disease
* Pregnancy or lactation period. Patients of childbearing potential must implement adequate non-hormonal contraceptive measures (barrier methods, intrauterine contraceptive devices) during study treatment.
* Prior malignancy (including invasive or ductal in-situ breast cancer) within 5 years prior to randomization, except curatively treated basal cell carcinoma of the skin and carcinoma in situ of the cervix.