Statement: A patient has recently been receiving Tamoxifen to treat breast cancer, they are excluded from the primary trial.
Primary_evidence: Inclusion Criteria:   Histologic Documentation of invasive breast cancer by core needle or incisional biopsy. Excess baseline biopsy tumor tissue sufficient to make three 5-micron sections must be available for molecular analyses as part of this study.   The invasive cancer must be estrogen receptor alpha (ER)-positive, with ER staining present in greater than 50% staining of invasive cancer cells by IHC.   The invasive cancer must be human epidermal growth factor receptor 2 (HER2) negative (IHC 0-1+, or with a fluorescence in situ hybridization (FISH) ratio of <1.8 if IHC is 2+ or if IHC has not been done).   Clinical stage I-III invasive breast cancer with the intent to treat with surgical resection of the primary tumor. Tumor must be  2cm to provide adequate tissue.   Patients with multi-centric or bilateral disease are eligible if the target lesions meet the other eligibility criteria. Samples from all available tumors are requested for research purposes.   Women  age 18, for whom adjuvant treatment with an aromatase inhibitor would be clinically indicated. Women must be either post-menopausal, or pre-menopausal having undergone oophorectomy.   Patients must meet the following clinical laboratory criteria:   Absolute neutrophil count (ANC) 1000/mm3 and platelet count  75,000/mm3. Total bilirubin  1.5 X the upper limit of normal range (ULN). Alanine aminotransferase (ALT) and aspartate aminotransferase (AST)  3 x ULN.   - Ability to give informed consent. Exclusion Criteria:   Prior endocrine therapy for any histologically confirmed cancer is not allowed. Prior endocrine therapy that was administered  5 years ago for the prevention of breast cancer in patients with no history of breast cancer is allowed.   Systemic drug treatment to induce ovarian suppression if woman is pre-menopausal.   Any other neoadjuvant therapy for breast cancer (i.e., treatment with any other anti-cancer agent besides Letrozole (10-21)days before surgical resection of the primary tumor).