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).