Gene Expression Following Short Term Exposure to Neoadjuvant Endocrine Therapy in Invasive Breast Cancer

Inclusion Criteria: * Postmenopausal patients, defined as one or more of the following criteria: Documented history of bilateral oophorectomy, Aged 60 years or more, Aged 45-59 years and satisfying one or more of the following criteria: Amenorrhea for at least 12 months and intact uterus Amenorrhea for less than 12 months and follicle stimulating hormone (FSH) concentrations within postmenopausal range including: patients who have had a hysterectomy and patients who have received hormone replacement therapy in the past. * Clinically palpable or non-palpable T1c or greater, ER positive invasive breast cancer diagnosed by outside core biopsy. * Outside diagnosis of invasive breast cancer confirmed at MSKCC * Palpable or non-palpable breast mass \> 1cm highly suspicious for invasive breast cancer based on mammographic, ultrasound and/or physical examination findings and amenable to diagnostic core biopsy. * Core biopsy diagnosis of ER positive invasive breast cancer performed at MSKCC * Breast conserving surgery or mastectomy scheduled at MSKCC. * Informed consent obtained. Exclusion Criteria: * Co-morbid conditions which would preclude use of aromatase inhibitors such as: previous hypersensitivity, severe hepatic insufficiency (SGOT or SGPT three times the upper limit of normal), severe renal insufficiency (creatinine clearance \<10ml/min). Prior radiation therapy to chest wall / breast. * Neoadjuvant chemotherapy * Hormone replacement therapy, tamoxifen or raloxifene treatment within past 3 months * ER negative breast cancer * History of active malignancy within the previous 5 years (except for nonmelanoma skin cancer and breast cancer)