1. Women with operable breast cancer with a 2-8 week preoperative waiting period will accept preoperative therapy trials and specifically taking a standard drug for breast cancer such as anastrozole in this study
2. Short term anastrozole treatment will induce measurable changes in biomarker levels (ER, PR, Her2, Ki67) within the tumor.
3. Degree of response to short term anastrozole varies with a) duration of treatment and b) breast cancer subtype (based on initial pre-treatment biomarker status)

Inclusion Criteria:

1. Newly diagnosed postmenopausal women (Women are considered post-menopausal and not of child bearing potential if they have had 12 months of natural (spontaneous) amenorrhea with an appropriate clinical profile (e.g., age appropriate, history of vasomotor symptoms) or six months of spontaneous amenorrhea with serum FSH levels > 40 mIU/mL and estradiol < 20 pg/mL or have had surgical bilateral oophorectomy (with or without hysterectomy) at least six weeks ago.)
2. Confirmation of estrogen receptor positive invasive carcinoma on core biopsy
3. Patients whose cancers are palpable and have been deemed to be "operable" by the surgeon
4. Surgery is planned for the next 2-8 weeks.

Exclusion Criteria:

1. History of hormone replacement therapy in the last 6 months
2. Previous treatment by tamoxifen or aromatase inhibitor treatment in six months
3. Known hypersensitivity or contraindications to aromatase inhibitors
4. Known metastatic disease on presentation
5. Recurrent breast cancer
6. Inability to give informed consent