Many women being treated for breast cancer are treated with drugs known as aromatase inhibitors, which stop the production of estrogen in post-menopausal women. Women taking these drugs often report muscle and joint aches. This is referred to as Aromatase Inhibitor Musculoskeletal Symptoms, or "AIMSS." The reasons for these muscle and joint pains are not known. This study is being conducted to determine the good and/or bad effects of an aromatase inhibitor known as anastrazole on post-menopausal women being treated for breast cancer, and whether their genes can help explain how anastrazole affects the body. The study also hopes to look at how the side efffects from hormone therapy influence patients' quality of life and willingness to continue hormonal treatment.