 The purpose of this pilot study is to see if taking anastrozole Arimidex for 10 days causes changes in breast cancer cells. Anastrozole Arimidex is a drug used in the treatment of a type of breast cancer that depends on estrogen to grow. This type of breast cancer is called estrogen receptor positive breast cancer. Anastrozole Arimidex works by blocking an enzyme in your body called aromatase. Aromatase is found in your muscles, fat, liver and in breast tumors. This enzyme is important for making estrogen in women who are no longer having menstrual periods. Anastrozole decreases levels of estrogen in the body. We are interested in seeing if taking anastrozole for 10 days will cause changes in breast cancer cells. We will study cell processes such as how they make new blood vessels, how quickly the cells multiply, and how soon they die. We will also study which genes in the breast cancer tissues are turned on or off by taking anastrozole for 10 days. 