You are invited to participate in a research study looking at metabolism (breakdown) and effects of aromatase inhibitors. The purpose of this research is to try to identify which women who take an aromatase inhibitor are more likely to have certain benefits or side effects from the drug. We will do so by determining whether there are differences that normally occur in genes that you have inherited from your parents that might influence the way individuals respond to medications. If you agree to participate in this study, you will be asked to sign this informed consent form.