Therefore, it may be advantageous to focus on a chemoprevention strategy that may have a more uniform impact on breast cancer development, such as estrogen exposure. Estrogen and its metabolites, both in the circulation and locally synthesized in the breast, are important in the pathogenesis of breast cancer. High levels of circulating estrogen in postmenopausal women have been associated with an increased risk of breast cancer (Clemons, 2001). Furthermore, local estrogen synthesis, i.e. aromatase activity, in the breast may also be important in the development of breast cancer.