Breast cancer worldwide affects nearly one million women per year and although current treatments do help many patients, more than 350 000 die from the disease. In the United States, 215 990 women were diagnosed with breast cancer in 2004 and 40 110 died from the disease (American Cancer Society, 2004). In Malaysia, breast cancer is the most frequent cancer amongst women. In 2002, the National Cancer Registry recorded 4337 new cases of breast cancer in Malaysia, which constituted 30.4% of all cancers in women (National Cancer Registry, 2003). The established risk factors for breast cancer include a family history of breast cancer, early menarche, late age at first childbirth, late age at menopause and history of benign breast disease. With the exception of the genetic predisposition to the disease, the rest of the risk factors point to the life time exposure of women to estrogen. Estrogen does not cause the disease, but is involved in the progression and development of breast cancer. Anti-estrogens are therefore used as therapy in the control of breast cancer progression.