Breast cancer is a hormone-dependent disease that relies on the mitogenic effect of
estrogen to increase tumorigenesis and tumor growth. Clinically significant levels of
estrogen-α receptor (ERα) expression are seen in 80% of human breast cancers,
whereas progesterone receptor is expressed in 55% of human breast cancers. These
data are one of the bases for the development of endocrine therapy. Endocrine
therapy is therapy that targets the pathway and synthesis of estrogen, by blocking it
via receptors, reducing circulating levels of estrogen, or suppressing estrogen
synthesis in the tissues of women diagnosed with breast cancer.