Two thirds or more of breast cancers are dependent on estrogen for growth. We use a number of estrogen-blocking medicines for treatment of metastatic breast cancer. The treatment response to these agents is unpredictable, however, and approximately one-third of patients with metastatic breast cancer with receptors for estrogen or progesterone have no benefit from hormonal therapy. Nearly all patients with metastatic breast cancer will eventually become resistant to hormonal therapy despite the fact that the hormone receptors are still present.