Approximately two-thirds of all breast cancers are positive for expression of the estrogen receptor.For patients whose tumors are positive for this receptor or the progesterone receptor, the preferred first-line treatment comprises blockade of estradiol synthesis or hormone receptor activity using aromatase inhibitors or antiestrogen agents. Although endocrine therapies are useful and well-tolerated, most patients respond to this form of treatment for about 12-18 months before developing refractory disease. New therapies able to provide additional benefit to patients with hormone receptor-positive, antiestrogen-refractory, advanced and metastatic breast cancer are required.