Approximately 75% of all invasive breast cancers are hormone sensitive [estrogen-receptor positive (ER+) or progesterone-receptor positive (PR+)] and patients with such cancers are candidates for endocrine therapy. Endocrine therapy is a central component of the treatment of hormone-sensitive breast cancer in the adjuvant and, increasingly, neoadjuvant settings. Knowledge of hormone receptor expression is essential for selection of appropriate therapy. Measurement of hormone-receptor expression [estrogen receptor (ER) or progesterone receptor (PR)] using in vitro assays of the tumor tissue at the time of primary diagnosis is standard of clinical care. However, the presence of these hormone receptors predicts for clinical benefit in only 30-50% of women with advanced disease receiving first-line endocrine therapy and 15-30% receiving second-line therapy (1-3). Thus, the presence of a hormone receptor does not indicate that the receptor is functional and essential to the growth of the cancer cell, nor does it imply that interference with receptor function will result in tumor cell kill. There are several shortcomings of the in vitro assays and neither quantitative nor qualitative receptor assays performed on samples of tumor tissue completely predict the response to antiestrogen therapy in breast cancers. In addition, none of the current clinical tools (serologies, prognostic factors, or radiologic studies) can accurately predict for clinical benefit from endocrine therapy. Accordingly, better methods for predicting clinical response to antiestrogen therapy need to be developed.