predictor for the level of success of the use of endocrine therapy in cancer treatment. Many of the endocrine therapies for breast cancer treatments involve the use of selective estrogen receptor modulators (SERMs). SERMs for example the drug tamoxifen, are ER antagonists in breast tissue. They are used in determining the sensitivity of breast cancer lesions to tamoxifen. Patients whose tumors are ER-positive are likely to respond well to these endocrine therapies. References External links Estrogen receptor test entry in the public domain NCI Dictionary of Cancer Terms Tumor markers