Estrogen and progesterone receptors are special proteins present on the surface of normal breast cells. These receptors bind with the appropriate "partner": estrogen in the case of the estrogen receptor (ER), and progesterone in the case of the progesterone receptor (PR). This binding results in growth and abnormal function of breast cells. These receptors (ER and PR) may also be present on breast cancer cells. In general the lower the tumor grade the more likely a tumor will be "positive" for ER and PR. Conversely the higher the tumor grade, the less likely a tumor will be ER and PR positive. In general it is better to have an ER/PR positive tumor. Every breast tumor is tested for the presence or absence of these receptors. If the tumor cells are ER /PR positive, then Tamoxifen may be prescribed to help reduce the chance of breast cancer recurrence. Tamoxifen is a pill that is taken once a day for 5 years. It is an anti-estrogen and shuts down tumor growth in ER positive tumors, but it does have some side effects (see Tamoxifen). It is ordinarily NOT prescribed for ER/PR negative breast cancers.