The role of estrogen receptors in breast cancer metastasis.

It has long been appreciated that the estrogen receptor (ER) plays an important role in the biology of breast cancer. It is an accepted factor predicting favorable disease outcome and treatment response, and as such is generally considered to represent a "good" prognostic marker in breast cancer. In this review we present data suggesting that the ER may also play a pivotal role in the metastatic behavior of breast cancer, and present an argument that the up-regulation of ER and/or the selection of specific ER mutations are early events important for facilitating tumor progression. Thus, ER could serve dual roles in breast cancer, acting as a "bad" prognostic marker later in the disease process.