The normal and malignant mammary gland: a fresh look with ER beta onboard.

Estrogens are important for the development and function of the normal mammary gland as well as for development of mammary cancer. The frontline therapy for treatment of estrogen receptor (ER alpha) positive breast cancer is antiestrogens. A second estrogen receptor (ER beta) is also expressed in the breast but it has not been measured because it is not detected by the immunoassays used to detect ER alpha. In many cell systems ER beta has actions which are opposite to those of ER alpha and this finding has raised questions about the role of ER beta in the development and treatment of breast cancer.