The Estrogen Receptors (ER), ERalpha and ERbeta, are members of the nuclear steroid hormone receptor family and are also found in the plasma membrane (8). Steroid hormone receptors are composed of three domains, an N-terminal domain required for maximal transactivation function (TAF), a DNA binding domain and a steroid binding domain. The DNA binding domain is separated from the steroid binding domain by a hinge region. The DNA binding domain has a high degree of homology amongst the various steroid hormone receptors, followed by the steroid binding domain. The N-terminus is the most divergent region among members of the steroid receptor family. A diagram showing the relative location of these domains is shown below: N H COOH + 1 595 2 S teroid DNA TA F h ing e H um an E strog en R e ce p to r 287 300 e p ito p e 302 180 263 Estrogen has pleiotropic effects on the growth, development and maintenance of diverse tissues (9). The ERalpha and ERbeta isoforms have similar structures, but show differences in tissue expression and can mediate overlapping but different sets of biological functions (10). ERalpha is predominantly expressed in breast, uterus, cervix, vagina, and other target organs. ERbeta has a more limited expression pattern and is primarily detected in ovary, prostate, testis, spleen, lung, hypothalamus, and thymus (12). Many other ER-interacting proteins confer distinct functionalities on the ERs, including activation and repression of transcription, integration of intracellular signalling pathways and control of cell cycle progression. The ERs function by integrating multiple signals from ligands and intracellular signalling pathways (9). ER knockout mice have been used to study the role of ERs with respect to brain degeneration, osetoporosis, cardiovascular diseases and obesity (11).