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Estrogen receptors (ERs1) are ligand-activated transcription factors that belong to the nuclear hormone receptor superfamily. In the late 1950s, the existence of a receptor molecule that could bind 17ß-estradiol was demonstrated by Jensen and Jacobsen (Jensen and Jordan, 2003). The first ER was cloned in 1986 (Green et al., 1986; Greene et al., 1986). This receptor was regarded as the only ER until a second ER was reported in 1996 (Kuiper et al., 1996). The two receptors are today known as ERɑ and ERß, respectively. ERɑ and ERß show a high degree of similarity when compared at the amino acid level. The amino acid sequence identity between ERɑ and ERß is approximately 97% in the DNA-binding domain and approximately 56% in the ligand-binding domain (LBD), whereas the N terminus is poorly homologous at 24%. Transcriptional activation by ERɑ is mediated by two distinct activation functions …