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The biological actions of estrogens are mediated by estrogen binding to one of two specific estrogen receptors (ERs) ERα and ERβ, which belong to the nuclear receptor superfamily, a family of ligand-regulated transcription factors. ERα and ERβ are products of different genes and exhibit tissue-and cell-type specific expression. The characterization of mice lacking ERα, or ERβ, or both has revealed that both receptor subtypes have overlapping but also unique roles in estrogen-dependent action in vivo. Additionally, ERα and ERβ have different transcriptional activities in certain ligand, cell-type, and promoter contexts. Both receptors, however, are coexpressed in a number of tissues and form functional heterodimers. The biological roles of ERα/β heterodimers in the presence of each respective homodimer are unknown. When coexpressed, ERβ exhibits an inhibitory action on ERα-mediated gene expression and in …