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The transactivation properties of the two estrogen receptors, ERα and ERβ, were examined with different ligands in the context of an estrogen response element and an AP1 element. ERα and ERβ were shown to signal in opposite ways when complexed with the natural hormone estradiol from an AP1 site: with ERα, 17β-estradiol activated transcription, whereas with ERβ, 17β-estradiol inhibited transcription. Moreover, the antiestrogens tamoxifen, raloxifene, and Imperial Chemical Industries 164384 were potent transcriptional activators with ERβ at an AP1 site. Thus, the two ERs signal in different ways depending on ligand and response element. This suggests that ERα and ERβ may play different roles in gene regulation.