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Estrogens control transcriptional responses through binding to two different nuclear receptors, estrogen receptor α (ERα) and β (ERβ). Since these two ER subtypes are thought to mediate different biological effects, there is intense interest in designing subtype-selective ER ligands. In this study, we evaluated the ERα and ERβ selectivity of 19 known estrogens and antiestrogens using reporter cell lines previously developed in our laboratory. The HELN-ERα and HELN-ERβ cells stably express full-length ERα and ERβ, respectively, and are derived from HELN cells (HeLa cells stably transfected with an ERE-driven luciferase plasmid). We report that 16α-LE2, PPT and 3β,5α-GSD have a high ERα-selective agonist potency while 8β-VE2, DPN, genistein and biochanin A show ERβ selectivity with 8β-VE2 being the most potent and selective ERβ agonist. We also tested ER antagonists and we showed that raloxifene and RU486 are ERα and ERβ-selective antiestrogens, respectively. In all cases, selectivity is due to differences in binding affinities as indicated by whole-cell ligand-binding assays. Very interestingly, we demonstrate that a combination of genistein and raloxifene produces a full-ERβ specific response. Together these results demonstrate the usefulness of our stably transfected cell lines to characterize ER ligands and indicate that treatments combining agonist/antagonist ligands produce full-ERβ selectivity.