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The finding that estrogen receptor ERβ agonists induce regression of ectopic endometrial implants in a rodent model has rekindled interest in the role of ERβ in the growth and maintenance of human endometriosis. We hypothesize that the ERβ agonist may have a direct effect on the human endometrium. We examined the mRNA and protein expression of ERβ in both eutopic and ectopic endometrium and performed a luciferase reporter assay for the presence of functional ERβ in human endometrial stromal cells. Diarylpropionitrile (DPN - a ERβ specific agonist) significantly inhibited endometrial stromal cell proliferation at 10 M by 25% but not at 10 nM. ERβ mRNA is present in both eutopic and ectopic endometrium; however, protein expression is absent in both stroma and epithelium. Our search for functional ERβ similarly showed an absence in endometrial stromal cells. These results suggest that the regression of endometriosis might be ERβ-independent.