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The estrogen receptor (ER) is a ligand-activated transcription factor that mediates the effects of the steroid hormone 17β-estradiol, in both males and females. Since the isolation and cloning of ER, the consensus has been that only one such receptor exists.

The finding of a second subtype of ER (ERβ) has caused considerable excitement amongst endocrinologists. In this article, we present data regarding the genomic structure and chromosomal localization of the human ERβ gene, demonstrating that two independent ER genes do exist in the human. Furthermore, we present data regarding the tissue distribution of human ERβ, showing that this receptor is expressed in multiple tissues. For instance, ERβ is found in developing spermatids of the testis, a finding of potential relevance for the ongoing debate on the effects of environmental estrogens on sperm counts. In addition, we find ERβ in ovarian granulosa …