AIM: Two oestrogen receptors (ERs) have been identified to date—the “classic” ERa and the more\ud recently described ERb. Although much is known about ERa at the mRNA and protein levels, our\ud knowledge of the expression and distribution of ERb protein is much more limited. The aim of this study\ud was to compare the cellular distribution of ERa and ERb in normal human mammary gland.\ud \ud \ud METHODS: Formalin fixed, paraffin wax embedded material was obtained from reduction\ud mammoplasty specimens, normal tissue adjacent to breast tumour, or fibroadenoma. Sections were\ud immunohistochemically stained for ERa, ERb, and the progesterone receptor. The staining pattern for\ud each antibody was evaluated and compared.\ud \ud \ud RESULTS: ERa was restricted to the cell nuclei of epithelial cells lining ducts and lobules. Although ERb\ud was also seen in these cells, additional strong staining was detected specifically in the cell nuclei of\ud myoepithelial cells. Occasional staining was seen in surrounding stromal and endothelial cell nuclei\ud and in lymphocytes.\ud \ud \ud CONCLUSIONS: ER subtypes have distinct distribution patterns in the normal mammary gland. The widespread\ud distribution of ERb suggests that it may be the dominant ER in the mammary gland where it may\ud be acting as a natural suppressor
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