The estrogen receptor (ER) is a ligand-activated transcription factor composed of several domains important for hormone binding, DNA binding, and activation of transcription. Alternative splicing results in several ER mRNA transcripts, which differ primarily in their 5-prime untranslated regions. Two isoforms of the human ER, ERA and ER-beta, occur, each with distinct tissue and cell patterns of expression. Pelletier and El-Alfy (2000) studied the immunocytochemical localization of ESRA and ESRB in human reproductive tissues. In the ovary, ERB immunoreactivity was found in nuclei of granulosa cells of growing follicles at all stages from primary to mature follicles, interstitial gland, and germinal epithelium cells. Nuclear staining for ERA occurred in thecal, interstitial gland, and germinal epithelium cells. In the uterus, strong ERA immunoreactivity was detected in nuclei of epithelial, stromal, and muscle cells.
