A protein, designated ERCoA3 is provided. The ERCoA3 protein interacts with the estrogen receptor and the progesterone receptor and causes activation of these receptors is provided. Also provided are polynucleotides which encode ERCoA3 or block translation of the mRNA which encodes ERCoA3. Antibiodies that bind to one or more epitopes in the human ERCoA3 protein are provided. The present invention also relates to methods of inhibiting or reducing tamoxifen or estrogen induced proliferation of cancer cells, particularly breast cancer cells, endometrial cancer cells and uterine cancer cells. The method comprises reducing the activity or levels of ERCoA3 in such.