Estrogens are a group of compounds important to menstruation and reproductive cycles. Estrogens are the primary female sex hormones. Like all steroid hormones, estrogens are able to readily diffuse across cell membranes, where they can bind and activate estrogen receptors. There they work to modulate the expression of many genes and to activate rapid-signaling membrane estrogen receptors.
Testing of estrogen and progesterone receptor activity are the most widely used predictive factors applied to certain cancers. The estrogen receptor (ER) antibody is directed against an epitope located on the nucleus of an ER positive normal and neoplastic cells. ER is a ligand-activated transcription factor belonging to the family of nuclear hormone receptors. ER protein is present in 50-70% of invasive breast cancers.
The progesterone receptor (PR) is used with ER as an important marker in breast cancer, due to its role in determining the functionality of estrogen receptors in human breast cancer. Because of its predictive value in patient response with receptor-positive breast cancers, multiple expert groups, such as the American Society for Clinical Oncology (ASCO), the National Academy of Clinical Biochemistry (NACB), and the National Institutes of Health (NIH), recommend that both ER and PR be assessed on all primary breast cancers.