Estrogen receptor is a specialized type of protein that allows a cell to respond to the hormone estrogen. After the estrogen receptor protein is made, it stays in the nucleus of the cell. Estrogen hormone that enters the cell can change the behavior of the cell by activating the estrogen receptor.
Not all cells make estrogen receptor. Cells that normally make estrogen receptor are found in the breast, ovaries, uterus, and cervix. Some types of cancer including those of the breast, ovary, uterus, and cervix also make estrogen receptor.
Pathologists can perform a test called immunohistochemistry to see if a group of cells are making the estrogen receptor. Cells that make estrogen receptor will be called positive or reactive while those that do not will be called negative or non-reactive.
Pathologists test all breast cancers to see if the tumour cells are making estrogen receptor. This test is performed because tumour cells that make estrogen receptor require the action of the estrogen hormone to help them grow.
Why is this important? A patient with an estrogen receptor producing breast cancer is more likely to benefit from hormone blocking medications. The response to the medication depends on the amount of estrogen receptor being produced. Tumours with higher levels of estrogen receptor are more dependent on estrogen for growth and respond best to treatment.