ER stands for estrogen receptor. Estrogen receptor is a specialized type of protein that allows a cell to respond to the hormone estrogen. After this 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 receptor.
Not all cells make ER. Cells that normally make this 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 ER.
Pathologists can perform a test called immunohistochemistry to see if a group of cells are making the ER. Cells that make this 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 ER. This test is performed because tumour cells that make this receptor require the action of the estrogen hormone to help them grow.
A patient with an ER producing breast cancer is more likely to benefit from hormone blocking medications. The response to the medication depends on the amount of the receptor being produced. Tumours with higher levels of ER are more dependent on estrogen for growth and respond best to treatment.