Breast cancers that are estrogen-receptor-positive (ER-positive) are those that have estrogen receptors present on many of the cancer cells. Estrogen receptors are protein molecules that bind to the hormone, estrogen. ER-positive cancers rely on a source of estrogen to encourage proliferation (increase the number) of cancer cells. About 60% of all breast cancers are ER-positive.
Because of their dependency on estrogen, most ER-positive cancers respond well to anti-estrogen therapies, such as Tamoxifen. The anti-estrogen therapies work by blocking the cancer cellsí estrogen receptors, effectively cutting off their nourishment. Finding out that your breast cancer cells do have estrogen receptors can be useful in determining an effective course of treatment.