An estrogen receptor assay is a laboratory test that is conducted on a biopsy specimen of breast cancer or ovarian cancer to determine whether the surface of the cancer cells contain receptors (molecules that bind) to estrogen. If the surface of the cancer cells contain estrogen receptors, the cancer cells are termed estrogen receptor-positive (ER-positive). If the surface of the cancer cells lack estrogen receptors, the cancer cells are termed estrogen receptor-negative (ER-negative).
When estrogen that circulates in the blood binds to the surface of ER-positive breast cancer cells or ER-positive ovarian cancer cells, the cancer cells are stimulated to multiply.
ER-positive breast cancer and ER-positive ovarian cancer may be treated with hormonal approaches that block the production, binding, or action of estrogen.