estrogen receptor assay
a lab test done on a sample of breast cancer cells to see whether they have estrogen receptors. The growth of normal breast cells and some breast cancers is stimulated by estrogen. Estrogen receptors are molecules that function as a cell’s “welcome mat” for estrogen circulating in the blood. Breast cancer cells without these receptors (called estrogen-receptor negative or ER-negative) are unlikely to respond to hormone therapy. Estrogen-receptor positive cancers are more likely to respond to hormone therapy. See also estrogen, hormone therapy.