Breast cancers negative for estrogen receptor but positive for progesterone receptor, a true entity?

By the conventional steroid-binding assay method for receptor, 3% of 1,095 primary breast cancers (or 10.6% of 263 premenopausal tumors) were classified as negative for estrogen receptor (ER), but positive for progesterone receptor (PR). The true ER status in this rare group of tumors was further investigated by the enzyme-immunoassay (EIA) or immunocytochemical (ICA) staining method using monoclonal antibodies H222 and D547. Immunoreactive ER was present in nine ER-/PR+ tumors studied, whereas it was not detectable in nine age-matched ER-/PR- tumors. Immunoreactive ER was also present in 24 ER+ breast cancers studied, and was particularly higher in tumors that were PR+. Measurement of immunoreactive ER by monoclonal antibody method provides certain advantages over the conventional dextran-coated charcoal (DCC) method, especially in ER-/PR+ tumors.