Oestrogen receptor negative-progesterone receptor positive phenotype in 1,211 breast tumours

From 1,211 breast cancers, 15 oestrogen receptor (ER) negative-progesterone receptor (PgR) positive breast cancers by conventional dextran coated charcoal steroid binding assays in cytosol were reassessed using Elisa techniques with monoclonal antireceptors antibodies in the cytosolic and nuclear fractions, and immunocytochemistry on cryostat sections. Three categories of results were found in this series. Two tumours were false negative ER due to receptor sites occupancy by hormonal contraceptive treatment. A second group of ten tumours, with high PgR concentrations and immunoreactive ER, corresponds to non ER-binding forms of receptors. One PgR positive tumour was found to be devoid of PgR by using monoclonal antiPgR antibodies might contain a progesterone binding cyst protein. Only two tumours were found to be true ER negative-PgR positive by all methods. This rare phenotype deserves further study of the regulation of the PgR gene.