A nuclear binding assay was developed for the purpose of having available a more predictive assay for hormone responsiveness in human cancers. The BNB assay identified specific and saturable steroid nuclear binding in human target tissues and human carcinomas. When the BNB assay was applied to a large set of breast and endometrial carcinomas, we speculate that nonfunctional receptors were detected in 20%-50% of the patients who were receptor-positive by the DCC assay. Lastly, as responsiveness to hormonal therapy in these cancer patients becomes known, the predictive value of the BNB assay can be established.