Abstract: Identification of defective ER and subclassifying ER+ breast cancers on the basis of the presence of defective ER is described as a tool potentially useful, as previously shown by the results of a pilot study, for predicting which among the ER+ tumors will respond and which will fail to repsond to hormonal modes of therapy. Improvements are introduced in the specimen sampling and ligand introduction steps of the immunohistochemical procedure which was developed for sub-classifying estrogen receptor-positive tumors of human breast cancers and other cancers of the estrogen target organs on the basis of the presence of defective estrogen receptors. Additionally, a new monoclonal antibody reagent which has potential use as a replacement for the polyclonal anti-ER antibody reagents is also described. The modified steps include the use of tumor imprints instead of cryosections; use of ligand coated slides instead of the original ligand layering step.