Estrogen receptor (ER) protein status was investigated in the MCF-7 cell line and 70 invasive ductal carcinomas of the breast. This was achieved by immunohistochemical assay (IHA) using two different monoclonal antibodies (ER-1D5 and AER311), which are able to recognize either the amino or carboxyl terminal. The staining results were assessed in terms of index score, and compared with the ERalpha mRNA expression, which was determined by reverse transcription-polymerase chain reaction for the positions of exons 5 and 7. MCF-7 showed similar immunoreactions with both antibodies, and expressed the wild-type (WT) ER mRNA coexpressing deletions of exons 5 and 7. Although there was a significant difference between the ER-1 D5 and AER311 indices in the tissue samples (20.5 +/- 27.2 and 5.7 +/- 16.4; P < 0.001), in the majority of cases ER mRNA expression patterns were similar to that of MCF-7, and WT ER mRNA was expressed in all cases that yielded PCR products. It was concluded that a number of palpable breast cancers lack the carboxyl terminal of the ER protein, regardless of WT ER mRNA expression. These results suggest that the incidence of WT ER mRNA in such cancers is lower than that in the MCF-7 cell line, or that WT ER is less stable.