Twenty cases of infiltrating ductal breast carcinomas were stained with the mouse monoclonal antibody H, an IgM, in order to estimate the distribution of the epitope (a carbohydrate moiety) recognized by this antibody in the malignant cells. The results showed that in all cases examined, the epitope was present in the cytoplasm of the neoplastic cells arranged mainly in a diffuse pattern. To identify the polypeptides that carry this epitope, we used extracts from the MCF-7 human breast carcinoma eel line in immunoblotting experiments. In addition to cytokeratin 8 this epitope was also found on five polypeptides originating from Triton X-100-soluble (M(r)x10(-3) of 232, 67, and 37) and from the Triton X-100-insoluble (M(r)x10(-3) of 51, and 50) fractions, respectively. The contribution, if any, of these polypeptides to the staining patterns is not known. These data reveal the existence of a common epitope between cytokeratin 8 and other cellular polypeptides. They also serve as a note of caution in the interpretation of immunohistochemical staining when anti-cytokeratin antibodies are used.

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