An estrogen-responsive murine Leydig cell tumor (M5480A) was examined for the presence of cross-reactive proteins to a monoclonal antibody directed against a Mr 24,000 estrogen-regulated protein in human breast cancer cells. Human breast tumor biopsies were used as controls for the cytosol preparations, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and Western blot conditions used in these experiments. The estrogen-regulated Mr 24,000 protein was detected in sodium dodecyl

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... olyacrylamide gels of cytosols from four human breast tumor biopsies examined. Larger amounts of the Mr 24,000 protein were present in the two estrogen-progesterone receptor-positive tumor biopsies in comparison to the two estrogen-progesterone receptor-negative samples. In addition, the two receptor-positive samples demonstrated an additional, less intense immunoreactive band at Mr 21,000. Under identical conditions, the same monoclonal antibody bound to two major protein bands from sodium dodecyl sulfate-polyacrylamide gels of Leydig cell tumor cytosols at Mr 56,000 and Mr 86,000. Antibodies prepared from BALB/c mouse ascites fluid of animals bearing the parent myeloma cell line (P3X63NS1) exhibited no immunoreactivity against the human breast or Leydig cell tumor proteins. In light of the high degree of specificity which this monoclonal antibody exhibits, our results suggest that similar antigenic determinants may exist in these proteins from two distinct tumors.