We have studied the histopathology and differential distribution of the c-myc protein (Myc) in human breast tissues including 17 cases of infiltrating mammary carcinoma, 4 cases of fibroadenoma, 5 cases with fibrocystic changes, and 1 case of reduction mammoplasty (as a control). Using a sensitive immunohistochemical method on frozen tissue sections, both a rabbit polyclonal anti-c-myc antibody and a mouse monoclonal anti-c-myc antibody, H51C116, produced high levels of Myc staining in the nuclei of epithelial cells of infiltrating mammary carcinomas (30-90% of cells stained). In contrast, the nuclei of epithelial cells of fibroadenomas, and breast tissues with fibrocystic changes stained infrequently. We studied benign tissue surrounding the tumors in four cases; three were essentially negative, and one showed nuclear epithelial cell staining throughout the lobules. Sixteen of the tumors were examined in parallel, using formalin-fixed, paraffin-embedded samples. Immunohistological procedures for Myc produced uniform, intense epithelial cell cytoplasmic staining (8 cases); light epithelial cell cytoplasmic staining (5 cases) or were unstained (3 cases). We argue that the differences between frozen and paraffin sections are incompatible with the notion of simple displacement of nuclear Myc to the cytoplasm during fixation. Elevated levels of nuclear Myc in tumor cells and subsets of benign tissue are consistent with a role for Myc in mammary cell proliferation and tumorigenesis.

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