We read with great interest the study of Kalof et al1 concerning immunostaining patterns of myoepithelial cells in breast lesions. The authors identified focal CD10 staining of breast epithelial cells. We recently saw a case of co‐existent intraductal papilloma, intraductal carcinoma and infiltrating ductal carcinoma of breast ((figsfigs 1 and 22).). The cytoplasm of epithelial cells in all components was pan‐cytokeratin positive and CD10 positive ((figsfigs 3 and 44)) in addition to myoepithelial CD10 staining in the intraductal papilloma. Kalof et al also mentioned the study of Moritani et al,2 which showed lack of CD10 positivity in both non‐neoplastic and neoplastic breast epithelia. Methodological differences were mentioned by Kalof et al as possible reason(s) for the discrepant results between their study and that of Moritani et al. Comparison of the methods used in the study of Kalof et al, that of Moritani et al and our case study (table 11)) indicates that CD10 immunostaining methods using 56C6 clone and/or non‐heat antigen retrieval detect CD10 expression in non‐neoplastic and neoplastic breast epithelial cells.
In their in vitro study of phenotypic and functional characterisation of a multipotent epithelial cell in the normal adult human breast, Stingl et al3 identified a bipotent human breast epithelial cell progenitor that can express MUC‐1, CD10 and epithelial‐specific antigen and can generate mixed colonies of both epithelial and myoepithelial cells. Therefore, it is possible to see CD10 expression in the breast luminal epithelial cells arising from these bipotent cells.
Although we cannot draw any conclusions about CD10 expression in breast epithelium based on the study by Kalof et al and our single case result, CD10 positivity in epithelial cells should not rule out the possibility of primary breast cancer during the work‐up of a metastatic adenocarcinoma. Larger studies using different CD10 antibody clones and different antigen retrieval methods could help clarify CD10 positivity in mammary epithelium.