Metaplastic carcinoma (MC) is a highly malignant tumor that is mainly classified as a triple-negative subtype exhibiting basal markers on immunohistochemical testing. Immunohistochemical findings of S-100 and SOX10 are useful and valuable in the differential diagnosis of Schwannian, melanocytic, and myoepithelial/basal cell epithelial neoplasms, particularly of unknown differentiation or origin. We report an extremely rare case of MC with a basal-HER2 phenotype (HER2 phenotype breast carcinoma

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... ith basal marker expression) that exhibited unusual S-100 and SOX10 immunoreactivity. A 54-year-old Japanese woman presented with a 2-month history of a painful, growing mass in her left breast. Core needle biopsy was performed and immunohistochemical results suggested poorly differentiated carcinoma or melanocytic neoplasm. Because the tumor was located in the breast, the immunohistochemical panel included the St. Gallen consensus markers, which showed focal expression of HER2 (score: 2+, equivocal) and a Ki-67 labeling index of 87.0%, but no expression of estrogen receptors or progesterone receptors. Fluorescence in situ hybridization identified HER2 gene amplification based on the 2013 ASCO-CAP HER2 Test Guidelines (HER2/CEP17 ratio: 3:3; HER2 copy number: 14.7 per cell). The histopathological results suggested that this was a primary breast carcinoma of a "basal-HER2" phenotype. After histological examination of the resected specimen, the tumor was ultimately diagnosed as metaplastic carcinoma. Tumors like this could potentially be mis-diagnosed as other S-100-positive and SOX10-positive malignant neoplasms based on limited sampling, such as that occurring with core needle biopsies. This case highlighted the importance of performing careful histological examinations of HER2 status to correctly diagnose a primary breast neoplasm.