An immunohistochemical study for S100 protein was performed on 95 human breast carcinomas. S100 protein positive carcinoma cells were recognized in 54 cases (57%). The S100 protein positive and negative carcinomas were then compared in terms of stage, histologic type, nuclear grade, appearance of minute calcification, immunoreactivity for estrogen receptor (ER), tumor recurrent pattern, and prognosis. The S100 protein positive cases had a tendency to show a low nuclear grade (P < 0.01). They were also more often seen in the ER positive group. For further analysis of the nature of S100 protein positive carcinoma cells, immunoreactivity for neuron‐specific enolase (NSE) and actin was investigated. The positive staining for NSE and actin was seen in 46 cases (48%) and 3 cases (3%), respectively. There was, however, no specific correlation among the S100 protein, NSE, or actin staining. The nature of S100 protein positive carcinoma cells still remains unclear. The S100 protein positive carcinomas, however, exhibit relatively better characteristics. © Wiley‐Liss, Inc.
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