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Evaluation of HER-2/neu status is important in the management of patients with breast carcinoma, especially in determining the possible application of trastuzumab, a humanized anti-HER-2/neu monoclonal antibody. Chromogenic in situ hybridization (CISH) detection of the HER-2/neu oncogene is a newly developed in situ hybridization method that utilizes a robust and unique-sequence DNA probe labeled with digoxygenin, and sequential incubations with antidigoxygenin fluorescein, antifluorescein peroxidase, and diaminobenzidine. In this study, we examined 20 archival specimens of human breast carcinoma using CISH, and we correlated findings with immunohistochemical findings for HER-2/neu. HER-2/neu immunohistochemistry was carried out with HercepTestTM, a standardized immunohistochemical examination system for HER-2/neu overexpression in surgical pathology specimens. CISH analysis could be done in 18 out of 20 cases examined. Gene copy signals for HER-2/neu were recognized as intranuclear brown dots in both neoplastic and non-neoplastic cells. Seven carcinomas showed an increased number or size of signals and were interpreted as being positive for HER-2/neu amplification. Eight cases were positive with the HercepTestTM. Seven out of eight carcinoma cases found to overexpress immunoreactive HER-2/neu also demonstrated HER-2/neu gene amplification following CISH analysis. There was a significant correlation between immunohistochemical and CISH analyses (P< 0.001). We found that CISH was a specific, sensitive and easily applicable method for the detection of HER-2/neu gene amplification, which may be used together with immunohistochemical examination for the evaluation of patients with breast carcinoma.