The combination of in situ hybridization and immunohistochemical techniques can successfully identify viral DNA/RNA in specific subsets of cellular populations. We recently modified this method to evaluate amplification of the oncogene Her2/neu and overexpression of its protein c-erbB-2 in a series of 15 breast carcinomas. This combination allows the simultaneous evaluation of the oncogene and its corresponding protein expression in single cells and specific cellular populations in histologic tissue sections. Double staining demonstrated heterogeneity within breast carcinomas. In addition, both nuclear and cytoplasmic signals were often detected in morphologically normal-appearing adjacent breast epithelium. The ability to view both the oncogene and its corresponding protein in single cells offers a unique look at the biology of c-erbB-2.