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HER2 gene amplification by fluorescence in situ hybridization and protein expression by immunohistochemistry (IHC) have been used for prognosis and guiding treatment of invasive ductal carcinoma of the breast with trastuzumab. Accurate evaluation of HER2 status is important in the management of patients with candidacy for the HER2-targeting therapy. Despite previous studies, effects of polysomy of chromosome 17, at which HER2 is located, on HER2 protein expression remains controversial. In this study, we calculated the average copy numbers of HER2 and chromosome 17 (CEP17) per nucleus in 109 cases of invasive breast carcinoma and analyzed their correlations with the HER2/CEP17 ratio and protein expression. As expected, there were close correlations between HER2 protein expression and the HER2/CEP17 ratio (CC: 0.49, P<0.001), along with the HER2 copy number per nucleus (CC: 0.48, P<0.001) and between the CEP17 copy number per nucleus and the HER2 copy number per nucleus (CC: 0.45, P<0.001). Correlation between the CEP17 copy number per nucleus and the HER2/CEP17 ratio was not significant (CC: 0.2, P>0.05). There was a weak, but statistically insignificant, correlation between the CEP17 copy number per nucleus and HER2 protein expression by IHC (CC: 0.26, P>0.05). The cases were then grouped on the basis of the amplification of the HER2 gene by fluorescence in situ hybridization. In the cases that showed no amplification, there was a significantly higher CEP17 copy number per nucleus in cases with strong HER2 protein expression (2.99) when compared with cases with weak (2.39) or absent (1.86) expression. In conclusion, a high HER2 gene copy number-associated polysomy 17 is a significant contributing factor in HER2 protein overexpression in unamplified invasive breast carcinomas. Cases without HER2 amplification, but carrying polysomy 17, should be further evaluated for HER2 protein overexpression by IHC. Those with polysomy 17-associated HER2 protein overexpression, like any other IHC+ ones, should be eligible candidates for trastuzumab therapy.