Results of HER2 gene-protein staining of FFPE breast cancer tissues exhibiting heterogeneity of HER2 positive tumor cell populations or isolated tumor cell populations. (A) The HER2 gene-protein assay demonstrated the heterogeneity of HER2 positive tumor cell populations in FFPE breast cancer tissues. In the sample shown, cell populations with HER2 IHC scores of 3+, 2+, and 1+ neighbor each other and all tumor populations present amplified HER2 gene. However, the HER2 IHC 3+ tumor cell population contains dispersed HER2 gene copies while the HER2 IHC 2+ and 1+ population contains clustered HER2 gene copies [40×]. (B) The HER2 gene-protein assay clearly visualized small groups of HER2 IHC 3+ breast cancer cells [4×]. The insert shows an isolated individual HER2 IHC positive tumor cell with HER2 gene amplification [100×].