ERBB2 and neoplasm: The ISH slide must be entirely scanned prior to counting 20 cells, or areas of HER2 amplification may be defined by using IHC. If visual estimation or image analysis of IHC or ISH slide reveals a second population of cells with increased HER2 signaling and >10% tumor cells, it warrants a separate count of at least 20 cells in this population and has to be reported. When using brightfield ISH, expert opinion must be obtained if the interpretation of the comparison of normal versus tumor cells is difficult due to artifactual patterns.