About 15–20% of breast cancers are HER2+, which is defined as having evidence of HER2 protein overexpression and determined by immunohistochemistry status (IHC3+), fluorescence in-situ hybridization (FISH) measurement of a copy number of six or more for the HER2 gene, or a HER2/CEP17 ratio of 2·0 or higher. Here, ERBB2 is linked to breast cancer.