ERBB2 and invasive breast carcinoma: Human epidermal growth factor receptor 2 (HER2) is a membrane tyrosine kinase receptor overexpressed in approximately 15% of invasive breast cancers.1 According to the National Comprehensive Cancer Network (NCCN) clinical oncology guidelines, all patients with newly diagnosed invasive breast cancer should undergo HER2 testing with immunohistochemistry (IHC) to evaluate protein expression levels.2 For those with equivocal results (IHC2+), reflex in situ hybridization (ISH) is required to quantify HER2 gene expression.