ERBB2 and cancer: Breast cancer is a heterogeneous disease that shows different behaviors according to clinical and pathologic factors as well as molecular subtypes.[1–3] When classified by gene expression profiles, breast cancers can be divided into the following types: luminal A and luminal B breast cancers, HER2-enriched breast cancers, and basal-like breast cancers.[4,5] Breast cancer is categorized because the appropriate treatment approach and potential outcomes vary depending on the specific type of cancer.