ERBB2 and breast cancer: The human epidermal growth factor receptor 2 (ERBB2) gene is amplified relatively frequently in breast cancer (BC), and is associated with disease relapse and patient survival.1 Ever since anti-ERBB2 targeted therapy was approved for the treatment of patients with BC, clinical evaluation of ERBB2 has become standard of care and is routinely assessed using immunohistochemistry (IHC) and in situ hybridization (ISH).