Regarding definitive diagnosis, an invasive tumour tissue biopsy followed by immunohistochemistry (IHC) of cancer-specific markers, such as oestrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor 2 (HER2), and Ki-67 (a nuclear protein associated with cellular proliferation for breast cancer) is a common technique. The gene discussed is ERBB2; the disease is cancer.