Breast cancer classification, based on hormone receptor expression, delineates four subtypes: Oestrogen Receptor-Positive (ER+), Progesterone Receptor-Positive (PR+), Human Epidermal Growth Factor Receptor 2-Positive (HER2+), and the notably aggressive triple-negative breast cancer (TNBC). The gene discussed is ERBB2; the disease is triple-negative breast carcinoma.