BC is categorized into four molecular subtypes, i.e., Luminal A, Luminal B, HER2-enriched, and triple-negative breast cancer (TNBC), based on the presence or absence of hormone receptors (estrogen and progesterone receptors) and the overexpression of the human epidermal growth factor receptor 2 (HER2). The gene discussed is ERBB2; the disease is breast cancer.