Subtyping of breast cancer, based on oestrogen receptor (ER), progesterone receptor (PR) and human epidermal growth factor receptor 2 (HER2) expression levels, classifies it into Luminal‐A, Luminal‐B, HER2‐positive and triple‐negative breast cancer (TNBC) subtypes. The gene discussed is ERBB2; the disease is breast cancer.