ESR1 and triple-negative breast carcinoma: In current clinical practice, the expression of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2) is determined by immunohistochemistry (IHC), with the expression patterns defining to which molecular subtype (luminal A, luminal B, HER2 positive or enriched and triple-negative breast cancer) individual tumors belong.