BCa is often classified clinically into four subtypes based on expression of ER, progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2, also known as ERBB2): ER+/PR+/HER2−, ER+/PR+/HER2+, ER−/PR−/HER2+, and ER−/PR−/HER2− (also known as triple negative breast cancer, TNBC). This evidence concerns the gene ESR1 and triple-negative breast carcinoma.