ESR1 and breast carcinoma: Triple-negative breast cancer (TNBC) is defined as a group of breast carcinomas that are negative for expression of hormone receptors (ER, PR) and HER2, we can distinguish between two groups: basal-like (ER-, PR-, Her2-, cytokeratin (CK) 5/6+ and/or Her11+) and unclassified subtype (ER-, PR-, Her2-, Her1- and CK5/6-).