ESR1 and cancer: Histological classification of breast cancer based on the expression of estrogen receptor (ER), progesterone receptor (PR), and/or human epidermal growth factor receptor‐2 (HER2) has been established as the gold standard of cancer diagnosis which is clinically used to classify breast cancer as luminal A, luminal B, HER2‐positive and basal‐like triple negative breast cancer (BL/TNBC).2