ESR1 and breast cancer: According to immunohistochemical and gene expression studies on ER, PR, HER2, and Ki67, human breast cancers are classified into the following subtypes: luminal A‐like (ER+/PR+/HER2−/low Ki67), luminal B/HER2‐negative‐like (ER+/PR+−/HER2−/high Ki67), luminal B/ HER2‐positive‐like (ER+/PR+−/HER2+), HER2‐positive (ER−/PR−/HER2+), and triple‐negative (ER−/PR−/HER2−) [1, 2, 3].