ESR1 and breast carcinoma: Breast cancer is categorized into five major subtypes by molecular and immunohistochemical profiling: luminal A (ER+PR+HER2−Ki67low), luminal B (ER+HER2−Ki67high or PR−), luminal B-like (ER+HER2+), ERBB2/HER2-enriched (ER−PR−HER2+), and triple negative (TN)/basal-like (ER−PR−HER2−) [58,59].