ESR1 and breast cancer: The characteristics of HER2 and ER can be used to roughly divide BC into four major molecular subtypes, including Luminal A (HER2 negative and ER positive), Luminal B (HER2 positive and ER positive), HER2-enriched (HER2 positive and ER negative), and Basal-Like (HER2 negative and ER positive) [3].