ESR1 and breast cancer: As the application of gene expression profiling (GEP) in daily practice is neither economical nor practical, many studies have proposed immunohistochemistry (IHC)-based MC to study BC, which has shown a comparable predictive and prognostic value [3].Using estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor 2 (HER2), and Ki-67 immunostains, BC can be generally categorized into four major IHC-based molecular subtypes, including luminal (A and B), HER2-enriched, and triple-negative subtypes.