ESR1 and breast cancer: Clinically, breast cancer is generally classified into four molecular subtypes according to the gene expression profiling of estrogen receptor alpha (ERα), progesterone receptor (PR), and human epidermal growth factor receptor 2 (Her2): Luminal (Lum) A, LumB, HER2-enriched (HER2), and triple-negative breast cancer (TNBC) [1].