ESR1 and breast neoplasm: At the molecular level, genomic and transcriptomic profiling—based on the expression of estrogen receptors (ER), progesterone receptors (PR), and HER2—classifies breast tumors into four main subtypes: luminal A (ER+ and/or PR+, HER2−, Ki67 <14%), luminal B (ER+ and/or PR+, HER2+ or HER2−, Ki67 >14%), HER2-enriched (ER−, PR−, HER2+), and basal-like/triple-negative (ER−, PR−, HER2−) (7).