ESR1 and breast neoplasm: Although molecular profiling has contributed to a greater understanding of breast tumor biology, therapeutic approaches are still largely guided by the presence or absence of three biomarkers: estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2) amplification, which broadly divide breast cancer into three groups: luminal (ER+ and PR+ or PR–), HER2-amplified, and triple-negative breast cancer (TNBC), which lacks ER, PR, and HER2 expression [6].