ESR1 and breast cancer: According to the distinction of ER, PR, HER2 level BC could be subtyped as Luminal A (ER+/PR+, HER2+), Luminal B (ER+/PR+, HER2-), HER2amp (ER-/PR-/HER2+), and Basal-like/triple-negative (ER-, PR-, HER2-) (4, 5).