ESR1 and breast cancer: Stratified by its molecular type, Luminal A BC (ER+, HER2−, and Ki67 < 14%) represents around 50%–60% of cancers [20], while Luminal B (ER+, HER2+, and Ki67 > 14%), triple negative BC (TNBC) (ER−, PR−, and HER2−), and HER2+ BC (HER2+, ER−, and PR−) represent, respectively, 15%–20% [21], 12%–17% [22], and 15%–20% [23] of BC diagnoses.