ESR1 and breast cancer: In addition, the molecular expression profiling of BC tissues has led to the classification of several intrinsic BC subtypes with different prognoses and survival: Luminal A (ER+ and/or PR+, HER2−, proliferation marker Ki-67 < 14%), Luminal B (ER+ and/or PR+, HER2−, Ki-67 ≥ 14%; or ER+ and/or PR+, HER2+, any Ki-67), HER2(+)/HER2-enriched (ER-, PR-, HER2+, any Ki-67), and basal-like (triple-negative) (ER−, PR−, HER2−, any Ki-67) [4,5].