The molecular BC subtypes are classified in luminal A (Lum A: ER+ and/or PR+, HER2–), luminal B (Lum B: ER+ and/or PR+, HER2+), HER2 enriched (HER2: ER–, PR–, HER2+) and triple-negative breast cancer (TNBC: ER−, PR−, HER2−) subtypes. Here, ESR1 is linked to breast cancer.