These molecular markers are also generally used to define BC subtypes, namely Luminal A (LA; ER/PR+HER2−), Luminal B (LB; ER/PR+HER2+ or ER/PR+HER2−Ki-67High), HER2-enriched (HER2+; ER/PR−HER2+), and basal-like, also termed as triple-negative (TN; ER/PR−HER2−) [5]. Here, ESR1 is linked to breast cancer.