ESR1 and breast cancer: In general, breast cancers are classified into six different intrinsic subtypes including luminal A, luminal B, HER2 enriched, normal-like, basal-like, and claudin-low based on the presence or absence of the three primary markers (ER, PR, and HER2) [4,5], basal marker (CK5/6, EGFR) [6], and Ki-67 proliferation index [7,8,9].