ESR1 and breast carcinoma: However, the modern classification of breast cancer on the basis of primary markers (ER, PR, and HER2) [76,77], the Ki–67 proliferation index [78], and basal markers (EGFR, CK5/6) [79] differentiates breast cancer into luminal A, luminal B, HER2 high, normal–like, basal–like, and claudin low.