As the concordance between the HG and Ki67 rate was high for HG1 and HG3 tumors and as the prognostic value of the Ki67 was significant in the ER+ HER2− HG2 subgroup (37% of the cases), we conclude that the Ki67 is a cost-effective decision marker for the indication of adjuvant therapy in more than one third of early-stage, pT1–pT2, pN0, breast cancer patients. Here, ESR1 is linked to breast cancer.