Currently, in clinical practice, the cytokeratin expression pattern of breast tumors usually is not assessed, and breast cancers are, as a rule, immunohistochemically classified into four immunophenotypes with very important implications [4, 5] in order to decide the more appropriate therapeutic approach: luminal A (ER + and/or PR+, HER2−), luminal B (ER + and/or PR+, and Ki67 > 14 % or HER2+), HER2 overexpressing (ER−, PR−, HER2+), and triple-negative (TN) phenotype (ER−, PR−, HER2−) [6]. The gene discussed is ESR1; the disease is breast cancer.