While other signaling pathways also modulate breast carcinogenesis, the expression status of ER, progesterone receptor (PR), human epidermal growth factor receptor 2 (HER2), and the Ki67 proliferation marker stratify breast cancers into several subtypes, such as luminal A (ER+ and/or PR+/HER2− and low Ki67 index), luminal B (ER+ and/or PR+ and higher Ki67 index), triple-negative (ER−/PR−/HER2− with higher Ki67 index), and HER2 enriched [2,3]. This evidence concerns the gene ESR1 and breast cancer.