According to gene expression profiles, breast cancers are classified into four types, including luminal A (estrogen receptor positive (ER+), progesterone receptor positive (PR+), human epidermal growth factor receptor 2 negative (HER2-)), luminal B (ER+, PR+, HER2+), non-luminal with HER2+ (ER-, PR-, HER2+) and basal-like or triple negative (ER-, PR-, HER2-) [3]. This evidence concerns the gene ESR1 and breast cancer.