According to specific protein expressions such as human epidermal growth factor receptor 2 (HER2), estrogen receptor (ER), and progesterone receptor (PR), four subtypes of BC were identified: HER2-enriched (HER2+), TNBC (ER−, PR−, HER2−, triple negative breast cancer), luminal A (ER+ or PR+, HER2−), and luminal B (ER+ or PR+, HER2+) [4, 5]. This evidence concerns the gene ESR1 and breast cancer.