Depending on the expression of cell receptors—estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2), breast cancer types are divided into four molecular subtypes: basal (triple negative, which does not express ER, PR, and HER2 (11 % of BC cases with an aggressive phenotype and lack of response to hormone replacement therapy), luminal A (ER+/PR+/HER2−, 60% of BC cases), luminal B (ER+/PR+/HER2+, 15% of BC cases), and HER2 overexpressing (ER-/PR−/HER2+, 14% of BC cases). Here, ESR1 is linked to breast cancer.