Despite notable improvement in survival rates of patients with breast cancer, this disease is still a major threat to women’s health, and especially patients with “triple-negative” breast cancer (TNBC), which is characterized by absent or minimal expression of estrogen receptor (ER), progesterone receptor (PR), or human epidermal growth factor receptor 2 (HER2) in tumor material (Couch et al., 2015). This evidence concerns the gene ESR1 and breast cancer.