For metastatic breast cancer (MBC), treatment is guided based on clinical subtypes determined by the expression of the estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2), often in the primary tumor21; endocrine therapies are prescribed to patients with ER-positive (ER + ) or PR-positive (PR + ) carcinomas while patients with HER2 positive tumors are prescribed anti-HER2 drugs. This evidence concerns the gene ESR1 and carcinoma.