The clinical classification of BC includes hormone receptor-positive (HR+) tumors with the expression of estrogen (ER) and/or progesterone (PR) receptors, human epidermal receptor 2 (HER2) -enriched tumors with overexpression of HER2 in the absence of HR expression as well as triple-negative tumors without the expression of these three receptors [4]. The gene discussed is ESR1; the disease is breast cancer.