Breast cancer is commonly classified into molecular subtypes based on the expression status of molecular markers, including ER, progesterone receptor (PR), hormone receptors (HRs), and HER2 (Luminal A, HR+ HER2-; Luminal B, HR+ HER2+; HER2: HR- HER2+; TNBC: HR- HER2-) [20]. The gene discussed is ESR1; the disease is breast cancer.