Surrogate intrinsic subtypes of breast cancer, classification regularly used in clinics, includes luminal A (positive for estrogen receptors/ER+ and/or positive for progesterone receptors/PR+, negative for human epidermal growth factor receptor 2/HER2−), luminal B HER2− (ER+ and/or PR+, HER2+), non-luminal HER2-enriched (ER−, PR−, HER2+), and triple-negative (ER−, PR−, HER2−) breast cancer [2,4]. The gene discussed is ESR1; the disease is breast cancer.