ER, PR, HER2, and Ki-67 proteins are IHC markers that are used for the identification of the following molecular subtypes of breast cancer: luminal A (ER+, PR ≥ 20%, HER2−, Ki-67 low), luminal B (ER+, PR < 20% and/ or HER2+ and/or Ki-67 high), HER2-overexpression (ER−, PR−, HER2+), and triple-negative breast cancer (ER−, PR−, HER2−). The gene discussed is ESR1; the disease is breast cancer.