ESR1 and cancer: In the new era of cancer therapy, breast cancers can be classified into four types according to the expression of estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor 2 (HER2), and Ki-67 (3): Luminal A [ER+ and/or PR+, HER2−, Ki-67 < 14%], Luminal B [ER+ and/or PR+, HER2+; ER+ and/or PR+, HER2−, Ki-67 > 14%], HER2 positive (HER2+) [ER−, PR−, HER2+], and triple-negative breast cancer (TNBC) [ER−, PR−, HER2−].