ESR1 and breast cancer: According to the expression of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor 2 (Her-2), the four main subtypes of BC are luminal A (ER+ and/or PR+, Her-2-), luminal B (ER+ and/or PR+, Her-2+), Her-2 overexpressing (ER- and PR-, Her-2+), and triple-negative breast cancer (TNBC; ER-, PR-, and Her-2-).