ESR1 and breast cancer: According to St Gallen [22], breast cancer is divided into four molecular subtypes, including 1 luminal A: ER positive, HER-2 negative, Ki67 < 14%, or Ki67 at 14%–19% and PR>20%; 2 luminal B: ER positive, HER-2 negative, Ki67 > 20%, or Ki67 at 14%–19% and PR negative or PR<20%, or ER positive, HER-2 positive; 3 HER-2 overexpression: ER negative, PR negative, and HER-2 positive; 4 triple negative: ER negative, PR negative, HER-2 negative.