ESR1 and breast cancer: According to the expression of estrogen receptor (ER), progesterone receptor (PR) and human epidermal growth factor receptor 2 (HER2), BC can be divided into hormone receptor positive breast cancer (ER + or/and PR +), HER2 positive breast cancer (ER-, PR-, HER2 +) and triple negative breast cancer (ER −, PR −, HER2 −) [126].