ESR1 and breast cancer: In this study, breast cancer molecular subtypes can be divided into the following: (1) HER2-positive: HER2 3+; (2) HER2-equivocal and HR-positive: HER2 2+ with at least one ER/PR-positive (including low positive); (3) HER2-equivocal and HR-negative: HER2 2+ with both ER/PR-negative; (4) HR-positive: HER2 1+ or 0 with at least one ER/PR-positive (including low positive); (5) triple-negative breast cancer (TNBC): HER2 1+ or 0 with both ER/PR-negative.