ESR1 and breast cancer: A subset analysis of the Cancer and Leukemia Group B (CALGB) 9344 trial, which evaluated the additive effect of paclitaxel after doxorubicin (Adriamycin) plus cyclophosphamide (AC) in postoperative patients with breast cancer, reported that additional treatment with paclitaxel was ineffective in patients with ER-positive, HER2-negative tumors [18].