ESR1 and breast cancer: Furthermore, we demonstrated that in addition to 5FU, other chemotherapies such as paclitaxel and doxorubicin exhibit additive to synergistic activity in combination with a SERD in tumors with an ESR1 mutation, suggesting the potential clinical benefit of continuing ER blockade in metastatic BC even after switching to second- or third- line chemotherapy or an antibody drug conjugate, such as trastuzumab-deruxtecan and sacituzumab-govitecan.