ERBB2 and breast cancer: The in vitro response of T-DXd was established in a panel of HER2-positive breast cancer cell lines and compared to the previously reported sensitivity to T-DM1 in the same cell panel.7 As T-DM1 and T-DXd differ with respect to cytotoxic payload and DAR, a head-to-head comparison of IC50 values in each cell line provides little information on cellular sensitivity.