ERBB2 and breast cancer: Patients’ gene alteration and gene expression data (Fig. 1), microarray data in TDM1R and TDXdR cell lines (Fig. 4A and Fig. S4A), and kinome RNAi screening results (Fig. 4C and D) indicated that the PI3K, cell cycle and DNA repair pathways are potential targets for combination therapy with T-DXd in TDM1R and TDXdR HER2+ BC.