To confirm whether HER2 expression was restored during tumor engraftment in mice, we stopped T-DM1 and T-DXd treatment in all cell lines for 1 month and measured total and cell surface HER2 expression levels by Western blotting and FACS analysis, respectively (Fig. S8A and S8B), as well as IHC staining in SUM190 and HCC1954 parent tissue samples (Fig. S8C). The gene discussed is ERBB2; the disease is neoplasm.