ERBB2 and breast cancer: We first evaluated the cytotoxicity of each drug against a panel of HER2-positive breast cancer cell lines and observed a spectrum of sensitivity to T-DM1, with IC50 values ranging from highly sensitive (4.4–10 ng/mL; e.g., SK-BR-3 and AU-565) to insensitive (>10,000 ng/mL; e.g., HCC-1419; Fig. 1A and B).