Three breast cancer cell lines, BT-474 (ErbB2-overexpression and trastuzumab-sensitive) (23), SK-BR-3 (ErbB2-overexpression and trastuzumab-resistant) (24), and MCF7 (ErbB2-low-expression) (25), were selected (SI Appendix, Fig. S1A) for trastuzumab treatment. Here, ERBB2 is linked to breast cancer.