ERBB3 and breast cancer: Since ErbB2 remained continuously expressed in a panel of breast cancer cell lines treated with either agent (Fig. 1a–c; Supplementary Fig. 1A,B), and no substantial increase in internalization nor degradation of ErbB2 was observed (Supplementary Fig. 1C–F), we inspected the phosphostatus of ErbB2 as well as of ErbB3.