ERBB2 and breast cancer: M6 cells are a HER2-overexpressing mammary cancer cell line derived from a basal-like GEMM that is enriched for Klf4/5, the C3(1) TAg model.43 Unlike many basal-like models, M6 cells overexpress both Egfr and Erbb2, the two RTKs targeted by lapatinib.