ERBB2 and breast cancer: Combination proliferation assays were carried out in three of the HER2+ breast cancer cell lines that were: (1) sensitive to trastuzumab/lapatinib (SKBR3); (2) resistant to the combination of trastuzumab/lapatinib/neratinib/afatinib (MDA-MB-453); and (3) sensitive to lapatinib/resistant to trastuzumab (HCC1569).