ERBB2 and breast cancer: Building on these pioneering findings and considering that trastuzumab sensitivity is notably restricted to luminal-HER2+ breast cancer cell lines, whereas all basal-HER2+ cell lines exhibit inherent primary resistance to trastuzumab [10], we recently proposed that a basal CK surrogate definition of HER2+ breast carcinomas might define subgroups of patients likely to display resistance to trastuzumab-based therapy.