The rate of inherent resistance to single-agent trastuzumab in HER2-overexpressing metastatic breast carcinomas is impressive at above 70%. Unfortunately, little is known regarding the distinctive genetic signatures that could predict trastuzumab refractoriness ab initio. The epithelial-to-mesenchymal transition (EMT) molecular features, HER2 expression status and primary responses to trastuzumab were explored in the public Lawrence Berkeley Laboratory (LBL) Breast Cancer Collection. Lentivirus-delivered small hairpin RNAs were employed to reduce specifically and stably the expression of EMT transcription factors in trastuzumab-refractory basal/HER2 + cells. Cell proliferation assays and pre-clinical nude mice xenograft-based studies were performed to assess the contribution of specific EMT transcription factors to inherent trastuzumab resistance. Primary sensitivity to trastuzumab was restricted to the SLUG/SNAIL2-negative subset of luminal/HER2 + cell lines, whereas all of the SLUG/SNAIL2-positive basal/HER2 + cell lines exhibited an inherent resistance to trastuzumab. The specific knockdown of SLUG/SNAIL2 suppressed the stem-related CD44 + CD24 -/low mesenchymal immunophenotype by transcriptionally upregulating the luminal epithelial marker CD24 in basal/HER2 + cells. Basal/HER2 + cells gained sensitivity to the growth-inhibitory effects of trastuzumab following SLUG/SNAIL2 gene depletion, which induced the expression of the mesenchymal-to-epithelial transition (MET) genes involved in promoting an epithelial phenotype. The isolation of CD44 + CD24 -/low mesenchymal cells by magnetic-activated cell sorting (MACS) confirmed their intrinsic unresponsiveness to trastuzumab. A reduction in tumor growth and dramatic gain in sensitivity to trastuzumab in vivo were confirmed when the SLUG/SNAIL2 knockdown basal/HER2 + cells were injected into nude mice. HER2 overexpression in a basal, rather than in a luminal molecular background, results in a basal/HER2 + breast cancer subtype that is intrinsically resistant to trastuzumab. EMT transcription factors might induce an enhanced phenotypic plasticity that would allow basal/HER2 + breast cancer cells to "enter" into and "exit" dynamically from trastuzumab-responsive stem cell-like states. The systematic determination of SLUG/SNAIL2 as a stem/CD44 + CD24 -/low cell-associated protein may improve the therapeutic management of HER2 + breast carcinomas.