Abstract Breast cancer remains a major health problem, despite advances in detection, diagnosis, and treatment. Four genomic subtypes of breast cancer have been identified: luminal A and B, HER2 positive, and basal type breast cancers. Three of these subtypes help to define the proper targeted therapeutic as breast cancers of luminal subtypes tend to be estrogen receptor positive and treatable with tamoxifen while breast cancers of HER2 positive subtype are treatable with trastuzumab. The fourth subtype, the basal type breast cancers, has a poor prognosis and has yet to have a molecular target identified. One molecule highly expressed in basal type breast cancers is epidermal growth factor receptor (EGFR). However, targeting EGFR kinase activity with small molecule kinase inhibitors has yet to show efficacy in the treatment of breast cancers. Recently, a panel of EGFR expressing breast cancer cell lines was characterized as sensitive or resistant to EGFR kinase inhibitors. One mechanism of resistance to EGFR kinase inhibitors was found to be the constitutive activation of Met, another molecule associated with basal type breast cancers. Here, the mechanism of constitutive Met activation in EGFR tyrosine kinase inhibitor resistant breast cancer cells is explored. Previously, constitutive Met activation was found to associate with acquired EGFR inhibitor resistance in lung cancer via genetic amplification of Met. In the EGFR TKI resistant breast cancer cells with constitutive Met phosphorylation, Met was not amplified or significantly overexpressed. In addition, constitutive Met phosphorylation was found to occur independent of ligand (hepatocyte growth factor (HGF)) stimulation, as HGF was not expressed in the breast cancer cell lines and inhibiting HGF association with Met via an inhibitory antibody failed to reduce Met phosphorylation. These data suggest a ligand-independent mechanism for Met constitutive phosphorylation. Interestingly, ligand-independent Met activation can be mediated by Met association with other membrane molecules including integrins, tetraspanins and receptor tyrosine kinases, including EGFR. Co-immunoprecipitation studies demonstrated that EGFR and Met co-associate and that this association occurs independent of EGFR or Met kinase activities. In addition, Met phosphorylation was reduced with EGFR TKIs as well as with downregulation of EGFR expression. Taken together, these data suggest that one potential mechanism of EGFR TKI resistance in breast cancer cells is the association and transphosphorylation of Met by EGFR. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 278.