Non-small cell lung cancers (NSCLCs), particularly those harboring certain EGFR or K-RAS mutations, are often unresponsive to molecularly targeted agents and have a poor prognosis. Mucin 1 (MUC1) is a transmembrane glycoprotein that is overexpressed in most NSCLCs. However, it is not known if MUC1 is of importance to NSCLC cell growth and survival. In this regard, there are no available genetically-engineered mouse models to study MUC1 involvement in NSCLC initiation, progression or maintenance. MUC1 consists of two subunits~ an N-terminal extracellular mucin subunit (MUC1-N) and a C- terminal oncogenic transmembrane subunit (MUC1-C). The MUC1-C cytoplasmic domain functions as a substrate for EGFR and MET, and interacts with effectors, such as PI3K, that have been linked to NSCLC development. Overexpression of MUC1-C induces transformation and associated gene signatures that are predictive of decreased disease-free and overall survival in NSCLC patients. Moreover, inhibition of the MUC1-C subunit in NSCLC cells is associated with downregulation of the PI3K->AKT pathway and loss of survival. The overall objective of the proposed work is to define the functional role of MUC1-C in NSCLC. Our hypothesis is that MUC1-C contributes to the pathogenesis of NSCLC and that MUC1-C function is essential for survival of NSCLC cells with EGFR and K-RAS mutations. The proposed work will address this hypothesis in a new MUC1-C-driven mouse model of NSCLC and through the use of recently developed MUC1-C inhibitors. The theoretical concept that MUC1-C is a target for the treatment of NSCLC is novel and could shift current research and clinical paradigms. The Specific Aims are: (1) To define involvement of MUC1-C in development of NSCLC in mouse models~ (2) To assess the role of MUC1-C in NSCLC with EGFR mutations~ (3) To determine whether MUC1-C is of importance to development of K-RAS mutant NSCLC~ and (4) To identify how MUC1-C contributes to NSCLC cell survival.