The goal of this protocol is to determine mechanisms of resistance to epidermal growth factor (EGFR) tyrosine kinase inhibitors (EGFR-TKI) in non-small cell lung cancer (NSCLC). A number of trials have shown small molecule kinase inhibitors to be active agents in the treatment of NSCLC [1]. Clinically these drugs have been noted to produce dramatic but infrequent responses. Mutations in the epidermal growth factor receptor have been shown to correlate with sensitivity to gefitinib and erlotinib[2,3]. However, we know that most patients who have initial responses to EGFR-TKI eventually progress. The mechanism of acquired clinical resistance to these inhibitors in patients incompletely understood.