Careful selection of these agents will be important, since lung cancer patients who respond initially to a first generation small-molecule tyrosine kinase inhibitor (TKI) such as erlotinib generally develop drug resistance in later stage cancer due to secondary mutations such as EGFR-T790M, which can in turn be treated effectively with a second generation TKI such as gefitinib [50–53]. This evidence concerns the gene EGFR and cancer.