Two general mechanisms of de novo resistance to EGFR TKI treatment in EGFR mutant NSCLC patients have been described to date: (1) secondary alterations in EGFR that prevent inhibition of EGFR by an EGFR TKI (drug resistant EGFR mutation), and (2) additional genetic alternations that can co-occur with an EGFR activating mutation in EGFR mutant NSCLC cells. This evidence concerns the gene EGFR and non-small cell lung carcinoma.