EGFR and non-small cell lung carcinoma: Another mechanism of drug resistance is the EGFR exon 20 T790M (EGFR T790M) substitution mutation found in approximately 50% of NSCLC genomes post-exposure to EGFR-TKIs.28 The EGFR T790M substitution changes the structure of the kinase domain, allowing an increase in the affinity of ATP to its’ binding pocket and overcoming EGFR-TKI inhibition.