Test Details: Mutations in the EGFR gene correlate with clinical responsiveness to the Tyrosine kinase (TK) inhibitor. About 90% of the mutations detected in EGFR are composed either of in-frame deletion in exon 19 or a specific missense mutation in exon 21. Screening for such mutation in lung cancer may identify patients responsive to gefitinib and erlotinib. In certain patients drug induced resistance is observed correlated with mutation in exon 20. A secondary point mutation that substitutes methionine for threonine at amino acid position 790 (T790M) produces a drug resistance to EGFR TK inhibitor.