EGFR and non-small cell lung carcinoma: The first discovered and most frequently observed cause of acquired resistance to gefitinib/erlotinib treatment in NSCLC patients with L858R or Del 19 mutations (50 % of all cases) is the emergence of clones carrying additional EGFR mutations involving the substitution of a threonine residue by a methionine in position 790 (T790M) (Engelman and Settleman 2008).