Advanced non-small-cell lung cancer (NSCLC) patients harbouring epidermal growth factor receptor (EGFR) mutations (del19 or L858R) show an impressive progression-free survival between 9 and 14 months when treated with erlotinib. However, the presence of EGFR mutations can only imperfectly predict outcome. The investigators hypothesize that progression-free survival could be influenced both by the pretreatment EGFR T790M mutation and by components of DNA repair pathways.