Since typical EGFR gene mutations (i.e., the deletion of typically five amino acids at codons 746-750 (ELREA) in exon 19 and a leucine-to-arginine mutation at codon 858 (L858R)) are a good predictor for tumor response to tyrosine kinase inhibitor, this present study is to tailor the patient's treatment according to his/her EGFR gene mutation status. Receptor tyrosine kinase inhibitor (erlotinib in this study) will be the suggested second-line drug of recommendation for typical EGFR gene mutation patients, and chemotherapy (pemetrexed in this study) will be the suggested second-line drug of recommendation for EGFR wild type patients.