EGFR and Pleural effusion: We analyzed either genomic DNAs from formalin-fixed paraffin embedded (FFPE) samples or pleural effusions, or circulating tumor DNAs (ctDNA) from plasma samples from 83 Chinese NSCLC patients with stage IV diseases at the time of developing drug resistance to the first generation of EGFR TKIs, erlotinib, gefitinib or icotinib.