Prior work in EGFR gene sequencing from repeat biopsies revealed that the EGFR activating mutation from the original adenocarcinoma remains in the SCLC cells that emerged during resistance [59], suggesting that these tumors have most likely undergone genuine phenotypic transformation from NSCLC to SCLC as opposed to developing drug-resistant SCLC de novo. This evidence concerns the gene EGFR and adenocarcinoma.