2.1. Patients demographics

Our study cohort consisted of patients with advanced or recurrent NSCLC whose initial biopsy indicated an EGFR‐activating mutation (either EGFR 19del or L858R), treated with first‐generation EGFR‐TKIs (gefitinib and erlotinib), developed PD, and underwent rebiopsy or liquid biopsy at our institution (Henan Cancer Hospital, Zhengzhou, Henan, China) between January 2016 and October 2017. We retrospectively reviewed the patient's molecular testing results and clinical characteristics. Clinical efficacy outcomes were determined by image testing with CT, MRI, chest X‐ray, or PET/CT on the basis of Response Evaluation Criteria in Solid Tumors. Collection and the use of tumor and blood samples of patients in this study were in accordance with the standards required by the Declaration of Helsinki and approved by the Ethical and Institutional Review Board of Henan Cancer Hospital. All patients enrolled in this study have signed informed consents prior the initiation of the experiments.