In the present study, EGFR T790M mutation in 20.4% (28/137) of the EGFR-mutated, treatment-naïve NSCLC tumors was only detected by using the ddPCR platform but not the cobas EGFR mutation Test, and the FA of EGFR T790M is lower than that of EGFR-activating mutations in baseline tissue and plasma samples, showing that EGFR T790M pre-existing as a minor subpopulation in treatment-naïve, EGFR-mutated NSCLC has undergone clonal expansion in response to the selection pressure by the first- or second-generation EGFR-TKIs treatment. The gene discussed is EGFR; the disease is non-small cell lung carcinoma.