In the present study, we directly compared the DNA methylation of nine selected genes in plasma-cfDNA and paired CTCs of NSCLC patients before and after osimertinib therapy, namely RAS-association domain family 1 isoform A (RASSF1A), Ras-association domain family 10 (RASSF10), Wnt inhibitory factor-1 (WIF-1), Adenomatous polyposis coli (APC), Breast cancer metastasis suppressor 1 (BRMS1), DNA/RNA helicase Schlafen-11 (SLFN11), SHISA3, retinoic acid receptor-beta (RARβ) and forkhead box protein A1 (FOXA1). This evidence concerns the gene APC and non-small cell lung carcinoma.