ctDNA in lung cancer patients carries variants that drive carcinogenesis, such as those in the epidermal growth factor receptor (EGFR), Kirsten rat sarcoma virus (KRAS), tumor suppressor gene TP53, anaplastic lymphoma kinase (ALK), reactive oxygen species 1 (ROS1), and epigenetic changes such as DNA methylation. This evidence concerns the gene KRAS and lung cancer.