In lung cancer, circulating tumor DNA (ctDNA) has been validated as a surrogate material to detect mutations in the gene encoding epidermal growth factor receptor (EGFR) at diagnosis [12–16] and to identify secondary EGFR mutations, such as p.T790M or p.C797S, at relapse in patients treated with EGFR inhibitors [16–20]. The gene discussed is EGFR; the disease is neoplasm.