Among patients with lung cancer, it has been shown that the effect of treatment with the EGFR inhibitor erlotinib is highest if the tumor cells have specific mutations in EGFR.1,2,17 In the current study, we examined EGFR mutations in plasma samples collected during erlotinib treatment from 23 patients with lung cancer in whom EGFR mutations were identified in the blood sample taken before the initiation of treatment. The gene discussed is EGFR; the disease is lung cancer.