It has been demonstrated that plasma DNA from patients with cancer contains DNA originating from the tumor.7–10 Comparison of sensitizing EGFR mutations found in biopsies and plasma DNA from the same patients has been performed and demonstrates varying degrees of correlation.11–14 The plasma DNA offers an opportunity to monitor the presence of EGFR mutations during the treatment of patients with lung cancer and to identify the emergence of resistance mutations. Here, EGFR is linked to lung cancer.