We found that a persistently high level of CEA after treatment with a reversible EGFR-TKI can successfully identify patients with NSCLC cells that are resistant (perhaps because of the occurrence in the EGFR kinase domain of a T790 M secondary mutation that prevents EGFR-TKI binding and subsequent growth arrest). The gene discussed is CEACAM5; the disease is non-small cell lung carcinoma.