In addition, serum level of tumor markers, such as carcinoembryonic antigen (CEA), cytokeratin-19 fragment (CYFRA21-1), and neuron-specific enolase (NSE), is commonly used to facilitate diagnosis of lung cancer, but the results are often nonspecific or unreliable; false-positive results are frequently caused by infection, benign tumors, pregnancy, or other factors (7). Here, CEACAM5 is linked to lung cancer.