Tumor markers that are currently available for lung cancer, such as progastrin-releasing peptide (ProGRP), carcinoembryonic antigen (CEA), and neuron-specific enolase (NSE), are not satisfactory for diagnosis at an early stage or for monitoring the disease because of their relatively low sensitivity and specificity in detecting the presence of cancer cells [6]–[8]. Here, CEACAM5 is linked to cancer.