For example, in lung cancer, serum tumor markers, including the carcinoembryonic antigen (CEA) [24,25,26,27], cytokeratin-19 fragment (CYFRA) [25,26,28], squamous cell carcinoma (SCC) antigen [27], progastrin-releasing peptide (ProGRP) [29], and neuron-specific enolase (NSE) [26,27,28,30] levels, have been considered to be predictive or prognostic, although no consensus has been reached on how to use them in daily clinical practice. Here, CEACAM5 is linked to lung cancer.