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. This evidence concerns the gene ENO2 and lung cancer.