CEACAM5 and lung cancer: Serum proteins, such as carcino-embryonic antigen, cytokeratin 19 fragment, cancer antigen 125, squamous-cell carcinoma antigen, neuron-specific enolase, progastrin-releasing peptide, tumour M2-pyruvate kinase, and C-reactive protein are potential markers for lung cancer, and their levels can signify the presence of tumours, facilitate histological analysis, and allow prediction of cancer progression.