CEACAM5 and small cell carcinoma: Carcinoembryonic antigen (CEA), squamous cell carcinoma antigen (SCC), neuron-specific enolase (NSE), glycoantigen 125 (CA125), cytokeratin-19 fragment (CYFRA21-1), and progranulin-releasing peptide precursor (proGRP) are widely used tumor markers for the screening or diagnosis of lung squamous, adenocarcinoma, and small cell carcinoma in clinical practice [12].