History, molecular features, and clinical importance of conventional serum biomarkers in lung cancer.

Serum biomarkers provide valuable information about the diagnosis and prognosis of a wide variety of malignant tumors. Despite the identification of several useful serum biomarkers in lung cancer, consensus on their utility has not yet been reached. Furthermore, guidelines and standard protocols to implement their use for patients with lung cancer are lacking, despite the accumulation of much data on the efficacy of several serum biomarkers over recent decades. In this review, we discuss the molecular features, functions, and clinical relevance of the conventional serum biomarkers for lung cancer, including carcinoembryonic antigen (CEA), cytokeratin 19 fragment 21-1 (CYFRA 21-1), tissue polypeptide antigen (TPA), carbohydrate antigen 19-9 (CA19-9), sialyl Lewisx (sLex), carbohydrate antigen 125 (CA-125), squamous cell carcinoma-related antigen (SCC-Ag), neuron-specific enolase (NSE), and pro-gastrin-releasing peptide (proGRP), aiming to provide a snapshot of the current landscape and their potential combined utility in the diagnosis and prognosis of lung cancer.