DKK1 and lung cancer: However, serum protein biomarkers of lung cancer, such as neuron-specific enolase, carcinoembryonic antigen (CEA), carbohydrate antigen (CA) 125, cytokeratin 19 fragment (Cyfra21), and Dickkopf-1 (Dkk1) may not be promising for detection lung cancer due to limited specificity and sensitivity and large number of false-positive findings [4–7].