This kit is used for quantitative detection of cytokeratin 19 fragment (CYFRA21-1) in human serum in vitro. Cytokeratin is the structural protein of epithelial cells. So far, 20 different cytokeratins have been found. Because of their special distribution, cytokeratin has become the index of tumor pathology. Complete cytokeratin peptides are difficult to dissolve, but the dissolved fragments can be detected in serum. Slight increase of CYFRA21-1 was occasionally seen in patients with benign liver disease and renal failure. CYFRA21-1 is mainly used to monitor the course of NSCLC. The significant increase of CYFRA21-1 level in blood indicates that the tumor has advanced stage or poor prognosis.