Cytokeratin 19 is a member of a family of at least twenty different cytokeratin polypeptides.

Cytokeratins form the intermediate filament structure of epithelial cells. Cytokeratin filaments are poorly soluble but following proteolytic degradation, soluble cytokeratin fragments are formed and released into body fluids.

CYFRA 21-1 is an immunoassay that determines the level of cytokeratin 19 fragments in serum. The CYFRA 21-1 EIA is based on two monoclonal antibodies (BM 19.21 and KS 19.1) specific for cytokeratin 193,7-8. Elevated levels of cytokeratin 19 fragments are seen in serum from patients with lung cancer and also in other cancers e. g. bladder cancer. In patients with lung cancer, CYFRA 21-1 has been reported to be useful for monitoring the course of disease during treatment and for detection of recurrence.

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