Cytokeratin 19 fragment (CYFRA 21-1) is a fragment of cytokeratin 19 that is typically associated with epithelial cell cancers, including NSCLC, and is typically associated with the SQLC type. Since cytokeratins are structural proteins of keratin-containing intermediate filaments found in the epithelial cells, their degradation produces soluble fragments which are measurable in the blood of lung cancer patients as a tumor marker. Other examples of cytokeratin fragments include tissue polypeptide antigen and tissue polypeptide-specific antigen. CYFRA 21-1 is known to be correlated with disease response and the prognosis of lung cancer but cannot be used to differentiate cancer patients from patients with respiratory diseases.