Cytokeratin 19 fragment (Cyfra 21-1) is an intermediate structural filament of cytoskeleton 19, typically associated with non-small cell lung cancer (NSCLC), especially of the squamous cell subtype. Cyfra 21-1 was first approved as a tumor marker by the FDA in 2011 to monitor disease progression in lung cancer patients. Its level of expression at the time of diagnosis correlates with the overall prognosis during treatment. It is also reported to be a potential biomarker for ovarian cancer and squamous cell carcinoma.