CYFRA 21-1 is a serum biomarker that measures fragments of cytokeratin 19, a protein present in the structural framework of epithelial cells. Elevated levels of CYFRA 21-1 are commonly associated with non-small cell lung cancer (NSCLC), particularly squamous cell carcinoma. This marker is often used alongside imaging and clinical evaluation to assist in diagnosis, prognosis, and monitoring of therapy response.