Cytokeratin 19 fragment antigen (Cy211) belongs to a cytokeratin family, which is normally expressed in epithelial tissues and forms the epithelial cells’ filament cytoskeleton. Cy211 is useful as a tumor marker, especially for non-small cell lung cancer (NSCLC) along with carcinoembryonic antigen (CEA) and squamous cell carcinoma–associated antigen (SCC), but also for other epithelial tumors such as bladder cancer. This elevation in tumors may be due to cell lysis, releasing cell contents to the blood, including Cy211 by the action of proteases that degrade the cytokeratin filaments