A type of protein found on epithelial cells, which line the inside and outside surfaces of the body. Keratins help form the tissues of the hair, nails, and the outer layer of the skin. They are also found on cells in the lining of organs, glands, and other parts of the body. Certain keratins may be found in higher than normal amounts in patients with different types of epithelial cell cancers, including lung, breast, colorectal, bladder, and head and neck cancers. Measuring the amount of specific keratins in the blood may help to plan cancer treatment or find out how well treatment is working or if cancer has come back. A keratin is a type of tumor marker. Also called cytokeratin.