A benign tumor of epithelium. Warts (caused by papilloma virus) are the most familiar example and each is a clone derived from a single infected cell. The epithelim is the covering of internal and external surfaces of the body, including the lining of vessels and other small cavities. It consists of cells joined by small amounts of cementing substances. Epithelium is classified into types on the basis of the number of layers deep and the shape of the superficial cells.