Twenty human keratins are resolved with two-dimensional gel electrophoresis into acidic (pI <5.7) and basic (pI >6.0) subfamilies. The acidic keratins have molecular weights of 56.5, 55, 51, 50, 50’, 48, 46, 45, and 40kDa. The basic keratins have molecular weights of 65-67, 64, 59, 58, 56 and 52kDa. Members of acidic and basic subfamilies are found together in pairs. The composition of keratin pairs varies with cell type, differentiation status and environment. Many studies have shown the usefulness of keratins as markers in cancer research and tumor diagnosis.