Markers produced by cancers include enzymes and isoenzymes, hormones, oncofetal antigens, carbohydrate epitopes recognized by monoclonal antibodies, receptors, and oncogene products.Enzymes were of the first groups of tumor markers identified. Their elevated activities were used to indicate the presence of cancer.Hormones as tumor markers were used for the detection and monitoring of cancer.Oncofetal antigens were discovered using conventional antisera produced against fluids from cancer - bearing animals or extracts of cancer tissues . The development of monoclonal antibody techniques allowed more sensitive and specific measurements of tumor antigens. More importantly, new antigens were discovered by developing monoclonal antibodies against tumor cell preparations. They appear to have better clinical sensitivity and specificity than do the oncofetal antigens .