1) Tumor marker is a substance present in or produced by a tumor or by the host, that can be used for differentiating neoplastic from normal tissue based on measurements in body fluids, secretions, cells, and/or tissues. Markers are used in diagnosis, staging and prognosis of cancer, provide an estimation of tumor burden, and serve for monitoring effects of therapy, detecting recurrence, localization of tumors, and screening in general populations. Tumor markers have been categorized as follows: enzymes, isoenzymes, hormones, oncofetal antigens, carbohydrate epitopes, oncogene products, and genetic changes. There is no identified tumor marker that fits the ideal specificity profile.

2) A substance sometimes found in the blood, other body fluids, or tissues. A high level of tumor marker may mean that a certain type of cancer is in the body. Examples of tumor markers include CA 125 (ovarian cancer), CA 15-3 (breast cancer), CEA (ovarian, lung, breast, pancreas, and gastrointestinal tract cancers), and PSA (prostate cancer). Also called biomarker.

3) Substances found in abnormal amounts in the blood, in other body fluids, or in tumor tissue of some patients with certain types of cancer.