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.

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