Definitions are adopted from the cancer.gov glossary.
A substance or protein 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 present 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 a biomarker.