tumor marker (TOO-mer MAR-ker)
A substance found in tissue, blood, or other body fluids that may be a sign of cancer or certain benign (noncancerous) conditions. Most tumor markers are made by both normal cells and cancer cells, but they are made in larger amounts by cancer cells. A tumor marker may help to diagnose cancer, plan treatment, or find out how well treatment is working or if cancer has come back. Examples of tumor markers include CA-125 (in ovarian cancer), CA 15-3 (in breast cancer), CEA (in colon cancer), and PSA (in prostate cancer).