A tumor marker is anything present in or produced by cancer cells or other cells of the body in response to cancer or certain benign (noncancerous) conditions that provides information about a cancer, such as how aggressive it is, whether it can be treated with a targeted therapy, or whether it is responding to treatment.
Tumor markers have traditionally been proteins or other substances that are made by both normal and cancer cells but at higher amounts by cancer cells. These can be found in the blood, urine, stool, tumors, or other tissues or bodily fluids of some patients with cancer. Increasingly, however, genomic markers such as tumor gene mutations, patterns of tumor gene expression, and nongenetic changes in tumor DNA, are being used as tumor markers.
Many different tumor markers have been characterized and are in clinical use. Some are associated with only one type of cancer, whereas others are associated with multiple different cancer types. No “universal” tumor marker has been found that can reveal the presence of any type of cancer.