# What is a tumor marker in cancer?

Source: https://oncologyistanbul.com/answers/what-is-a-tumor-marker
Last updated: 2026-07-12

According to the U.S. National Cancer Institute (NCI), a tumor marker is anything present in or produced by cancer cells — or by other cells of the body in response to cancer or certain benign (noncancerous) conditions — that provides information about a cancer. NCI notes that tumor markers have traditionally been proteins or other substances made at higher amounts by cancer cells than by normal cells. These can be found in the blood, urine, stool, tumors, or other tissues or bodily fluids of some people with cancer. Doctors may use tumor markers to help diagnose cancer, guide treatment planning, and monitor whether a tumor is responding to treatment or has returned. Importantly, NCI states that having an elevated tumor marker level does not by itself mean that someone has cancer. This is general information, not personal medical advice.

## Verify at
- [Tumor Markers fact sheet](https://www.cancer.gov/about-cancer/diagnosis-staging/diagnosis/tumor-markers-fact-sheet) — National Cancer Institute (NCI) (checked 2026-07-12)

Related: [The biomarkers we cover](https://oncologyistanbul.com/biomarkers)