Tumor markers are substances, often proteins, produced by cancer cells or by the body in response to cancer. They can be found in blood, urine, or tissues and are used to help diagnose certain types of cancer, monitor treatment effectiveness, and check for recurrence. Common tumor markers include CA-125 for ovarian cancer and PSA for prostate cancer. While tumor markers can provide valuable information, they are not definitive for diagnosing cancer. Elevated levels may also occur in non-cancerous conditions, so they are typically used alongside other diagnostic methods, such as imaging tests and biopsies, for a comprehensive evaluation.