A tumor marker is a substance found in the blood, urine, or body tissues that can be elevated in cancer, among other tissue types. There are many different tumor markers, each indicative of a particular disease process and they are used in oncology to help detect the presence of cancer. An elevated level of a tumor marker can indicate cancer; however, there can also be other causes of the elevation. Tumor markers can be produced directly by the tumor or by non-tumor cells as a response to the presence of a tumor. Determination of tumor marker levels by serum testing are useful for: diagnosis of cancer, prognosis cancerous tumors progression, indicating of metastases, detection of residual disease (after surgery), assessment of cancers treatment.