Tumor markers are substances that create cancer cells or other cells in response to cancer or other diseases. Although tumor markers can be produced by healthy and cancerous cells, there are higher levels in cancer. These substances are found in blood, urine, stool, tissue tumors or other tissues of the body. They serve to aid in the diagnosis of cancer, but sometimes, some benign situations can cause an increase in the concentration of tumor markers or cancer patients will not have a high concentration of a tumor marker associated with cancer. Therefore, although some cancers help it is not sufficient to diagnose cancer itself, but must be combined with other tests such as biopsies. In some cancer patients the concentration of a tumor marker may give information about the stage or extent of the tumor and the patient's prognosis. However, they must be carried out several studies periodically to check the progress of the disease.