Cancer is a generic term for malignant tumors. Tumor markers can be normal in cancer patients. Tumor markers include two types of conditions. One type of marker is a product specific to tumor cells and is not or rarely produced by normal tissues, so it is also called tumor-specific antigen, for example, methemoglobin produced by primary liver cancer. Another type of marker is not specific to tumor tissues, but the indicator is increased in tumor patients, and is called tumor-associated antigen, such as carcinoembryonic antigen. Tumor markers are not the gold standard for cancer diagnosis, but only play an auxiliary value for early diagnosis of cancer as well as prognosis assessment and evaluation of treatment effect. The gold standard for cancer diagnosis needs to be based on the results of pathological examination.