Carcinoembryonic antigen (CEA for short) is a protein synthesized in the gastrointestinal tract, liver and pancreas during embryonic life. It is also synthesized in small amounts in the adult gastrointestinal tract, but is excreted through the gastrointestinal tract instead of entering the blood system, so there are only trace amounts of CEA in the serum of normal adults. In patients with gastrointestinal tumors (e.g., stomach and colon cancer), serum carcinoembryonic antigen is significantly elevated. However, other malignant tumors such as lung cancer, pancreatic cancer, breast cancer, and some benign diseases such as non-specific colitis, collagen diseases, and cardiovascular disorders may also be elevated. Carcinoembryonic antigen is not elevated in hepatocellular carcinoma, and it is not a marker of hepatocellular carcinoma. Since malignant tumors of the gastrointestinal tract can easily metastasize to the liver, it is necessary to test for carcinoembryonic antigen when differentiating primary liver cancer from secondary liver cancer.