CEA was first identified in human colon carcinoma tissue extracts and is known to play a critical role as a ligand in cancer dissemination. Elevated serum CEA is found in 17∼47% of colorectal cancer patients. CEA was originally thought to be a specific marker for colon cancer but levels may also be raised in gastric, pancreatic, lung, breast and medullary thyroid carcinomas, as well as some non-neoplastic conditions such as inflammation, pancreatitis, ulcerative colitis, cirrhosis, COPD, Crohn's disease, hypothyroidism and in smokers. For this reason, it is not useful as a general cancer screening tool, but can be useful in evaluating the response to cancer treatment, to monitor metastasis of colorectal cancer to the liver and to indicate recurrence. Elevated CEA levels should return to normal after successful surgical removal of the tumour and can be used in follow up, especially of colorectal cancers. CEA elevation is known to be affected by multiple factors and it varies inversely with tumor grade; well-differentiated tumours secrete more CEA.