a substance normally found in certain fetal tissues. If found in the blood of an adult, it may suggest that a cancer, especially one starting in the digestive system, may be present. Blood tests for this substance may help doctors find out if a colorectal cancer has come back after treatment. The test is not helpful for screening for colorectal cancer because of the large number of false positives and false negatives. See also digestive system, false negative, false positive, screening, tumor marker, colorectal cancer.