A protein found on epithelial cells, which line the inside and outside surfaces of the body. It is part of a larger protein called MUC 1. CA 27.29 may be found in higher than normal amounts in patients with some types of cancer, including breast cancer. Measuring the amount of CA 27.29 in the blood may help to find out how well cancer treatment is working or if cancer has come back. CA 27.29 is a type of tumor marker.