Carcinoembryonic Antigen (CEA), also known as CD66e, is a cell surface glycoprotein that exhibits several functions, including regulation of intercellular adhesion, differentiation and anoikis, cell polarization and tissue architecture. CEA is present in fetal colon and many types of epithelial tumors, including adenocarcinomas of the GI tract, lung and breast. Antibody to CEA is useful in differentiating lung adenocarcinoma (positive) from mesothelioma (negative). CEA has been helpful in monitoring tumor progression.