CD66a/CEACAM1 is a member of the carcinoembryonic antigen (CEA) gene family, which belongs to the immunoglobulin superfamily. This protein was originally described in bile ducts of liver as biliary glycoprotein. Subsequently, it was found to be a cell–cell adhesion molecule detected on leukocytes, epithelia, and endothelia. It also mediates cell adhesion via homophilic as well as heterophilic binding to other proteins of the subgroup. Multiple cellular activities have been attributed to the protein, including roles in the differentiation and arrangement of tissue three-dimensional structure, angiogenesis, apoptosis, tumor suppression, metastasis, and the modulation of innate and adaptive immune responses.