Carcinoembryonic antigen (CEA) is one of the most commonly used tumor markers in serum immunoassay determinations of carcinoma. Members of the CEACAM (carcinoembryonic antigen-related cell adhesion molecule) family contain a single N domain, with structural homology to the immunoglobulin variable domains, followed by a variable number of immunoglobulin constant-like A and/or B domains. CEACAM21 (carcinoembryonic antigen-related cell adhesion molecule 21) is a 293 amino acid single-pass type I membrane protein that belongs to the CEACAM family and contains one Ig-like C2-type domain. CEACAM21 exists as three alternatively spliced isoforms that are encoded by a gene which maps to human chromosome 19.