CDO (CAMrelated/down-regulatedby oncogenes) is a member of the Immunoglubulin (Ig) superfamily, Ig/Fibronectin (FN)type III repeat family of cell surface proteins. Human CDO is a type I transmembrane (TM) glycoprotein. It is synthesized as a 1287 amino acid (aa) precursorthat contains a 25 aa signal sequence, a 938 aa extracellular domain (ECD), a 21 aa TM segment and a 303 aa cytoplasmic region. The ECD contains five C2-typeIglikedomains, followed by three FN type III repeats. The ECD of human CDO is 85% aa identical to mouse CDO ECD.CDO is found on muscle precursor and neural progenitor cells of the embryo. It likely promotes muscle differentiation, and contributes to axon guidance andneuronal patterning.