CDO (CAMrelated/down-regulated by 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) precursor that 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-type Iglike domains, 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 and neuronal patterning.