Cell adhesion molecule-related/down-regulated by oncogenes (CDO) and BOC (brother of CDO) are members of the immunoglobulin/fibronectin type III repeat family and act as cell surface receptors. CDO is a component of a cell-surface receptor complex which also contains BOC, NEO1, CTNNB1 and cadherins and which acts as a mediator of cell-cell interactions between muscle cells. CDO and BOC are single pass membrane proteins that play a role in myogenic cell differentiation. Together, CDO and BOC participate in a positive feedback loop with MyoD, a myogenic transcription factor. The 1,242 amino acid rat CDO protein has a 24 residue signal sequence, five Ig V-like repeats, a 25 residue membrane-spanning region, three FNIII-like repeats and a cytoplasmic region of 256 amino acids containing a proline-rich stretch. The human protein contains 1,225 amino acid residues and shares significant homology with the domain structures of the rat protein.