Brother of CDO is a protein that in humans is encoded by the BOC gene. CDON and BOC are cell surface receptors of the immunoglobulin (Ig)/fibronectin type III repeat family involved in myogenic differentiation. CDON and BOC are coexpressed during development, form complexes with each other in a cis fashion, and are related to each other in their ectodomains, but each has a unique long cytoplasmic tail.