CDO is a cell surface receptor-like protein that positively regulates myogenic differentiation. Reported here is the identi®cation of BOC, which, with CDO, de®nes a newly recognized subfamily within the immunoglobulin superfamily. cdo and boc are coexpressed in muscle precursors in the developing mouse embryo. Like CDO, BOC accelerates differentiation of cultured myoblast cell lines and participates in a positive feedback loop with the myogenic transcription factor, MyoD. CDO and BOC form complexes in a cis fashion via association of both their ectodomains and their intracellular domains. A soluble fusion protein that contains the entire BOC ectodomain functions similarly to full-length BOC to promote myogenic differentiation, indicating that the intracellular region is dispensable for its activity in this system. Furthermore, a dominant-negative form of CDO inhibits the pro-myogenic effects of soluble BOC, suggesting that BOC is dependent on CDO for its activity. CDO and BOC are proposed to be components of a receptor complex that mediates some of the cell±cell interactions between muscle precursors that are required for myogenesis.

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