Myelin/oligodendrocyte glycoprotein (MOG) is a primary target autoantigen in experimental autoimmune encephalomyelitis, a widely used animal model for autoimmune demyelinating diseases such as multiple sclerosis. We have isolated several rat MOG cDNAs and confirmed their identity by comparison with MOG N-terminal peptide sequence. As expected, MOG mRNA expression is CNS-specific and peaks during active myelination. Our studies show that full length MOG mRNA is approximately 1.6 kb and encodes a signal peptide of 27 amino acids, followed by 218 residues for mature MOG (24,962 MW). A single site for N-glycosylation is found at Asn-31. Rather than the ubiquitous AAUAAA polyadenylation signal, a series of three overlapping, rare poly A signals were identified. The N-terminal half of mature MOG shares 52% identity with bovine butyrophilin, a possible lipid receptor. This same region has 39% identity with chicken B-G antigen, a major histocompatibility complex antigen involved in B cell selection and immune repertoire development. We show that both MOG and butyrophilin, each exhibiting a single Ig-like variable region domain, meet criteria for inclusion in the immunoglobulin superfamily. Moreover, MOG appears to represent a unique member of this superfamily in that it possesses two potential transmembrane domains, in contrast to a single membrane-spanning domain or glycophospholipid anchor found in all other members of Ig superfamily members.