Interferons (INFs) are cytokines that exert a plethora of biological activities by interacting with receptors and inducing the synthesis of effector proteins. One such effector is an antiviral protein called Mx, widely distributed among vertebrates. Mx proteins belong to the superfamily of large GTPases, which includes dynamins, products of the Drosophila shibire gene, the yeast Vps1p, and others. Oligodendrocytes (OLGs) synthesize, assemble, and maintain myelin in the central nervous system. In an effort to identify genes implicated in establishing a myelinogenic phenotype, we isolated a 2,494 bp cDNA that encodes a protein of 654 amino acids, which are 95% identical to ovine Mx (from endometrium of pregnant sheep) and 83% similar to human MxA. Cultured OLGs express the mRNA and protein. We present structural features of this protein and speculate on the origin of its induction and the functional significance of its presence in OLGs. The finding that OLGs express an IFN-susceptible protein raises interesting possibilities in light of the evidence that beta-IFN is therapeutically beneficial for the treatment of multiple sclerosis.