Mutations within the gene for myelin proteolipid protein (PLP), a major myelin structural protein, result in abnormal glial differentiation, suggesting that the PLP gene products play some other functional roles. Transcripts from the PLP gene were analyzed in the developing mouse brain by a sensitive method using polymerase chain reaction. The mRNA for DM-20, an alternatively spliced transcript from the PLP gene, was detected in the embryonic mouse brain as early as embryonic day 11, long before the appearance of oligodendrocytes, which were considered to be responsible for PLP production. PLP gene expression was analyzed in various cell lines to determine whether synthesis of the DM-20 mRNA is restricted to those of glial cell lineage. All of the nervous system cell lines examined, including nonglial cell lines, produced DM-20 mRNA but no or very little PLP mRNA. Peripheral sciatic nerve from adult Wistar rats also produced mainly DM-20 mRNA. These results indicate that DM-20 is not only a myelin structural protein, but it also plays other roles in the nervous system that seem to relate, at least in part, to glial differentiation.