Two homologous cDNAs were previously isolated by expression cloning with a monoclonal antibody that recognized a CNS neuronal membrane protein. Both cDNAs, M6a and M6b, bore significant homology with the major myelin proteolipid protein, PLP/DM20. Our initial studies of M6 gene expression in the adult mouse brain showed that M6a was present in neurons, PLP/DM20 in oligodendrocytes, and M6b in both neurons and glia. This led to the recognition of a novel gene family that included the oligodendrocyte-specific PLP/DM20 gene and the neuronal M6 genes. These observations supported the idea that PLP/DM20 may have functions other than myelination. In this report, we describe the spatial and temporal patterns of expression of M6a, M6b, and PLP/DM20 in the developing nervous system. PLP expression was limited to the white matter. M6a appeared in post-mitotic neurons of the brain and spinal cord as early as E10, and later in the hippocampus, cerebral cortex, and the granule cells of the cerebellum. In contrast, M6b was expressed at early embryonic stages in the ventricular zone of the spinal cord, and later during development in both neurons and glia. The early appearance of M6a and M6b mRNAs in the murine CNS suggested that these molecules might play an important role in the development of a variety of neural cell types.