The mouse proteolipid protein (PLP) gene was cloned into the lambda bacteriophage Charon 4A. The organization and the nucleotide sequence of the exons of the mouse PLP gene were quite similar to those of their human counterparts, consisting of seven exons. The transcription of the PLP gene started from multiple sites. There was a unique sequence tandemly repeated four times, sharing homology with the herpes simplex virus DR2 sequence, upstream from the transcribed region. Expression of the myelin basic protein (MBP) is also restricted to the oligodendrocytes in the central nervous system as is the PLP expression. Homology search against the mouse MBP gene revealed that several boxes in the 5'-flanking region of PLP show a high degree of homology with the sequence present in the MBP 5'-flanking region, possibly of importance in the concomitant expression of both genes in the central nervous system. PLP-mRNA in jimpy mutant mice does not contain exon 5 and its content is greatly reduced. We analyzed the jimpy PLP-mRNA and showed that the transcription initiated from the same sites as those in normal mice. Cloning and sequencing of the 5'-flanking region of the jimpy PLP gene revealed that there were no mutations in the promoter region of the jimpy PLP gene. Therefore, it is likely that a mutation, presumably existing within the jimpy PLP gene, caused the skipping of exon 5 and directly affected the mRNA level.