A clone specific for the rat myelin proteolipid protein (PLP) was isolated from a cDNA library made in pUC18 from 17-day-old rat brain stem mRNA. This clone corresponded to the carboxyl-terminal third of the PLP-coding region. The clone was used to identify PLP-specific mRNAs in mouse brain and to establish the time course of PLP mRNA expression during mouse brain development. Three PLP-specific mRNAs were seen, approximately 1,500, 2,400, and 3,200 bases in length, of which the largest was the most abundant. During brain development, the maximal period of PLP mRNA expression was from 14 to 25 days of age, and this was a similar time course to that for myelin basic protein mRNA expression. When the jimpy mouse, an X-linked dysmyelination mutant, was studied for PLP mRNA expression, low levels of PLP mRNA were seen which were approximately 5% of wild-type levels at 20 days of age. When jimpy brain RNA was analyzed by Northern blotting, the PLP-specific mRNA was shown to be 100 to 200 bases shorter than the wild-type PLP-specific mRNA. This size difference was seen in the two major PLP mRNAs, and it did not result from a loss of polyadenylation of these mRNAs.The biosynthesis of the myelin sheath is a crucial part of nervous system development, and as a result, the developmental expression of the myelin protein genes is highly regulated. The normal pattern of myelin protein expression in rodents indicates that little myelin protein expression occurs prenatally or in neonatal animals (2,5,6,8,23,26,28) and that myelin protein synthesis and accumulation increase significantly in the brain at about 10 days after birth, reaching a peak between 15 and 22 days after birth (2,5,6,8,23).Central nervous system myelin contains two major classes of proteins, myelin basic protein (MBP) and proteolipid protein (PLP), which together constitute as much as 80% of the total myelin protein (15,30). The four MBPs that are present in rodent myelin have homologous amino acid sequences, including identical amino-and carboxyl-terminal sequences but internal segments of unique amino acid sequence (3). They are translated from four different mRNAs (50), and the four MBP mRNAs can be produced from a single MBP gene by alternative mRNA splicing (12,44). Two PLPs are present in myelin, the major PLP, which has an apparent molecular weight of 25,000, and the closely related proteolipid DM20 (1), which has an apparent molecular weight of 20,000. PLP and DM20 may have a relationship similar to that found among the four MBPs, since they have identical amino-and carboxyl-terminal sequences (46), and they are very closely related by amino acid composition and by antigenic characteristics. The expression of these myelin proteins correlates closely with the formation of myelin in the developing rat brain, and mouse mutants that are defective in myelin formation show lowered levels of these myelin proteins (4,14,20,25 expression and the genetic deficiency in the shiverer mouse myelination mutant (12,34,44). Several groups have recently isolated cDNA ...