The mouse mutant jimpy carries an X chromosome-linked recessive gene defect that affects the formation of myelin in the central nervous system. To understand the molecular basis of the jimpy mutation, we have examined the expression of mRNAs encoding myelin proteolipid protein (PLP). PLP mRNAs were detectable in jimpy brain RNA at 21 days after birth but were severely reduced in abundance compared to wild-type littermates. Nucleotide sequence analysis of cDNA clones for PLP mRNA, isolated from a cDNA library of jimpy brain poly(A)+ RNA, revealed that the PLP mRNA expressed in jimpy contained a deletion of 74 nucleotides with respect to the wild-type sequence. This deletion causes a frameshift in the open reading frame resulting in an altered carboxyl terminus for jimpy PLP. Probes specific for the deleted sequence, however, hybridize with equal efficiency to genomic DNA from jimpy and wild-type littermates, suggesting that the defect in the jimpy PLP mRNA is generated by aberrant RNA processing rather than by deletion of genomic sequences. We conclude that a mutation in the gene for PLP that leads to an incorrectly spliced RNA transcript is the primary defect of this genetic disorder.