The myelin proteolipid proteins PLP and DM20 are essential for the compaction of central nervous system myelin and they play an important role in the maturation of the oligodendrocyte. The specific function of the less abundant DM20 isoform is still unknown, but rescue experiments previously indicated that both isoforms are necessary for oligodendrocyte maturation. In vitro experiments have suggested DM20 may assist in the translocation of PLP into the membrane. We tested this hypothesis in vivo, by investigating whether wild-type PLP derived from a transgene could be incorporated into the myelin membrane of Plp mutant rumpshaker mice. We previously demonstrated that expression of the PLP transgene alone in a more severe Plp mutant, jimpy mouse, did not result in PLP incorporation into the myelin. Here we report that there was significantly more PLP in white matter from rumpshaker expressing the PLP transgene than their nontransgenic rumpshaker littermates and that myelin structure was improved. The delay in oligodendrocyte development was not alleviated by expression of the PLP transgene however, supporting an essential role for DM20 in oligodendrocyte maturation.