Abstract: A cDNA (M41) corresponding to a mouse myelin basic protein (MBP) mRNA with a longer 5′-untranslated region than predicted from earlier studies of MBP gene structure has been isolated and characterized. The additional 5′-untranslated region is encoded by two previously unidentified exons upstream of the major transcription start site of the gene. Using a DNA probe specific for M41-MBP mRNAs, Northern blot analysis indicated that expression of this transcript follows a developmental course in mouse brain similar to that of the majority of MBP mRNAs, but that the level of expression varies between brain and spinal cord. Expression of MBP mRNAs similar to the mouse M41-MBP also was identified in rat brain. The results suggest that the structure of the MBP gene is more complex than originally thought, containing at least two more exons. There appears to be at least one more MBP gene promoter that directs the synthesis of a subset of MBP mRNAs with a unique 5′-untranslated region.