A mouse cosmid library was screened with a cDNA clone for rat small (M, 14,000) myelin basic protein (MBP). The entire MBP gene was isolated. Five exons were detected with the cDNA clone, which encode the Mr 14,000 MBP. A sixth exon, detected using a synthetic oligonucleotide, encodes the 41 amino acids specific to the Mr 18,500 form of MBP. Splicing together all six exons would give rise to Mr 18,500 MBP, while an mRNA omitting exon V would be translated to produce Mr 14,000 MBP. Further experiments indicate that there is probably a single MBP gene in the mouse genome, and that there is a single major 5' end for mouse MBP transcripts, 47 bp 5' of the initiator methionine codon.