Proto-myb is a highly conserved cellular gene that is closely related to v-myb, the transforming gene of the avian myeloblastosis virus. We have isolated lambda clones encompassing 21 kbp of mouse DNA that contains portions of the proto-myb gene. Also, we have isolated a cDNA clone containing a 2.5 kbp insert corresponding to mouse proto-myb mRNA. By analyzing both the cloned DNAs and mouse genome DNA, we have constructed a restriction map for mouse proto-myb that extends over 50 kbp.