The oncogene (v-myb) of avian myeloblastosis virus and a large portion of its cellular homolog (c-myb) have been molecularly cloned and sequenced. The portion of c-myb we analyzed contains seven interspersed segments (or exons). Fusion of these exons creates a continuous nucleotide sequence that is remarkably similar to the sequence of v-myb and that potentially encodes a protein very similar to that specified by v-myb. Comparisons between the sequences of v-myb and c-myb indicate that transduction of c-myb to form v-myb probably resulted from an initial DNA rearrangement and the subsequent use of a spliced RNA as an intermediate.