C-myb proto-oncogene: evidence for intermolecular recombination of coding sequences.

We have characterized a novel chicken c-myb exon whose sequences are specifically expressed in thymic cells. In situ hybridization experiments indicate that this thymus-specific coding exon is localized on a small chromosome, distinct from the large acrocentric chromosome 3 on which we recently mapped the bulk of 15 exons, common to the c-myb mRNA species expressed in hematopoietic cells of both B and T lineages. These observations indicate that intermolecular recombination is required for the tissue-specific expression of the c-myb proto-oncogene. We also show that these thymus-specific sequences are conserved in human DNA and lie on chromosome 17q25, whereas the human c-myb locus is localized on chromosome 6q22-23. Sequencing data obtained from genomic DNA and PCR analyses performed with c-myb mRNA species expressed in chicken thymic cells strongly suggest that a repeated decameric sequence plays a key role in the recombination process.