1. The (8;21) translocation, which is consistently associated with a subgroup of acute myeloid leukaemia, involves two loci: runt on chromosome 21 and MTG8 on chromosome 8.
2. Breakpoints in runt fall within a single intron that is located immediately downstream of a phylogenetically conserved DNA-binding domain (the ‘runt box’).
3. We now show that most breakpoints on chromosome 8 fall within a region between two alternative 5′ MTG8 exons. Thus, we predict that chimaeric genes on both the derivative(8) and the derivative(21) chromosomes have the potential to be transcriptionally active.