In the t(8;21) of acute myeloid leukemia (AML) M2, breakpoints are clustered on both chromosomes. The chromosome 21 breakpoint cluster region (bcr) falls within the runt locus, in the intron immediately downstream of the exons encoding an evolutionary conserved domain (the runt box). Transcripts in which the runt box is fused in frame to a novel sequence derived from chromosome 8 (MTG8) have been previously identified and have been assumed to constitute a critical leukemogenic event. Here we show physical linkage of the chromosome 8 bcr to the MTG8 locus. Unexpectedly, not only does the bcr map upstream of the most 5′ MTG8 exon found in runt/MTG8 fusion transcripts, but it also maps upstream of a further 5′ exon. In addition, we demonstrate the presence of alternative transcripts, originating from the der(8) chromosome, in which runt is out of frame with MTG8. Thus, runt truncation per se, rather than its fusion to MTG8, may be the crucial leukemogenic event.