The t(8;21) translocation is one of the most frequent chromosome abnormalities in acute myeloid leukemia. It has been shown that the t(8;21) breakpoints on chromosome 21 cluster within a single specific intron of the AML1 gene, which is highly homologous to the Drosophila segmentation gene runt. Here we report that this translocation juxtaposes the AML1 gene with a novel gene, named MTG8, on chromosome 8, resulting in the synthesis of an AML1-MTG8 fusion transcript. The fusion protein predicted by the AML1-MTG8 transcript consists of the runt homology region of AML1 and the most part of MTG8, which contains putative zinc finger DNA binding motifs and proline-rich regions constituting a characteristic feature of transcription factors. The MTG8 gene is not expressed in normal hematopoietic cells, whereas AML1 is expressed at high levels. Our results indicate that the production of chimeric AML1-MTG8 protein, probably a chimeric transcription factor, may contribute to myeloid leukemogenesis.