In the t(8;21)(q22;q22) of acute myelogenous leukemia (AML), the breakpoint on chromosome 21 disrupts the AML1 gene, generally in the intron between exons 5 and 6. To isolate fusion transcripts of AML1, and an as yet unidentified gene on chromosome 8 involved in the rearrangement, we used rapid amplification of cDNA ends (RACE) and primers for AML1 exons 5 and 6. A fusion transcript was identified by 3' RACE in the RNA of t(8;21) leukemic cells that also express multiple normal AML1 transcripts. This result clearly indicates that at least one transcriptionally active chimeric gene is generated by the chromosome translocation. This gene on the 8q- derivative represents the fusion between the 5' portion of the AML1 gene with the 3' portion of a chromosome 8 gene that contains a region of sequence homology with the cyclin D2 gene, here referred to as the CDR gene (cyclin D-related gene). The chimeric gene is probably responsible for the pathogenesis of the 8;21 AML. This finding makes it possible to detect the translocation at the molecular level, thus improving the diagnosis and monitoring of the disease in leukemic patients.