Although the BCR-ABL hybrid gene on chromosome 22q-plays a pivotal role in the pathogenesis of chronic myeloid leukemia (CML), little is known of the reciprocal chimeric gene, ABL-BCR, formed on chromosome 9q+. By reverse transcription/polymerase chain reaction amplification (RT/PCR) we have detected ABL-BCR mRNA in cells from 31 of 44 BCR-ABL positive CML patients and 3 of 5 CML cell lines. Of the 34 positive samples, 31 had classical t(9;22) (q34;q11) translocations; in 3 samples there was no Philadelphia (Ph) and/or 9q+ chromosomes. ABL-BCR expression consisted of ABL(Ib)-BCR mRNA in 26 patients and of both ABL(Ib)-BCR and ABL(Ia)-BCR mRNA species in 6 patients. The ABL-BCR transcripts encoded one or, more rarely, both of the two potential junctions, designated ABL-b3 and ABL-b4, which differed in size by 75 bp. In 2 patients, the BCR exon b3 was not present in either the BCR-ABL or the corresponding ABL-BCR transcript, whereas in 5 patients exon b3 was present in both transcripts. Direct sequencing of PCR fragments representing the full-length coding sequence of ABL-BCR cDNAs type Ib- b3, Ia-b3, Ib-b4, and Ia-b4 showed an open reading frame predicted to encode fusion proteins of 370 to 414 amino-acids. If an ABL-BCR gene product is produced in CML cells, it may be relevant as a mechanism for deregulating the GTPase activating protein (GAP) function of BCR.