A DNA region on chromosome 22, designated M-BCR, contains the chromosomal breakpoint of the Philadelphia (Ph) translocation in all Ph positive CML patients studied to date. M-BCR is part of a gene, BCR, oriented with its 5' end towards the centromere of chromosome 22. All of the CML DNAs analysed have a breakpoint within introns of the BCR gene. As a consequence of the Ph translocation the 3' end of the BCR gene has been translocated to chromosome 9, while the 5' part remains on the Ph chromosome. The remaining BCR sequences act as an acceptor for a chromosome 9 gene, the ABL oncogene: the ABL oncogene is fused in a head-to-tail fashion to the chromosome 22 sequences. This genomic configuration results in the transcription of a novel chimeric mRNA consisting of 5' BCR sequences and 3' ABL oncogene sequences. In K562, a cell line derived from a CML patient, and in five CML patients such chimeric BCR/ABL transcripts have been demonstrated. An abnormally sized ABL protein has been detected in the cell line K562 and in leukaemic cells from patients. This protein represents the translational product of the chimeric mRNA. The role of the BCR part of the fusion protein is unknown; it is possible that the BCR moiety could alter the structure of the ABL protein and unmask its tyrosine kinase activity. By analogy with the gag/v-abl polyprotein, the CML-specific BCR/ABL protein might have transforming activity and could play an essential role in the generation and/or maintenance of CML.