The Philadelphia (Ph') chromosome, an abnormal chromosome 22 (ref. 1), is one of the best-known examples of a specific human chromosomal abnormality strongly associated with one form of human leukaemia, chronic myelocytic leukaemia (CML). The finding that a small region of chromosome 9 which includes the c-abl oncogene is translocated to chromosome 22 prompted studies to elucidate the molecular mechanisms involved in this disease. We have demonstrated previously that the chromosome 9 of one patient with CML contains a breakpoint 14 kilobases (kb) 5' of the most 5' v-abl-homologous exon. These data suggest a role for c-abl in CML, a theory supported by the presence of an abnormally sized abl messenger RNA and protein in the CML cell line K562. The region involved in the translocation on chromosome 22 has also been identified: all Ph'-positive patients examined to date have a breakpoint within a 5.8-kb region, for which we have proposed the name 'breakpoint cluster region' (bcr). To determine whether bcr contains protein-encoding regions, probes from bcr were tested for their ability to hybridize to complementary DNA sequences. A 0.6-kb HindIII/BamHI bcr restriction enzyme fragment proved suitable for isolating several cDNA clones from a human fibroblast cDNA library. Using bcr cDNA sequences, we obtained data strongly suggesting the presence of a chimaeric bcr/abl mRNA in the leukaemic cells of Ph'-positive CML patients. The recent isolation of cDNA clones containing bcr and abl sequences confirms this finding. Because the bcr part of the chimaeric mRNA could be required to induce the transforming activity of the human c-abl oncogene, we have now initiated studies to characterize the normal 'bcr gene' and to determine the effect of a translocation within its coding domain. We demonstrate that as a result of the Ph' translocation, a variable number of bcr exons are included in the chimaeric bcr/abl mRNA. The bcr gene sequences in this mRNA could be responsible for the transition of the abl cellular proto-oncogene into an oncogene.