We report the sublocalization of the breakpoint in chromosome 22 in 33 patients with chronic myeloid leukemia (CML) who also had unusual marrow cytogenetics. In 23 patients, the leukemic clones were characterized by Philadelphia (Ph') chromosomes that arose through complex translocations that involved three or more chromosomes. In the remaining ten patients, there were no detectable Phi chromosomes despite molecular evidence for the presence of rearrangements in the major breakpoint cluster region (bcr) of chromosome 22 in all cases. There was no significant difference between the two groups with respect t o location of the breakpoints within the bcr. When these two groups of patients were combined, there was a significant excess of breakpoints in one segment of the bcr when compared t o the distribution of breakpoints seen in I 19 patients with simple 9 2 2 translocations. The difference in breakpoint distributions did not appear to be entirely attributable t o differences between groups in disease duration at the time of study. These data support the notion that the unusual genetic recombinations that give rise to BCWABL fusion genes in CML involve specific DNA sequences of BCR (and possibly ABL) and additional, recombinogenic sequences, at least some of which are present in loci known t o be nonrandomly involved in complex Ph I translocations.