Chromosome translocations that disrupt or alter gene function have been implicated in the pathogenesis of a variety of malignancies. Therefore, identification of a translocation breakpoint has become a more important means by which to identify genes involved in cellular transformation. A common site of translocation in myeloid and lymphoid malignancies involves 11q23. One human protooncogene, ETS1, has been localized to this chromosomal segment, and several tumors with 11q23 translocations have been shown to have altered ETS1 DNA migration after restriction enzyme digestion. Two laboratories, however, have recently localized the 11q23 breakpoint region to a small region of DNA telomeric of the CD3 loci, a region at considerable distance from the ETS1 gene locus. Therefore, it is difficult to reconcile the studies that suggest altered migration of fragments associated with ETS1 and lack of a localization of the breakpoint to a region near the ETS1 gene. Recently, in our studies to characterize the promoter/enhancer region of the ETS1 protooncogene, we had the opportunity to analyze DNA from 18 patients with acute leukemia involving chromosome 11q23 aberrations. We were unable to demonstrate rearrangement of the ETS1 gene in this group, thus confirming that the 11q23 breakpoint does not involve ETS1 protooncogene. In one patient, however, a DNA break in the region of the ETS1 promoter was detected reproducibly. This DNA break was mapped to the major DNaseI hypersensitive site in the ETS1 promoter. Mapping from both sides of the break demonstrated that the break must have occurred during processing of the leukemic cells for DNA analysis. Therefore, artifactual DNA breaks can occur at nuclease-hypersensitive sites of active genes. These data suggest that previous reports of chromosomal translocations involving the ETS1 protooncogene may have resulted from DNA breaks at nuclease hypersensitive sites. This mechanism may account for sporadic case reports of altered restriction enzyme fragment migration involving genes that are not ultimately shown to be associated with the chromosome translocation being examined.