Chromosomal rearrangements occur in S. cerevisiae rfa1 mutator mutants due to mutagenic lesions processed by double-strand-break repair

Suppression of spontaneous chromosomal rearrangements by S phase checkpoint functions in Saccharomyces cerevisiae

Cancer cells show increased genome rearrangements, although it is unclear what defects cause these rearrangements. Mutations in Saccharomyces cerevisiae RFC5, DPB11, MEC1, DDC2 MEC3, RAD53, CHK1, PDS1, and DUN1 increased the rate of genome rearrangements up to 200-fold whereas mutations in RAD9, RAD17, RAD24, BUB3, and MAD3 had little effect. The rearrangements were primarily deletion of a portion of a chromosome arm along with TEL1-dependent addition of a new telomere. tel1 mutations increased the proportion of translocations observed, and in some cases showed synergistic interactions when combined with mutations that increased the genome rearrangement rate. These data suggest that one role of S phase checkpoint functions in normal cells is to suppress spontaneous genome rearrangements resulting from DNA replication errors.

Impairment of lagging strand synthesis triggers the formation of a RuvABC substrate at replication forks

The holD gene codes for the psi subunit of the Escherichia coli DNA polymerase III holoenzyme, a component of the gamma complex clamp loader. A holD mutant was isolated for the first time in a screen for mutations that increase the frequency of tandem repeat deletions. In contrast to tandem repeat deletions in wild-type strains, deletion events stimulated by the holD mutation require RecA. They do not require RecF, and hence do not result from the recombinational repair of gaps, arguing against uncoupling of the leading and lagging strand polymerases in the holD mutant. The holD recBC combination of mutations is lethal and holD recBts recCts strains suffer DNA double-strand breaks (DSBs) at restrictive temperature. DSBs require the presence of the Holliday junction-specific enzymes RuvABC and are prevented in the presence of RecBCD. We propose that impairment of replication due to the holD mutation causes the arrest of the entire replisome; consequently, Holliday junctions are formed by replication fork reversal, and unequal crossing over during RecA- and RecBCD-mediated re-incorporation of reversed forks causes the hyper-recombination phenotype.

SGS1, the Saccharomyces cerevisiae homologue of BLM and WRN, suppresses genome instability and homeologous recombination

The Escherichia coli gene recQ was identified as a RecF recombination pathway gene. The gene SGS1, encoding the only RecQ-like DNA helicase in Saccharomyces cerevisiae, was identified by mutations that suppress the top3 slow-growth phenotype. Relatively little is known about the function of Sgs1p because single mutations in SGS1 do not generally cause strong phenotypes. Mutations in genes encoding RecQ-like DNA helicases such as the Bloom and Werner syndrome genes, BLM and WRN, have been suggested to cause increased genome instability. But the exact DNA metabolic defect that might underlie such genome instability has remained unclear. To better understand the cellular role of the RecQ-like DNA helicases, sgs1 mutations were analyzed for their effect on genome rearrangements. Mutations in SGS1 increased the rate of accumulating gross chromosomal rearrangements (GCRs), including translocations and deletions containing extended regions of imperfect homology at their breakpoints. sgs1 mutations also increased the rate of recombination between DNA sequences that had 91% sequence homology. Epistasis analysis showed that Sgs1p is redundant with DNA mismatch repair (MMR) for suppressing GCRs and for suppressing recombination between divergent DNA sequences. This suggests that defects in the suppression of rearrangements involving divergent, repeated sequences may underlie the genome instability seen in BLM and WRN patients and in cancer cases associated with defects in these genes.

Identification of a 428-kb homozygously deleted region disrupting the SEZ6L gene at 22q12.1 in a lung cancer cell line

Frequent allelic losses on chromosome 22q in small cell lung carcinomas (SCLCs) and advanced non-small cell lung carcinomas indicate the presence of tumor suppressor gene(s) on this chromosome arm. We detected a homozygous deletion at 22q12.1 in a SCLC cell line, Lu24. Cloning of the breakpoints of the Lu24 deletion revealed that the deletion was interstitial and 428, 131 bp in size. The deleted region contained the SEZ6L (Seizure 6-like) gene, whose structure had been partially determined by the chromosome 22 sequencing project. We determined the full length cDNA sequence for the SEZ6L gene based on the genomic sequence for the SEZ6L locus using the GENSCAN program and the RT - PCR method. The deduced SEZ6L protein was a transmembrane protein of 1024 amino acids with multiple domains involved in protein - protein interaction and signal transduction. SEZ6L expression was detected in a variety of human tissues, including lung, while its expression was detected in 14 (30%) of 46 lung cancer cell lines examined. Missense mutations were detected in three (7%) of the 46 cell lines, and a 1 bp deletion in the polypyrimidine tract preceding exon 4 was detected in one (2%) of 46 primary lung cancers. Therefore, it is possible that genetic and/or epigenetic SEZ6L alterations are involved in the development and/or progression in a subset of lung cancer, although functional analysis of the SEZ6L gene as well as molecular analysis of other genes in the homozygously deleted region is necessary to understand the pathogenetic significance of 22q deletions in human lung carcinogenesis.

Spontaneous loss of heterozygosity in diploid Saccharomyces cerevisiae cells

To obtain a broad perspective of the events leading to spontaneous loss of heterozygosity (LOH), we have characterized the genetic alterations that functionally inactivated the URA3 marker hemizygously or heterozygously situated either on chromosome III or chromosome V in diploid Saccharomyces cerevisiae cells. Analysis of chromosome structure in a large number of LOH clones by pulsed-field gel electrophoresis and PCR showed that chromosome loss, allelic recombination, and chromosome aberration were the major classes of genetic alterations leading to LOH. The frequencies of chromosome loss and chromosome aberration were significantly affected when the marker was located in different chromosomes, suggesting that chromosome-specific elements may affect the processes that led to these alterations. Aberrant-sized chromosomes were detected readily in approximately 8% of LOH events when the URA3 marker was placed in chromosome III. Molecular mechanisms underlying the chromosome aberrations were further investigated by studying the fate of two other genetic markers on chromosome III. Chromosome aberration caused by intrachromosomal rearrangements was predominantly due to a deletion between the MAT and HMR loci that occurred at a frequency of 3.1 x 10(-6). Another type of chromosome aberration, which occurred at a frequency slightly higher than that of the intrachromosomal deletion, appeared to be caused by interchromosomal rearrangement, including unequal crossing over between homologous chromatids and translocation with another chromosome.

Involvement of very short DNA tandem repeats and the influence of the RAD52 gene on the occurrence of deletions in Saccharomyces cerevisiae

Chromosomal rearrangements, such as deletions, duplications, or Ty transposition, are rare events. We devised a method to select for such events as Ura(+) revertants of a particular ura2 mutant. Among 133 Ura(+) revertants, 14 were identified as the result of a deletion in URA2. Of seven classes of deletions, six had very short regions of identity at their junctions (from 7 to 13 bp long). This strongly suggests a nonhomologous recombination mechanism for the formation of these deletions. The total Ura(+) reversion rate was increased 4.2-fold in a rad52Delta strain compared to the wild type, and the deletion rate was significantly increased. All the deletions selected in the rad52Delta context had microhomologies at their junctions. We propose two mechanisms to explain the occurrence of these deletions and discuss the role of microhomology stretches in the formation of fusion proteins.

Increased rates of genomic deletions generated by mutations in the yeast gene encoding DNA polymerase delta or by decreases in the cellular levels of DNA polymerase delta

In Saccharomyces cerevisiae, POL3 encodes the catalytic subunit of DNA polymerase delta. While yeast POL3 mutant strains that lack the proofreading exonuclease activity of the polymerase have a strong mutator phenotype, little is known regarding the role of other Pol3p domains in mutation avoidance. We identified a number of pol3 mutations in regions outside of the exonuclease domain that have a mutator phenotype, substantially elevating the frequency of deletions. These deletions appear to reflect an increased frequency of DNA polymerase slippage. In addition, we demonstrate that reduction in the level of wild-type DNA polymerase results in a similar mutator phenotype. Lowered levels of DNA polymerase also result in increased sensitivity to the DNA-damaging agent methyl methane sulfonate. We conclude that both the quantity and the quality of DNA polymerase delta is important in ensuring genome stability.

Checkpoint-dependent activation of mutagenic repair in Saccharomyces cerevisiae pol3-01 mutants

The Saccharomyces cerevisiae DNA polymerase delta proofreading exonuclease-defective mutation pol3-01 is known to cause high rates of accumulating mutations. The pol3-01 mutant was found to have abnormal cell cycle progression due to activation of the S phase checkpoint. Inactivation of the S phase checkpoint suppressed both the pol3-01 cell cycle progression defect and mutator phenotype, indicating that the pol3-01 mutator phenotype was dependent on the S phase damage checkpoint pathway. Epistasis analysis suggested that a portion of the pol3-01 mutator phenotype involves members of the RAD6 epistasis group that function in both error-free and error-prone repair. These results indicate that activation of a checkpoint in response to certain types of replicative defects can result in the accumulation of mutations.

BCR-ABL directed immunotherapy: a virtual reality?

Nearly ten years of research on the feasibility of specific immunotherapy targeting the junctional regions of BCR-ABL has considerably increased our knowledge of which MHC alleles might present BCR-ABL peptides, yet has failed to provide us with definite proof of appropriate processing of the hybrid oncoprotein into such antigenic peptides. This paper intends to provide an overview of the current state of affairs as well as to delineate limitations and future directions of this line of research.

Gross chromosomal rearrangements and genetic exchange between nonhomologous chromosomes following BRCA2 inactivation

Cancer-causing mutations often arise from gross chromosomal rearrangements (GCRs) such as translocations, which involve genetic exchange between nonhomologous chromosomes. Here we show that murineBrca2 has an essential function in suppressing GCR formation after chromosome breakage. Cells that harbor truncated Brca2spontaneously incur GCRs and genomic DNA breaks during division. They exhibit hypersensitivity to DNA damage by interstrand cross-linkers, which even at low doses trigger aberrant genetic exchange between nonhomologous chromosomes. Therefore, genetic instability in Brca2-deficient cells results from the mutagenic processing of spontaneous or induced DNA damage into gross chromosomal rearrangements, providing a mechanistic basis for cancer predisposition.

Links between replication, recombination and genome instability in eukaryotes

Double-strand breaks in DNA can be repaired by homologous recombination including break-induced replication. In this reaction, the end of a broken DNA invades an intact chromosome and primes DNA replication resulting in the synthesis of an intact chromosome. Break-induced replication has also been suggested to cause different types of genome rearrangements.

Replication and recombination intersect

A bacterial housekeeping function, which requires both recombination and replication enzymes, has been identified that re-establishes inactivated replication forks under normal growth conditions. Some long-tract gene-conversion events initiated by double-strand breaks in yeast and mammalian cells can be attributed to recombination-directed DNA replication. Double-strand break repair in yeast has been shown to require both leading- and lagging-strand DNA synthesis. These observations suggest that the recombination and replication machinery cooperate to maintain genomic integrity.

DNA repair protein Ku80 suppresses chromosomal aberrations and malignant transformation

Cancer susceptibility genes have been classified into two groups: gatekeepers and caretakers. Gatekeepers are genes that control cell proliferation and death, whereas caretakers are DNA repair genes whose inactivation leads to genetic instability. Abrogation of both caretaker and gatekeeper function markedly increases cancer susceptibility. Although the importance of Ku80 in DNA double-strand break repair is well established, neither Ku80 nor other components of the non-homologous end-joining pathway are known to have a caretaker role in maintaining genomic stability. Here we show that mouse cells deficient for Ku80 display a marked increase in chromosomal aberrations, including breakage, translocations and aneuploidy. Despite the observed chromosome instabilities, Ku80-/- mice have only a slightly earlier onset of cancer. Loss of p53 synergizes with Ku80 to promote tumorigenesis such that all Ku80-/- p53-/- mice succumb to disseminated pro-B-cell lymphoma before three months of age. Tumours result from a specific set of chromosomal translocations and gene amplifications involving IgH and c-Myc, reminiscent of Burkitt's lymphoma. We conclude that Ku80 is a caretaker gene that maintains the integrity of the genome by a mechanism involving the suppression of chromosomal rearrangements.

The importance of repairing stalled replication forks

The bacterial SOS response to unusual levels of DNA damage has been recognized and studied for several decades. Pathways for re-establishing inactivated replication forks under normal growth conditions have received far less attention. In bacteria growing aerobically in the absence of SOS-inducing conditions, many replication forks encounter DNA damage, leading to inactivation. The pathways for fork reactivation involve the homologous recombination systems, are nonmutagenic, and integrate almost every aspect of DNA metabolism. On a frequency-of-use basis, these pathways represent the main function of bacterial DNA recombination systems, as well as the main function of a number of other enzymatic systems that are associated with replication and site-specific recombination.

The breast cancer susceptibility gene, BRCA2: at the crossroads between DNA replication and recombination?

The identification and cloning of the familial breast cancer susceptibility gene, BRCA2, has excited much interest in its biological functions. Here, evidence is reviewed that the protein encoded by BRCA2 has an essential role in DNA repair through its association with mRad51, a mammalian homologue of bacterial and yeast proteins involved in homologous recombination. A model is proposed that the critical requirement for BRCA2 in cell division and the maintenance of chromosome stability stems from its participation in recombinational processes essential for DNA replication.

Caretaker Brca1: keeping the genome in the straight and narrow

Inheritance of germline BRCA1 mutations is associated with a high risk of breast and ovarian cancers. A multitude of cellular functions has been ascribed to BRCA1, including transcription activation and various aspects of DNA repair. So far, indirect evidence has indicated a role for BRCA1 in the repair of double-strand breaks. Recently, an elegant gene targeting design was used to provide definitive evidence that BRCA1 promotes homologous recombination and limits nonhomologous mutagenic repair processes. This reaffirms the role of BRCA1 as caretaker in preserving genomic integrity.

The onset and extent of genomic instability in sporadic colorectal tumor progression

Cancer cell genomes contain alterations beyond known etiologic events, but their total number has been unknown at even the order of magnitude level. By sampling colorectal premalignant polyp and carcinoma cell genomes through use of the technique inter-(simple sequence repeat) PCR, we have found genomic alterations to be considerably more abundant than expected, with the mean number of genomic events per carcinoma cell totaling approximately 11,000. Colonic polyps early in the tumor progression pathway showed similar numbers of events. These results indicate that, as with certain hereditary cancer syndromes, genomic destabilization is an early step in sporadic tumor development. Together these results support the model of genomic instability being a cause rather than an effect of malignancy, facilitating vastly accelerated somatic cell evolution, with the observed orderly steps of the colon cancer progression pathway reflecting the consequences of natural selection.

Saccharomyces cerevisiae pol30 (proliferating cell nuclear antigen) mutations impair replication fidelity and mismatch repair

To understand the role of POL30 in mutation suppression, 11 Saccharomyces cerevisiae pol30 mutator mutants were characterized. These mutants were grouped based on their mutagenic defects. Many pol30 mutants harbor multiple mutagenic defects and were placed in more than one group. Group A mutations (pol30-52, -104, -108, and -126) caused defects in mismatch repair (MMR). These mutants exhibited mutation rates and spectra reminiscent of MMR-defective mutants and were defective in an in vivo MMR assay. The mutation rates of group A mutants were enhanced by a msh2 or a msh6 mutation, indicating that MMR deficiency is not the only mutagenic defect present. Group B mutants (pol30-45, -103, -105, -126, and -114) exhibited increased accumulation of either deletions alone or a combination of deletions and duplications (4 to 60 bp). All deletion and duplication breakpoints were flanked by 3 to 7 bp of imperfect direct repeats. Genetic analysis of one representative group B mutant, pol30-126, suggested polymerase slippage as the likely mutagenic mechanism. Group C mutants (pol30-100, -103, -105, -108, and -114) accumulated base substitutions and exhibited synergistic increases in mutation rate when combined with msh6 mutations, suggesting increased DNA polymerase misincorporation as a mutagenic defect. The synthetic lethality between a group A mutant, pol30-104, and rad52 was almost completely suppressed by the inactivation of MSH2. Moreover, pol30-104 caused a hyperrecombination phenotype that was partially suppressed by a msh2 mutation. These results suggest that pol30-104 strains accumulate DNA breaks in a MSH2-dependent manner.

RAD51 as a candidate parathyroid tumour suppressor gene on chromosome 15q: absence of somatic mutations

OBJECTIVE

Loss of heterozygosity (LOH) at chromosome 15q is frequent in parathyroid adenomas, but no tumour suppressor gene of importance to parathyroid tumour development has been isolated from this region. The RAD51 gene has been localized to chromosome 15q and possesses regulatory functions involving DNA stability and cell proliferation, suggesting its possible role in tumorigenesis. Additionally, mutations in the RAD51 gene cause reduced resistance to ionizing radiation, which is a major risk factor for primary hyperparathyroidism. RAD51 was therefore analysed as a candidate tumour suppressor gene in a group of parathyroid adenomas for which mutations in a 15q tumour suppressor should be most readily detectable.

PATIENTS AND DESIGN

From a total of 55 parathyroid adenomas, nine were selected based on their LOH pattern showing DNA loss at chromosome 15q in the vicinity of the RAD51 gene. RAD51 mRNA expression was investigated by reverse transcription-polymerase chain reaction (RT-PCR), and sequence analysis of the entire coding region of the RAD51 cDNA was performed in all nine adenomas.

RESULTS

RAD51 mRNA expression was substantiated in all parathyroid adenomas. Compared with the normal RAD51 cDNA sequence, no point mutations or microdeletions could be found in the parathyroid tumor cDNA.

CONCLUSION

These observations suggest that somatic inactivating mutations of the RAD51 gene are uncommonly, if ever, associated with parathyroid tumourigenesis.

Analysis of the RAD54 gene on chromosome 1p as a potential tumor-suppressor gene in parathyroid adenomas

Parathyroid adenomas causing primary hyperparathyroidism (pHPT) frequently exhibit allelic loss of DNA markers on the short arm of chromosome 1, indicating the presence of one or more tumor-suppressor genes on 1p. Since the development of pHPT is enhanced in individuals exposed to ionizing radiation to the neck, it could be anticipated that genes involved in DNA repair and recombination may be special targets for mutation in parathyroid tumorigenesis, whether irradiation-associated or not. RAD54 is a member of a family of genes involved in such functions, and RAD54 knockout mice show increased sensitivity to ionizing radiation. The localization of the RAD54 gene to 1p32 has therefore elevated it to a most compelling candidate parathyroid tumor-suppressor gene. Twelve parathyroid adenomas demonstrating allelic loss at chromosome 1p were selected from 55 parathyroid adenomas previously analyzed for loss of heterozygosity using polymorphic microsatellite markers. All 18 exons of the RAD54 gene were fully analyzed by automated sequencing for detection of point mutations or micro-deletions in each parathyroid adenoma. No mutational aberrations were detected in the RAD54 gene, strongly suggesting that complete somatic inactivation of RAD54 is infrequently, if ever, associated with the development of parathyroid adenomas. Whether genes controlling DNA repair and recombination are involved in parathyroid neoplasia remains to be determined.

Gross chromosomal rearrangements in Saccharomyces cerevisiae replication and recombination defective mutants

Cancer progression is often associated with the accumulation of gross chromosomal rearrangements (GCRs), such as translocations, deletion of a chromosome arm, interstitial deletions or inversions. In many instances, GCRs inactivate tumour-suppressor genes or generate novel fusion proteins that initiate carcinogenesis. The mechanism underlying GCR formation appears to involve interactions between DNA sequences of little or no homology. We previously demonstrated that mutations in the gene encoding the largest subunit of the Saccharomyces cerevisiae single-stranded DNA binding protein (RFA1) increase microhomology-mediated GCR formation. To further our understanding of GCR formation, we have developed a novel mutator assay in S. cerevisiae that allows specific detection of such events. In this assay, the rate of GCR formation was increased 600-5, 000-fold by mutations in RFA1, RAD27, MRE11, XRS2 and RAD50, but was minimally affected by mutations in RAD51, RAD54, RAD57, YKU70, YKU80, LIG4 and POL30. Genetic analysis of these mutants suggested that at least three distinct pathways can suppress GCRs: two that suppress microhomology-mediated GCRs (RFA1 and RAD27) and one that suppresses non-homology-mediated GCRs (RAD50/MRE11/XRS2).

Interstrand cross-links induce DNA synthesis in damaged and undamaged plasmids in mammalian cell extracts

Mammalian cell extracts have been shown to carry out damage-specific DNA repair synthesis induced by a variety of lesions, including those created by UV and cisplatin. Here, we show that a single psoralen interstrand cross-link induces DNA synthesis in both the damaged plasmid and a second homologous unmodified plasmid coincubated in the extract. The presence of the second plasmid strongly stimulates repair synthesis in the cross-linked plasmid. Heterologous DNAs also stimulate repair synthesis to variable extents. Psoralen monoadducts and double-strand breaks do not induce repair synthesis in the unmodified plasmid, indicating that such incorporation is specific to interstrand cross-links. This induced repair synthesis is consistent with previous evidence indicating a recombinational mode of repair for interstrand cross-links. DNA synthesis is compromised in extracts from mutants (deficient in ERCC1, XPF, XRCC2, and XRCC3) which are all sensitive to DNA cross-linking agents but is normal in extracts from mutants (XP-A, XP-C, and XP-G) which are much less sensitive. Extracts from Fanconi anemia cells exhibit an intermediate to wild-type level of activity dependent upon the complementation group. The DNA synthesis deficit in ERCC1- and XPF-deficient extracts is restored by addition of purified ERCC1-XPF heterodimer. This system provides a biochemical assay for investigating mechanisms of interstrand cross-link repair and should also facilitate the identification and functional characterization of cellular proteins involved in repair of these lesions.

Mutator phenotypes conferred by MLH1 overexpression and by heterozygosity for mlh1 mutations

Loss of DNA mismatch repair due to mutation or diminished expression of the MLH1 gene is associated with genome instability and cancer. In this study, we used a yeast model system to examine three circumstances relevant to modulation of MLH1 function. First, overexpression of wild-type MLH1 was found to cause a strong elevation of mutation rates at three different loci, similar to the mutator effect of MLH1 gene inactivation. Second, haploid yeast strains with any of six mlh1 missense mutations that mimic germ line mutations found in human cancer patients displayed a strong mutator phenotype consistent with loss of mismatch repair function. Five of these mutations affect amino acids that are homologous to residues suggested by recent crystal structure and biochemical analysis of Escherichia coli MutL to participate in ATP binding and hydrolysis. Finally, using a highly sensitive reporter gene, we detected a mutator phenotype of diploid yeast strains that are heterozygous for mlh1 mutations. Evidence suggesting that this mutator effect results not from reduced mismatch repair in the MLH1/mlh1 cells but rather from loss of the wild-type MLH1 allele in a fraction of cells is presented. Exposure to bleomycin or to UV irradiation strongly enhanced mutagenesis in the heterozygous strain but had little effect on the mutation rate in the wild-type strain. This damage-induced hypermutability may be relevant to cancer in humans with germ line mutations in only one MLH1 allele.

Mutator phenotype induced by aberrant replication

We have identified thermosensitive mutants of five Schizosaccharomyces pombe replication proteins that have a mutator phenotype at their semipermissive temperatures. Allele-specific mutants of DNA polymerase delta (poldelta) and mutants of Polalpha, two Poldelta subunits, and ligase exhibited increased rates of deletion of sequences flanked by short direct repeats. Deletion of rad2(+), which encodes a nuclease involved in processing Okazaki fragments, caused an increased rate of duplication of sequences flanked by short direct repeats. The deletion mutation rates of all the thermosensitive replication mutators decreased in a rad2Delta background, suggesting that deletion formation requires Rad2 function. The duplication mutation rate of rad2Delta was also reduced in a thermosensitive polymerase background, but not in a ligase mutator background, which suggests that formation of duplication mutations requires normal DNA polymerization. Thus, although the deletion and duplication mutator phenotypes are distinct, their mutational mechanisms are interdependent. The deletion and duplication replication mutators all exhibited decreased viability in combination with deletion of a checkpoint Rad protein, Rad26. Interestingly, deletion of Cds1, a protein kinase functioning in a checkpoint Rad-mediated reversible S-phase arrest pathway, decreased the viability and exacerbated the mutation rate only in the thermosensitive deletion replication mutators but had no effect on rad2Delta. These findings suggest that aberrant replication caused by allele-specific mutations of these replication proteins can accumulate potentially mutagenic DNA structures. The checkpoint Rad-mediated pathways monitor and signal the aberrant replication in both the deletion and duplication mutators, while Cds1 mediates recovery from aberrant replication and prevents formation of deletion mutations specifically in the thermosensitive deletion replication mutators.

Double-strand break repair by interchromosomal recombination: suppression of chromosomal translocations

To directly determine whether recombinational repair of double-strand breaks (DSBs) can occur between heterologous chromosomes and lead to chromosomal rearrangements in mammalian cells, we employed an ES cell system to analyze recombination between repeats on heterologous chromosomes. We found that recombination is induced at least 1000-fold following the introduction of a DSB in one repeat. Most (98%) recombinants repaired the DSB by gene conversion in which a small amount of sequence information was transferred from the unbroken chromosome onto the broken chromosome. The remaining recombinants transferred a larger amount of information, but still no chromosomal aberrations were apparent. Thus, mammalian cells are capable of searching genome-wide for sequences that are suitable for DSB repair. The lack of crossover events that would have led to translocations supports a model in which recombination is coupled to replication.