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.