Two breaks make a translocation

There are multiple ways in which double-stranded breaks (DSBs) in DNA can be repaired or recombine with other DNA molecules. Under some of these conditions it is theoretically possible that a single DSB could invade a region of homology and cause a translocation. But in the 8 June Nature Richardson and Jasin find that mouse cells with a single DSB often repair the break with homologous sequences from another location, but only cells with two DSBs experience translocation events (Nature 2000, 205:697-700). Richardson and Jasin introduce DSBs by adding a rare-cutting restriction enzyme gene and allowing the enzyme to act on a site within an introduced drug-resistance gene. This system should help in studies of how to suppress translocation events.