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BREAK-UPS are tough, but getting back together can be even harder.

When DNA strands get torn, the pieces can drift apart. Cells have a repair system that tries to reconnect the loose ends, but it sometimes goes wrong and DNA from different chromosomes is spliced together. These mix-ups – known as chromosome translocations – can lead to harmful mutations and are a . But the details of the process are unclear.

So at the National Cancer Institute in Bethesda, Maryland, and colleagues engineered mouse DNA to split when exposed to a yeast enzyme and tagged the break points with fluorescent proteins to make them visible. They then watched thousands of cells repair themselves over 24 hours ().

They found that an enzyme called DNA-dependent protein kinase plays an important part in correctly reuniting broken strands. Removing this enzyme made a translocation 10 times as likely. The team eventually hopes to find ways to prevent DNA repair going wrong, potentially helping to fight certain cancers.

This article appeared in print under the headline “DNA mix-ups hint at how to halt cancer”