"The advantage of a site-specific recombinase is that the enzyme does everything," says Stark, who is also investigating the potential of ZFRs in genetic engineering.

When the intended gene, which would have been inserted at the same time, comes along, the recombinase recognises it, and binds the DNA to the ends, repairing the break (see diagram).

As proof of principle, Barbas's team has taken human cells and inserted a gene that their ZFR would recognise. They then used the recombinase to insert the gene into the cells' genome.