A popular genome-editing technique has been modified so researchers can use light to control when editing occurs and in which cells.

To better control how the CRISPR–Cas9 system cuts and alters DNA sequences, Moritoshi Sato and his team at the University of Tokyo in Japan split the Cas9 DNA-cutting enzyme in two and fused each part to a protein. These proteins join together when hit with blue light, reactivating Cas9. They found that the modified enzyme could still edit the genome accurately in human cells in the lab. The frequency of edits increased with longer light exposures and no editing was seen in cells kept in the dark.

The team also shone patterns of light onto the cells and found that genome editing occurred along the same patterns. The technique could help to improve the precision of CRISPR, the authors say.

Nature Biotech. http://dx.doi.org/10.1038/nbt.3245 (2015)