Gene editing takes another step forward

SINCE its discovery in 2012 CRISPR-Cas9, a gene-editing technique, has gone from strength to strength. This tool, developed from a bacterial defence system that cuts up the DNA of invading viruses, permits genetic material to be edited easily and precisely. It has transformed research in biology, and promises to have wide applications in agriculture and medicine. But it is not ideal. One of its flaws is that its ability to replace genes works best in cells that are replicating, and thus have the...