Introducing a number of mutations to mice is usually tedious and time-consuming. Researchers introduce a genetic change into embryonic stem cells, mix them into embryos and transplant these into surrogate mothers. Resulting pups are then interbred for generations.

A group led by Rudolf Jaenisch at the Whitehead Institute for Biomedical Research in Cambridge, Massachusetts, has succeeded in making several genetic changes at once by using a bacterial system called CRISPR/Cas that normally serves to destroy viruses. The technique allowed the authors to target both copies of a number of genes in embryonic stem cells and to manipulate genes in fertilized eggs. The method could allow researchers to create mice with a host of mutations in months rather than years, the authors say.