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The bacterial clustered, regularly interspaced, short palindromic repeats (CRISPR)/Cas9 system can be engineered to achieve site-specific DNA cleavage in several experimental systems. Wang et al. now demonstrate its use for multiplex gene modification in the mouse, performing the simultaneous mutagenesis of eight alleles of five genes (two of the genes being on the Y chromosome) in mouse embryonic stem cells. They further showed that the CRISPR/Cas9 system can be used via direct injection into mouse zygotes to rapidly generate animals with biallelic mutations in two genes. Finally, by supplying donor sequence with a single-stranded oligonucleotide, they demonstrated the precise biallelic insertion of short sequences in two genes via homologous recombination. The CRISPR/Cas9 system should greatly improve the efficiency of genetic engineering in the mouse.