The researchers then transferred genetically modified embryos into surrogate females, one of which gave birth to a set of twins. By sequencing the twins' genomic DNA, they found mutations in two of the target genes. Moreover, the CRISPR/Cas9 system did not produce mutations at genomic sites that were not targeted, suggesting that the tool will not cause undesirable effects when applied to monkeys. "With the precise genomic targeting of the CRISPR/Cas9 system, we expect that many disease models will be generated in monkeys, which will significantly advance the development of therapeutic strategies in biomedical research," Ji says.