A new study highlights the limits of gene editing in human embryos. Researchers have used CRISPR-Cas9 in an attempt to correct genetic errors in human embryos. However, embryonic cells have difficulty repairing broken DNA strands, leading to further mutations. In one study, researchers fertilized donated eggs with donated sperm using intracytoplasmic sperm injection (ICSI) to create 84 embryos. In 33 of the embryos, they used CRISPR-Cas9 to create breaks in the two strands that make up the DNA molecule. However, only nine percent of the targeted sites were repaired using the clinically useful process of homology-directed repair. Fifty-one percent of the broken DNA strands underwent non-homologous end-joining, producing mutations where the strands were reconnected. The remaining 40% of broken DNA strands were not repaired, leading to the loss or duplication of large chunks of chromosome from the site of the break to the chromosome end. Abnormalities of this kind affect the viability of embryos. The researchers point out that this raises concerns about the use of gene editing in human embryos, particularly to eliminate serious hereditary diseases. Technical improvements are needed to reduce the risks and increase the efficiency of gene editing in embryos.