Imagine editing a manuscript, only to find random paragraphs from other books slipping into the text. That is the risk revealed in a new study of the celebrated genome-editing tool called CRISPR-Cas9: various unintended snippets of DNA sometimes get inserted in the edited region where they do not belong.

KAUST scientists uncovered this side effect in a detailed analysis of human embryonic stem cells edited with different types of donor DNA, whereby helper sequences provide a template for fixing the genome during the CRISPR process. They found that the genome editing platform, while efficient in cutting DNA, can sometimes leave behind large fragments of genetic material that were not part of the plan.

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