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A recent study has identified another potential hazard for developers of genome editing therapies based on CRISPR–Cas9. The double-strand DNA breaks introduced during CRISPR editing could result in chromothripsis, an extremely damaging form of genomic rearrangement that results from the shattering of individual chromosomes and the subsequent rejoining of the pieces in a haphazard order. Although most cells do not remain viable after undergoing such a dramatic alteration, those that do could, in theory, express oncogenic fusion proteins or give rise to dysregulated expression of particular genes that could cause problems.