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Last October, a 27-year-old man with Duchenne muscular dystrophy died after receiving a CRISPR-based treatment custom built to treat his particular genetic mutation.

Results of a detailed investigation released on Wednesday suggest that the patient, Terry Horgan, likely died of a previously undocumented adverse effect of the virus used to deliver the CRISPR machinery to his cells.

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These viruses, known as adeno-associated viruses, or AAVs, are used in several approved drugs and hundreds of clinical trials to deliver genes into patients, because they are generally benign. But at high doses they can cause side effects and, occasionally, deaths through one of several mechanisms, such as directly injuring the liver or triggering a type of immune response known as complement activation.

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