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takyon writes:

A new study has contradicted a finding of CRISPR-Cas9 off-target activity, instead finding that "unexpected mutations" were due to two closely related mice:

In May, a study claimed that the revolutionary CRISPR gene editing technique can cause thousands of unwanted and potentially dangerous mutations. The authors called for regulators to reassess the safety of the technique. But doubts were raised about these claims from the very beginning, not least because it was a tiny study involving just three mice. Some critics have called for the paper to be withdrawn. Now a paper posted online on 5 July has proposed a simple and more plausible explanation for the controversial results. If it's right, the authors of the original study were wrong.

[...] When Stephen Tsang of Columbia University Medical Center and colleagues compared the entire genomes of two CRISPR-edited mice with a third one, they found thousands of shared mutations in the two edited mice. Tsang and co attributed to these mutations to CRISPR, and issued a widely-covered press release that suggested CRISPR is far riskier than dozens of other studies had suggested.

[...] But there is a much simpler explanation, says the latest study: the two CRISPR-edited mice just happened to be more closely related and thus shared more mutations. [...] "I agree the two mice are indeed more likely to be closely related," says geneticist Gaetan Burgio of the Australian National University, one of the many critics of the original paper. He says its publication in a prominent journal was a failure of peer review.

"Unexpected mutations after CRISPR-Cas9 editing in vivo" are most likely pre-existing sequence variants and not nuclease-induced mutations (open, DOI: 10.1101/159707) (DX)

The disputed press release from May, and the study it was based on:

Unexpected mutations after CRISPR–Cas9 editing in vivo (DOI: 10.1038/nmeth.4293) (DX)

Original Submission