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The CRISPR/Cas9 gene editing approach might not be as precise as previously thought, according to Stanford University's Vinit Mahajan and his colleagues. Mahajan, along with Columbia University's Stephen Tsang, previously used the editing approach to fix the mutation behind retinitis pigmentosa in mice.

In a letter to Nature Methods, the researchers now report that they sequenced the whole genomes of two mice that had undergone CRISPR/Cas9 editing to 50X coverage and an unedited control mouse to 30X coverage. In the edited mice, they found that the targeted alleles were repaired, but they also found other indels and SNPs in the edited mice that weren't present in the control mouse and hadn't been predicted by their algorithm as possible off-target sites.

As Gizmodo notes, there was nothing obviously wrong with the mice, but the researchers urge further refinement of the approach. "More work may be needed to increase the fidelity of CRISPR/Cas9 with regard to off-target mutation generation before the CRISPR platform can be used without risk, especially in the clinical setting," the researchers write. 

Their finding, though, led the stocks of CRISPR-based companies to fall, according to the Motley Fool. It reports that shares of Editas Medicine, CRISPR Therapeutics, and Intellia Therapeutics fell as much as 16 percent, 6.9 percent, and 14.9 percent, respectively, yesterday.

However, Tsang tells New Scientist that he and his colleagues are optimistic about the CRISPR gene-editing approach — they just want other researchers to avoid unnecessary mutations. "This is going to clinical trials, and is being used in crops," Tsang says. "Perhaps the USDA and FDA should require our method prior to approval of CRISPR guides in humans and food?"