A new research paper is stirring up controversy among scientists interested in using DNA editing to treat disease.

In a two-page article published in the journal Nature Methods on May 30, a group of six scientists report an alarming number of so-called “off-target mutations” in mice that underwent an experimental gene repair therapy.

CRISPR, the hot new gene-editing technique that’s taken biology by storm, is no stranger to headlines. What is unusual, however, is a scientific article so clearly describing a potentially fatal shortcoming of this promising technology.

The research community is digesting this news—with many experts suggesting flaws with the experiment, not the revolutionary technique.

Unwanted DNA Changes

The research team sought to repair a genetic mutation known to cause a form of blindness in mice. This could be accomplished, they showed, by changing just one DNA letter in the mouse genome.

They were able to successfully correct the targeted mutation in each of the two mice they treated. But they also observed an alarming number of additional DNA changes—more than 1,600 per mouse—in areas of the genome they did not intend to modify.

The authors attribute these unintended mutations to the experimental CRISPR-based gene-editing therapy they used.