In a blow to the gene-editing super-tool CRISPR, scientists have found that it causes big, dangerous-looking changes to DNA in mouse and human cells.

What happened: A team at the Wellcome Sanger Center in the UK described how CRISPR can lead to massive genetic typos—big chunks of DNA getting deleted, reversed, or moved entirely.

Is that bad? Yes. The team says if CRISPR is used in a gene therapy to edit billions of cells, bad results are “likely.” “The multitude of different mutations generated makes it likely that one or more edited cells in each protocol would be endowed with an important pathogenic lesion,” according to geneticist Allan Bradley, who headed the study, published in the journal Nature Biotechnology.

The risk: The danger is that unplanned DNA alterations could start turning cells cancerous. That could throw a wrench into plans to use CRISPR to treat inherited illnesses like hemophilia—and it makes “designer babies” seem less likely than ever.

Still in the toolbox: Startups working on CRISPR cures dismissed the findings as irrelevant. A spokesperson for Editas Medicine, a startup developing a CRISPR treatment for a rare eye disease, told Stat that the problems are not “specifically problematic in our work to make CRISPR-based medicines.” Tom Barnes, a senior vice president at Intellia Therapeutics, told Genetic Engineering News that the new paper is “a little bit alarmist” and said his company had “been thinking about this topic all along.”