Editing cells’ genomes with CRISPR-Cas9 might increase the risk that the altered cells, intended to treat disease, will trigger cancer, two studies published on Monday warn — a potential game-changer for the companies developing CRISPR-based therapies.

In the studies, published in Nature Medicine, scientists found that cells whose genomes are successfully edited by CRISPR-Cas9 have the potential to seed tumors inside a patient. That could make some CRISPR’d cells ticking time bombs, according to researchers from Sweden’s Karolinska Institute and, separately, Novartis.

CRISPR has already dodged two potentially fatal bullets — a 2017 claim that it causes sky-high numbers of off-target effects was retracted in March, and a report of human immunity to Cas9 was largely shrugged off as solvable. But experts are taking the cancer-risk finding seriously.

The CEO of CRISPR Therapeutics, Sam Kulkarni, told STAT the results are “plausible.” Although they likely apply to only one of the ways that CRISPR edits genomes (replacing disease-causing DNA with healthy versions) and not the other (just excising DNA), he said, “it’s something we need to pay attention to, especially as CRISPR expands to more diseases. We need to do the work and make sure edited cells returned to patients don’t become cancerous.”