This doesn't mean that the medical community is about to hit the brakes on stem cell research. There's still some review necessary to decide what happens next. And there are ways to make sure cells are healthy before they're used. However, this raises the possibility that there are other, less common mutations that haven't been caught. And these stem cell lines have been in use for nearly 20 years -- that's a lot of time for risks to go unchecked.

If the discovery holds up, researchers may have little choice but to look for mutations through DNA sequencing, which is expensive at about $1,000 for every genome. That screening could soon be government-mandated, in fact. Still, it might be necessary to make sure that stem cell treatments aren't just substituting one disease for another.