If you've followed the latest medical research, you know that stem cells are a big deal. They let you repurpose cells so that you can theoretically grow them into whatever you need. However, scientists just got a good reason to be more cautious than they have in the past. A Harvard team has discovered that five of the 140 human embryonic stem cell lines registered for research use in US labs have cells whose mutations can cause cancer. Two of the lines have been used in human trials, too. None of those patients has developed cancer, thankfully, but there's a "very real risk" it could happen.