Not Wanted

A Stanford University study suggests that, like organ transplants, human embryonic stem cells may come with a high probability of rejection. Joseph Wu and his colleagues injected hESCs into mice with compromised immune systems and into normal mice. The mice with compromised immune systems did well while the others died within 10 days. "[This result is] not a disappointment, it's more of a reality check," Wu says at sciam.com. "I think there's some promise [to hESCs], but you don't want to be foolish and say these cells are going to cure things in the next five years."