"There's a real danger here: if you make a gamete that's not right, rather than helping someone, you harm them," says Peter Donovan, codirector of the Sue and Bill Gross Stem Cell Research Center at the University of California, Irvine. "This points out that we may need to be even more careful about this."

Clark believes that the quality of iPS cell gametes has much to do with how the original cell line was formed in the first place: two years ago researchers used a method that employs a virus to induce the genetic changes needed to reprogram an adult cell. That virus also inserts itself into the cell's DNA and can cause cancer. However, newer techniques have created iPS cells without integrating viral DNA. "We'd like to be able to use these more modern, contemporary iPS cells to see if the molecular integrity of the germ cells we derive from them is improved," Clark says.

"It's very interesting," says Renee Reijo Pera, the director of the Stanford University Center for Human Embryonic Stem Cell Research and Education. "I think it's going to be a marvelous tool for human genetics and for some potential treatments." But she cautions that it's a very early step. Scientists have been working with embryonic stem cells for far longer, and are still only able to coax them to develop into germ precursor cells. "The big step is still to come, and that's getting a mature egg or sperm cell," she says. "That's turned out to be a roadblock so far."

Even with all the caveats, Donovan and Reijo Pera agree that the study represents a big step forward. Studying iPS-induced germ cells in vitro could allow researchers to understand the intricate process through which gametes are formed. "That itself could have a big impact on understanding underlying mechanisms of infertility," Donovan says. And that, in turn, could help elucidate the effects of toxins on a developing embryo. "If we understand how the embryonic and fetal periods are affected by toxic environments, we can understand how to protect the germ line during development," he says.