That realization led Fuchs and Mombaerts to wonder whether the skin stem cells they studied could be reprogrammed to generate other tissues. In the February 20, 2007, issue of the Proceedings of the National Academy of Sciences, the scientists reported the first successful cloning of healthy mice from adult stem cells. They achieved that goal by using the technique of nuclear transfer: replacing the nucleus of an unfertilized oocyte with the nucleus of an adult skin stem cell. After transfer to a mouse's uterus, such hybrid cells were capable of developing into healthy adult mice, showing that the adult skin stem-cell nuclei could be reprogrammed to produce all tissues in the adult mouse.

Mouse cloning from other somatic stem cells has been attempted before, but the few cloned mice that resulted were not normal and almost always died soon after birth. Fuchs' and Mombaerts' cloning experiments had success rates as high as 5.4 percent, and half of their cloned mice lived out normal life spans.

Instead of introducing a mouse hybrid embryo into a mother to produce a cloned offspring, Fuchs points out, the embryo could be grown in a culture dish with the goal of producing embryonic stem cells. If scientists are able to adapt this strategy to human skin stem cells, she says, this technology might prove to be clinically important in the future. "You might then be able to tailor-make embryonic stem cells to a particular patient and thereby avoid rejection by the immune system," she says. "Additionally, you might be able to study a patient's neurodegenerative disease by creating neurons from the embryonic stem cells generated from a skin biopsy."

Does Fuchs still do crossword puzzles as a diversion? She says they no longer interest her. "What I used to like about crossword puzzles is that you knew when you had solved the problem. In biology, you can never solve the problem. But that's what I now find fascinating. With the results of each new experiment comes the next question to address."

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