But scientists urge the continued funding of embryo-based methods until more is known about these new types of cells. "Until the alternative is shown to produce the same kind of extremely versatile, normal cells that we derive from previously frozen human blastocysts, it would be unfair to patients to renounce that approach," says the Harvard Stem Cell Institute's Melton.

While the new cells look and act like embryonic stem cells, it's not yet clear just how similar they are. "Most of the markers we know of in embryonic stem cells are expressed by these stem cells," says the University of Wisconsin's Yu. "But we really have no idea if there is a significant difference." Initial experiments from Yamanaka's Kyoto lab suggest that at least some differences exist: a screen of the expression of 30,000 genes showed that the pluripotent cells are similar but not identical to embryonic stem cells.

Both teams used viral DNA to introduce genes for four transcription factors--proteins that turn on other genes in the cell--into fibroblasts, a type of skin cell. (Two of these transcription factors were the same in both groups; two were different. All had previously been identified in embryonic stem cells.) Scientists theorize that when expressed in the adult cells, the transcription factors activate a genetic cascade that returns the cell and its DNA to an embryonic-like state.

When implanted into mice, the cells generated a ball of tissue containing multiple differentiated cell types, a standard test for cell pluripotency. Yamanaka's team also showed that the cells could differentiate into muscle and nerve cells, employing the same protocols used with embryonic stem cells. The findings were published online today in the journals Cell and Science.

Before these cells can be considered for human therapeutics, the researchers will need to develop an alternative way to express the transcription factors. The viruses currently used can integrate into the genome and pose potential safety concerns. It's not yet clear how difficult this will be to achieve, but Thomson says that his group and others are already working on this problem.

While much work remains to be done, Thomson says that the findings are likely to speed the pace of research by encouraging more scientists to study stem cells and by increasing funding for the field. "My personal barometer of optimism has gone up a lot," he says. "I think young investigators avoided getting into this field because of the ethical issues ... Now I believe more and more labs will move to this method."