Advance May End Stem Cell Debate

At the same time, Thomson, Junying Yu and colleagues were racing ahead. Working from an initial list of 14 genes that seemed to make human cells embryonic, they gradually narrowed their recipe to just four genes, too.

"It took us forever to get to the finish line," Thomson said. A lot of that time was spent checking for the emergence of slow-growing, embryo-cell-like colonies in dishes, so "there was no eureka moment. It was a drawn-out thing."

His cells passed the same tests as Yamanaka's, though in his final recipe, two of the genes he used were different.

"Apparently there are various ways to get to Rome," said Rudolf Jaenisch, a stem cell researcher at the Whitehead Institute for Biomedical Research in Cambridge, Mass. "We don't have to do it like the egg. We can do it differently."

Some of the hurdles to medical applications have already begun to be overcome. One of the genes that Yamanaka used, called c-myc, can initiate cancers, for example. But Thomson's recipe does not include c-myc, and in recent studies Yamanaka has succeeded with a three-gene cocktail that excludes c-myc.

More generally, retroviruses are a problem because they disrupt a cell's DNA in random locations, which can trigger tumor growth.

Both Thomson and Yamanaka said they are now testing methods that don't involve retroviruses. Among them are adenoviruses and fatty bubbles called liposomes, which deliver genes to cells without harming DNA, or even direct-injecting the biochemicals that the added genes produce inside cells.

Scientists differed on how big a challenge it would be to transcend retroviruses, but several said they were not concerned. "I don't think it is a big hurdle," Jaenisch said.