Two teams of scientists reported today that they had developed lines of human stem cells almost identical to embryonic ones, using a procedure that does not involve destroying actual embryos.

The two groups, at Kyoto University and at the University of Wisconsin at Madison, followed up on experiments in mice last year that showed how adult cells could be tricked into acting like embryonic cells. In the current round of experiments, Kyoto’s Shinya Yamanaka and his colleagues used viruses to implant the genes for four proteins into the DNA of adult human skin cells. That turned the clock back on the cells, and they behaved like embryonic stem cells, with the ability to grow into all three of the major tissue types in the human body, the team reported in the journal Cell.

The second group, led by Wisconsin’s Junying Yu, used viruses to insert a different set of four genes into cells collected from human fetuses and from babies’ foreskins. Those genes also reprogrammed the cells, the group reported in Science.

Because it does not destroy embryos, the new technique avoids the main ethical hurdle blocking studies of embryonic stem cells. President Bush cited the earlier mouse studies when he vetoed a bill this year that would have expanded work on embryonic stem cells.

But the scientists behind the new studies contend that the reprogramming efforts are not as advanced as work on embryonic stem cells and that scientists should pursue both. One concern with the new technique is that the viruses used to insert genes can cause cancer. “As long as we use those viruses, I don’t think we can say that the [reprogrammed] cells are as safe as embryonic stem cells,” said Dr. Yamanaka.

He and others are working to reprogram adult cells without using viruses to alter a cell’s DNA. While Dr. Yamanaka and many other stem-cell scientists continue to back embryonic-stem-cell research, the reprogramming technique has eroded support for cloning, which was previously seen as the best way to develop stem cells tailored to individual patients. Reprogramming offers a similar ability to tailor stem cells, but it does not require vast numbers of donated eggs or the destruction of embryos.

Ian Wilmut, who created the first mamallian clone, Dolly the sheep, said he supported the new technique. “I am sure that in the end, reprogramming will be more effective,” he wrote in an e-mail message. —Richard Monastersky