While both groups used just four genes to reprogram human skin cells, two of the genes used differed from group to group. All the genes in question, though, act in a similar way — they are master regulator genes whose role is to turn other genes on or off.

The reprogrammed cells, the scientists report, appear to behave very much like human embryonic stem cells but were called “induced pluripotent stem cells,” meaning cells that can change into many different types.

“By any means we test them they are the same as embryonic stem cells,” Dr. Thomson says.

He and Dr. Yamanaka caution, though, that they still must confirm that the reprogrammed human skin cells really are the same as stem cells they get from embryos. And while those studies are under way, Dr. Thomson and others say, it would be premature to abandon research with stem cells taken from human embryos.

Another caveat is that, so far, scientists use a type of virus, a retrovirus, to insert the genes into the cells’ chromosomes. Retroviruses slip genes into chromosomes at random, sometimes causing mutations that can make normal cells turn into cancers.

One gene used by the Japanese scientists actually is a cancer gene.

The cancer risk means that the resulting stem cells would not be suitable for replacement cells or tissues for patients with diseases, like diabetes, in which their own cells die. But they would be ideal for the sort of studies that many researchers say are the real promise of this endeavor — studying the causes and treatments of complex diseases.

For example, researchers could make stem cells from a person with a disease like Alzheimer’s and turn the stem cells into nerve cells in a petri dish. Then they might learn what goes awry in the brain and how to prevent or treat the disease.

But even the retrovirus drawback may be temporary, scientists say. Dr. Yamanaka and several other researchers are trying to get the same effect by adding chemicals or using more benign viruses to get the genes into cells. They say they are starting to see success.