Researchers have developed a safer way to make powerful stem cells from ordinary skin cells, taking one more step toward so-called regenerative medicine.

They used a common cold virus to carry transformative genes into ordinary mouse cells, making them look and act like embryonic stem cells.

If the same can be done with human cells, it may offer a safe way to test cell therapy to treat diseases such as sickle cell anemia or Parkinson's, according to a report by Konrad Hochedlinger of Massachusetts General Hospital and Harvard Medical School in the journal Science on Thursday.

Stem cells are the body's master cells, giving rise to all the tissues, organs and blood. Embryonic stem cells are considered the most powerful kinds of stem cells, as they have the potential to give rise to any type of tissue.

But they are difficult to make, requiring the use of an embryo or cloning technology. Many people also object to their use and several countries, including the United States, limit funding for such experiments.

In the past year, several teams of scientists have reported finding a handful of genes that can transform ordinary skin cells into induced pluripotent stem cells, or iPS cells, which in turn look and act like embryonic stem cells.

Retroviruses have been used to get these genes into the cells because they integrate their own genetic material into the cells they infect. This can be dangerous and can cause tumours and other effects.

Hochedlinger's team used a much more harmless virus, called an adenovirus, to carry the required transformative genes into the cells.

"The nice thing about adenoviruses . . . is they deliver proteins inside the cells but they will never, ever integrate their DNA into the cells," Hochedlinger said.

As the cells divide, they dilute the virus until it disappears, he said. But the genetic changes remain.