Human iPS cells created without using viruses

From the story:

A team at the University of Wisconsin said they made the so-called induced pluripotent stem cells, or iPS cells, from human cells without using viruses or exotic genes, which leave behind genetic material that might pose risks if the cells were used as medical therapies.

James Thomson of the University of Wisconsin, whose study appears in the journal Science, said the finding represents the first time researchers have made human induced pluripotent stem cells without inserting potentially problematic new genes into their DNA....

Thomson said the new method uses a circle of DNA called a plasmid, which carries the genes needed to transform a skin cell into an iPS cell.

Over time, the plasmid disappears naturally from the cell population, avoiding the danger posed by using viruses, which can insert harmful genes into the cells' genetic material.

"That means they are less likely to form tumors, less likely to destroy the function of some important gene," Thomson said in a telephone interview.