Scientists have taken another important step toward using ordinary skin cells that are made to behave like embryonic stem cells to find treatments for conditions like Parkinson's disease.

Researchers at the Whitehead Institute for Biomedical Research in Massachusetts removed a stumbling block in using so-called induced pluripotent stem cells, or iPS cells, by taking out potentially cancer-causing genes.

Writing in the journal Cell on Thursday, the scientists said they then turned these iPS cells into brain cells involved in Parkinson's disease.

The iPS stem cells could be made from a patient's own skin cells, reducing the chances that the body's immune system might reject the cells as it sometimes does with organ transplants.

Transplanting healthy cells made from iPS cells to replace cells damaged by disease or injury may be possible in the future. But a more immediate use for these cells may be in lab dishes testing the effects of new drugs, according to Dirk Hockemeyer, one of the Whitehead Institute researchers.

"For transplantation applications, we are further away," Hockemeyer said in a telephone interview.

Scientists have learned that just a handful of genes can reprogram a cell back to a state in which, like an embryonic stem cell, it can generate any type of cell in the body.

But these genes have the potential to cause cancer, and also may interact with unpredictable results with thousands of other genes in the cell, the researchers said.

The Whitehead Institute team used viruses to transfer three genes into the skin cells of Parkinson's patients, then removed them after they had done their job.

The result was a batch of cells that looked like embryonic stem cells from Parkinson's patients, without the extra genes.

They then used the resulting iPS cells to create dopamine-producing nerve cells. They are the brain cells that die in people with Parkinson's, causing telltale symptoms such as tremors, slow movement and balance problems.

It is the first time scientists have created human iPS cells that have kept their embryonic stem-cell-like properties after the removal of reprogramming genes.

"Other labs have reprogrammed mouse cells and removed the reprogramming genes, but it was incredibly inefficient, and they couldn't get it to work in human cells," Whitehead Institute scientist Rudolf Jaenisch said in a statement.

"We have done it much more efficiently, in human cells, and made reprogrammed, gene-free cells," Jaenisch added.

The DNA of the iPS cells ended up as nearly identical to the DNA of the original skin cells, the researchers said.