Borrowing a biological cut-and-paste trick from bacteria, scientists have created the first personalized stem cells for patients that are free of the cancer-causing viruses and genes needed to make them, according to a study published today in the journal Cell.

The stem cells, derived from skin samples provided by five patients with Parkinson's disease, were first transformed back to the undecided state of cells in an early embryo. Then they were used to make the dopamine-manufacturing neurons that are lost to disease.

The technique removes a key barrier to using a special class of stem cells called an induced pluripotent stem cell, or iPS cell, to create replacement parts for patients that could be transplanted without risk of rejection -- the ultimate goal of regenerative medicine.

The reprogramming of skin cells into iPS cells, which have the potential to become any type of cell in the body, is one of the hottest areas of biological research.

The cells seem to offer all the benefits of embryonic stem cells without any of the ethical drawbacks.

They are also ideally suited to making genetically matched tissues for patients, such as insulin-secreting islet cells for people with diabetes or brain tissue to treat stroke victims.

Dr. Andras Nagy['s research team] at Mount Sinai Hospital in Toronto's is perfecting a related method to make iPS cells that do not use viruses but rely on DNA sequences called transposons. After the reprogramming is complete, the transposons can be deleted by an enzyme.

Nagy and his colleagues described their technique this week in the journal Nature, and they are now applying it to adult human cells.

Jaenisch has already validated his procedure by reprogramming cells taken from Parkinson's patients and turning them into replacement neurons, but several technical issues must be resolved before it would be safe to use the cells therapeutically, he said.