Cells taken from cloned mouse embryos have been used to successfully treat a condition similar to Parkinson's disease in humans.
The research was carried out on mice
The breakthrough, by US researchers, could assist the search for a cure for the common brain condition.
The embryonic "stem cells", reports the journal Nature Biotechnology, were grown into new tissue which was implanted into the mouse brain.
However, many obstacles stand in the way of human treatments, say experts.
The team, from the Memorial Sloan-Kettering Cancer Center in New York, is not the first to use embryonic stem cells to treat Parkinsonism in mice, but they are the first to use cells which were cloned from the "patient".
Although embryonic stem cells - taken from a five-day-old embryo - are all the same, they have the ability, when placed in the right biochemical conditions, to be transformed into any cell type in the body.
In theory, they could provide an inexhaustible supply of different tissue types to replace those lost through injury or illness.
The advantage of using a cloned embryo is that the cells will be a perfect genetic match for the transplant recipient, removing the need for extra treatments to suppress the immune system.
However, the use of cloning technology, even for therapeutic reasons, remains controversial, as does the use of any embryo in this kind of research.
In the latest experiments, mice were bred specially to suffer a condition which mimics many aspects of Parkinson's disease.
In the human diseases, patients lose key brain cells which produce a chemical vital for controlling muscle movements - and are left with gradually increasing stiffness, fatigue and jerky movement.
Cells were taken from the tail of the mouse, then their genetic material extracted and used to create cloned embryos.
The stem cells were taken from these and, in a laboratory culture, and body chemicals used to "persuade" them to shift, stage by stage, from this state into the type of brain cells that their Parkinsonism mice were lacking.
These were then selected and implanted back into the mouse brain to see if they made a difference.
The researchers found that the mouse symptoms disappeared.
Dr Lorenz Studer, head of the Stem Cell and Tumor Biology Laboratory at the centre, and the lead researcher on the study, told BBC News Online that the study was "proof of principle" that cloned embryonic stem cells could be reliably transformed into a variety of useful cell types.
He said: "There are potential applications not just in Parkinson's disease, but in many other disease types as well."
However, he said that his team was still a long way from moving into human research, as scientific and political obstacles had yet to be overcome.
He said: "We don't know if we would be able to do the same thing in humans - there is some research, which is controversial - that suggests that it might actually be impossible."
However, he said that embryonic stem cells remained the most obvious source of cells for such transplants, as foetal tissue - used in some experiments - was scarce, and so-called "adult stem cells" had never been converted into the right type of brain cells, despite 15 years of research.