Researchers have found a way to breach the body's natural defences and deliver genes and drugs into the brain.
The brain is highly protected
The method shows promise for treating a host of brain disorders such as Parkinson's disease.
The brain is protected by a membrane formed by tight connections between the cells that line the blood vessels that supply it.
This is known as the blood-brain barrier, and is designed to ensure that the brain is immune from attack from foreign substances circulating in the blood stream.
Only a few molecules that are recognised by the cell receptors are allowed to pass through.
However, it also means that it has proved virtually impossible to deliver genes or drugs into the brain, unless they are injected directly into its tissue.
Even this is flawed, because it is difficult to ensure an even distribution throughout the brain's tissue.
New Scientist magazine reports that a team from the University of California, Los Angeles has come up with a way to solve the problem.
So far their research has only been tested on primates - but the same principle should apply for humans.
The US team has been able to smuggle genes past the brain's defences by hiding them inside fatty spheres called liposomes.
These liposomes are coated with a polymer which is impregnated with antibodies similar to those produced by the immune system to fight disease.
The presence of these antibodies effectively tricks the cell receptors into letting the liposomes pass, where they can deliver their cargo to brain cells.
The US team initially tested the technique in rats, but found that it produced even better results in rhesus monkeys.
Dr Savio Woo, director of gene therapy at the Mount Sinai School of Medicine in New York, told New Scientist: "To reach the central nervous system through the blood-brain barrier in a non-human primate with this kind of efficiency - that's absolutely fantastic."
The liposomes do not appear to have any toxic side effects.
They do deliver genes to other organs besides the brain, but scientists are able to switch gene activity on and off so that they are active only in the desired tissues.
The technique has already showed promise as a possible treatment for Parkinson's disease.
In lab tests, rats with Parkinson's symptoms showed substantial improvement after they were injected with liposomes containing a gene that boosts production of a crucial enzyme.
Separate tests on mice have shown that the liposomes can also deliver treatment which can block the development of brain tumours.
Dr Russell Lonser, of the US National Institute of Neurological Disorders and Stroke who has also carried out research into how to cross the blood-brain barrier, told BBC News Online the latest study was promising.
He said: "It may have several exciting potential uses, but additional studies and questions must be answered to determine the precise clinical applications of this technology."
Robert Meadowcroft, Director of Information, Policy and Research at the Parkinson's Disease Society said: "This research appears to offer a major step forward in terms of our understanding of how we can transport genes to the affected parts of the brain.
"However, the Parkinson's Disease Society remains cautious of the effectiveness of gene therapy's affect on the overall progression of the disease itself, until findings of large scale clinical trials are published."