Scientists say they have found a safer way of controlling genes to treat disease.
A special sequence of DNA was inserted
Current methods of switching genes on and off usually involve complicated systems and have serious side effects like cancer.
Scientists at the Children's Hospital in Boston say their new system should help overcome some of these problems.
They told Nature it could be used to treat a broad range of diseases and conditions in the future.
Researchers have been looking at ways to treat diseases due to faulty genes, like cystic fibrosis, using gene therapy.
For example, a gene could be added to replace a broken one.
These "therapeutic" genes then need to be controlled - switched on and off as required.
Current methods used to control genes usually involve a complicated three-step process.
It requires a "promoter", which is a piece of DNA coding, a special activating protein that makes this coding work, and a drug that, in turn, enables the protein to work.
There are concerns that the activating protein could switch on other systems as well, leading to unwanted side effects like leukaemia - a cancer of the blood cells.
Also, these methods only work with a handful of specific drugs.
In contrast, Dr Richard Mulligan and colleagues say their new method would work with any drug or chemical change in the body.
It is much simpler, involving only one step.
It is also easier to turn on and off than current methods, allowing treatment to be stopped if unwanted side effects do occur.
The new method involves inserting a special DNA sequence into the patient's own genes or into the therapeutic gene introduced by gene therapy.
This piece of DNA holds coding for making something called ribozyme.
Ribozyme contains the instructions for switching genes on, but has a natural tendency to split itself in half meaning the instructions cannot be read and the genes remain switched off.
Drugs can stop ribozyme splitting so the "therapeutic" genes are switched on.
Dr Mulligan said: "With recent concerns about the development of leukaemia in several children treated with gene therapy, this new method adds an important new safety feature to the gene therapy toolbox."
Professor Andrew Read, professor of human genetics at Manchester University, said: "This is a clever trick of doing it with just the one gene.
"It's a better switch than we had before.
"If you can get a gene into a cell this gives you a much better way of controlling it."
He said having better control should allow scientists to switch off the gene if there were safety concerns.
"When you can do gene therapy it means you can do it with rather more confidence in its safety than you could before," he said.