Anthony Perry explains his research

The method is promising because it is technically simpler and has a higher success rate than existing methods. These inject genes directly into animals or deliver them using a virus.

The new method may also allow larger pieces of DNA to be inserted into an animal's genetic make-up.

"The interesting thing now is to see if we can develop the technique," Professor Anthony Perry at the University of Hawaii told the BBC.

"We want to put other genes into the mice and apply our methods to other species - that's our hope and expectation," he said.

Professor Perry and his colleagues in Honolulu were the first to clone mice and to breed multiple generations of the rodents.

Sperm insertion

Their new experiment involved first freeze-drying mouse sperm, then inserting DNA from another species into it and finally injecting the altered sperm into mouse eggs.

Those eggs were placed in female mice using standard embryo implantation techniques.

The gene inserted in this case was from a jellyfish and makes the sea creature glow fluorescent green.

It was used because it is then easy to see if the gene has been successfully incorporated in the mice - the mice glow too.

Up to 80% of the embryos created in the laboratory took up the green gene. But only one in five survived pregnancy and expressed the gene.

However, this overall 20% success rate is significantly higher than currently-used methods.

Forever green

The new technique also delivers the genes in such a way that they can be inherited by future generations, i.e. offspring of the green mice can also be green.

Scientists want to add new genes to animals for a number of reasons:

• To produce human proteins in animals which can be used to treat, for example, haemophilia or cystic fibrosis
• To create mice with human genes so that new drugs can be tested on a human-like subject
• To create pigs whose organs will closely resemble human organs and can therefore be transplanted

"We hope and believe that the technique will be useful in medical research and in development of xenotransplant donors," said Professor Perry.