This really is a tremendous confluence of very, very challenging technology

Dr George Daley

Embryo clones of the mice were created in the lab and their stem cells harvested and modified to correct the genetic defect that had led to the immune problem in the first place.

The stem cells were then injected back into the mice, where they improved the health of the animals.

"This really is a tremendous confluence of very, very challenging technology, wrapping them all together into a model therapy," said Dr George Daley, from The Whitehead Institute for Biomedical Research in Cambridge, Massachusetts, US. "We are the first to do this all the way."

The research has been published in the journal Cell.

Tail skin

The Whitehead research comes as news reports from China say scientists there have successfully extracted stem cells from human embryo clones.

The Whitehead research is published in the journal Cell

Many researchers hope the cloning procedure demonstrated in the Massachusetts rodents could pave the way for similar therapies in humans to treat diseases such as muscular dystrophy and Parkinson's.

The Whitehead team worked on mice with a severe immune deficiency resulting from the absence of a working Rag 2 gene. The genetic defect prevented the rodents from manufacturing antibodies to fight off disease.

First, the team made embryo clones of the mice by injecting their genetic code, extracted from skin cells on their tails, into mouse eggs that had been emptied of their nuclear DNA.

The clones were then allowed to develop until they could be mined for their embryonic stem cells, the "master cells" that have the ability to become virtually any type of cell in the body.

Journal restriction

The scientists added a working Rag 2 gene to the stem cells and a HoxB4 gene that drove the cells into becoming effective immune cells.

The goal was to give the mice stem cells that would make healthy blood cells. The process worked - to an extent.

Disease fighting cells known as B cells and T cells ordinarily make up about 40% of the animals' blood. After the experiment, they comprised about 3%.

"That was still enough to generate antibodies in these mice," Dr Daley said. "We would expect this level of reconstitution would provide significant immune function and have significant benefit."

Professor Lu Guangxiu, from the Xiangya Medical College in Changsa, China was unable to talk about her cloning work to the BBC on Monday.

She said an agreement had been reached with an international journal to publish her team's work and she was not allowed to speak to the media until the research paper appeared.