A genetically modified "supermouse" which can run twice as far as a normal rodent has been created by scientists working in the US.
It also lives longer, and breeds later in life compared with its standard laboratory cousin.
The research has been conducted at Case Western Reserve University in Cleveland, Ohio.
Details of the scientists' new transgenic animals are published in the Journal of Biological Chemistry.
The mice were produced to study the biochemistry at play in metabolism and could aid the understanding of human health and disease.
The GM rodents can run five to six kilometres at a speed of 20 meters per minute on a treadmill, for up to six hours before stopping.
"They are metabolically similar to Lance Armstrong biking up the Pyrenees; they utilise mainly fatty acids for energy and produce very little lactic acid," said Professor Richard Hanson, the senior author on the journal article.
The performance was tested on rodent treadmills
He told BBC News: "The muscles of these mice have many more mitochondria. These are the little 'engines' in the cell that produce energy. For some reason, the number of mitochondria are around 10 times more than we see in the muscle of their littermates."
The mice over-express a gene responsible for the enzyme phosphoenolypyruvate carboxykinases (PEPCK-C). Normal expression is in the liver, in the production of glucose.
The scientists found their new mice would eat twice as much as normal mice - but weigh half as much. They could also give birth at three years old - which in human terms is akin to an 80-year-old woman giving birth.
Other research groups have produced similar novel rodents by altering different aspects of their genetics. One criticism of the work is that it could open the door to abuse, with the spectre of athletes resorting to gene therapy to try to improve their performance.
The research is featured in the Journal of Biological Chemistry
But Professor Hanson played this down. "Right now, this is impossible to do - putting a gene into muscle. It's unethical. And I don't think you'd want to do this. These animals are rather aggressive, we've noticed."
Scientists say such work is more likely to help them understand human conditions, such as those which lead to wasting of the muscles.