Scientists are expected to conduct 5,000 urine and 1,000 blood tests at the Games - the most comprehensive testing program at any Olympics.

And now a new study says that athletes who use drugs that make muscles bigger and more efficient
could be caught if forced to supply muscle biopsies, but not through the analysis of urine or blood samples.

It was authored by Mauro Giacca of the International Centre for Genetic Engineering and Biotechnology in Trieste, Italy, who was asked by the World Anti-Doping Agency to look into how to screen for gene doping.

According to newscientist.com, Giacca's team created mice loaded with extra copies of the muscle-boosting gene IGF-1 by injecting its limbs with a virus that implants IGF-1 into muscle cells. They then tested the animals' endurance by recording how long they could swim before exhaustion.

Theys say they doped mice swam for three times as long as mice that received the virus but not IGF-1.

Autopsies showed that the extra IGF-1 triggered the production of 10 times more protein than normal in the muscles. Giacca says he also saw activity soar in genes controlling energy production, contraction of muscles and respiration. Also detectable in the muscle were traces of the virus used to deliver the genes. However, the gene, protein and virus were undetectable in blood or urine from the mice.

Giacca doubts it is possible to achieve such results through exercise alone.

But, "from a muscle biopsy, it would be possible to distinguish a well-trained athlete from one who'd been gene-doping for a month," he says.