A team of US scientists have used gene therapy to regenerate inner ear 'hair' cells - the culprit in many forms of deafness. The team, lead by John Brigande, who is himself profoundly deaf, injected mice with a version the Atoh1 gene modified so that it fluoresces green. Atoh1 is thought to orchestrate the formation of cochlear hair cells, which are fundamental to hearing and involved in producing brain signals in response to sound waves.

The mice, who were injected with the gene whilst still in the womb, converted non-sensory cells into cells producing green fluorescent hairs in exactly the right segment of the cochlea. The researchers then went on to prove that the hair cells were functional - converting air movement into electrical signal.

A similar study carried out in 2003 successfully used gene therapy with Atoh1 to revive hearing in chemically deafened guinea pigs, but there was controversy at the time as to the origin of the new hair cells produced. The new study proves that the cells generated are as a result of gene therapy.

Whilst excited about the findings, Brigande urged caution.

'I'd like to hear, and I would love to be a member of the research team or community that does define an efficacious therapy, but I think it needs to be approached with enormous caution,' he says.

Stanford University cell biologist Stefan Heller, who specialises in hearing research, agrees: 'No human subject committee would allow this kind of experiment to be done, given that the process would involve injecting a foetus with a foreign gene.'

Deafness can be the result of prolonged exposure to loud noise, infection, old age or gene mutations damaging the 'cochlear hair cells', with the only current treatment being a cochlear implant which bypasses defective hair cells tapping directly into auditory nerves, giving users the sensation of sound. Although not a cure for deafness, it is hoped that these findings will contribute to scientist's understanding of deafness, perhaps helping with the development of new drugs to treat the condition, offering the hope of one day finding a cure.