Researchers have discovered a gene mutation which prevents otherwise healthy carriers from feeling pain.
Members of three Pakistani families could feel no pain
The University of Cambridge team made the discovery after studying three related families with a rare genetic disorder in northern Pakistan.
One family member, a 10-year-old boy, worked as a street performer, placing knives through his arms.
Writing in Nature, the researchers said the finding might aid development of more effective pain treatments.
In total, the Cambridge team found six people from the three related families all carried the same mutated gene.
None had experienced pain at any time in their lives.
Detailed neurological examinations revealed that there was no evidence of any sort of disease which could explain this deficit.
And they were able to perceive a number of sensations, such as touch, temperature, tickle and pressure.
All six had sustained a variety of injuries as a result of their inability to sense pain, including damage to their lips and tongue as a result of biting themselves when young.
The researchers pinned down the problem to a mutation in a gene which plays a role in stimulating sensory nerve cells by controlling the movement of sodium through them.
They believe it could be possible to develop new painkilling drugs which target the same process.
Researcher Dr Geoffrey Woods said: "This paper shows that rare diseases can still be of great importance, because of the insights they give into biological and developmental processes."
Dr John Wood, from University College London, said the finding provided scientists with an exciting new target for pain-killing drugs.
"Potentially this is as important as the identification of the morphine receptors."
Dr Wood said a different mutation in the same gene had been linked to over-sensitivity to pain in some inherited disorders.
Dr Joan Hester, a consultant pain clinician and president of the British Pain Society, said the perception of pain was necessary for survival, and the latest findings were significant.
"This could account for some of the differences in pain perception between one individual and another."
She said some painkillers already worked by blocking sodium channels - but they acted on all of them, not just the precise variety highlighted in the new study.
At present, she was not aware of drugs that had such a specific mechanism of action.
"They may be useful but would only modify one part of the pain pathway, their usefulness remains to be seen."