Gene-editing has been attempted on cells inside a patient, in a world first by doctors in California.
Brian Madeux, 44 from Arizona, was given the experimental treatment to try to correct a defect in his DNA that causes Hunter’s syndrome.
Mr Madeux says he was prepared to take part in the trial as he is “in pain every second of the day”.
It is too soon to know whether or not the gene-editing has worked in Mr Madeux’s case.
Hunter’s syndrome is rare. Patients are born without the genetic instructions for an enzyme that breaks down long sugary molecules called mucopolysaccharides.
Instead, they build up in the body and damage the brain and other organs. Severe cases are often fatal.
“I actually thought I wouldn’t live past my early 20s,” said Mr Madeux.
Patients need regular enzyme replacement therapy to break down the mucopolysaccharides.