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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.
But Mr Madeux has been given an experimental treatment to rewrite his DNA to give him the instructions for making the enzyme.
The therapy was infused into his bloodstream on Monday at Oakland's UCSF Benioff Children's Hospital.
The therapy contains two molecular scissors - called zinc finger nucleases - that cut the DNA at two precise spots.
This creates an opening for a new piece of DNA, containing the desired instructions, to be inserted into the patient's genetic code.
The genetic therapy has been designed so it becomes active only once it gets inside Mr Madeux's liver cells.
Dr Chester Whitley, one of the doctors working on the trial, told the BBC: "If works as well as it does in mice, this has huge ramifications. "I'm very optimistic we have a both safe and efficacious way of providing gene therapy." His long-term hope is to perform gene-editing shortly after birth, because an "untreated baby loses 20 IQ points per year".