Scientists for the first time have tried editing a gene inside the body in a bold attempt to permanently change a person's DNA to try to cure a disease.
The experiment was done in California on Monday on Brian Madeux, 44.
Through an IV, he received billions of copies of a corrective gene and a genetic tool to cut his DNA in a precise spot.
Mr Madeux - who has a metabolic disease called Hunter syndrome - described it as "kind of humbling" to be the first person to try the procedure.
He added: "I'm willing to take that risk. Hopefully it will help me and other people."
Signs of whether it is working may come in a month while tests will show for sure in three months.
If it is successful, it could give a major boost to the fledgling field of gene therapy.
Scientists have edited people's genes before, altering cells in the lab that are then returned to patients.
There are also gene therapies that do not involve editing DNA.
However, these methods can only be used for a few types of diseases and some give results that may not last.
Some others supply a new gene like a spare part but cannot control where it inserts in the DNA, possibly causing a new problem like cancer.
This time, the gene tinkering is happening in a precise way inside the body, described as like sending a mini-surgeon along to place the new gene in exactly the right location.
Dr Sandy Macrae, president of Sangamo Therapeutics, the California company testing it for two metabolic diseases and haemophilia, said: "We cut your DNA, open it up, insert a gene, stitch it back up. Invisible mending.
"It becomes part of your DNA and is there for the rest of your life."
It also means there is no going back, no way to erase any mistakes the editing might cause.
Independent expert Dr Eric Topol, of the Scripps Translational Science Institute in San Diego, described the experiment as "toying with mother nature".
He added that the risks cannot be fully known but the studies should move forward because these are incurable diseases.
Protections are in place to help ensure safety and animal tests were very encouraging, said Dr Howard Kaufman, a Boston scientist on the National Institutes of Health panel that approved the studies.
Dr Kaufman added: "So far there's been no evidence that this is going to be dangerous. Now is not the time to get scared."
People with Hunter syndrome lack a gene that makes an enzyme that breaks down certain carbohydrates. These build up in cells and cause havoc throughout the body.
Patients may have frequent colds and ear infections, distorted facial features, hearing loss, heart problems, breathing trouble, skin and eye problems, bone and joint flaws, bowel issues and brain and thinking problems.
Weekly IV doses of the missing enzyme can ease some symptoms but cost $100,000 to $400,000 (£300,000) a year.