Updated on November 15 at 12:37 p.m ET

On Monday, 44-year-old Brian Madeux spent three hours hooked up to an IV and made scientific history.

The clear liquid that dripped into his arm set off a chain of events that is supposed to end with the precise insertion of a gene that Madeux has lacked since birth into the DNA of his liver cells. With that, he might be cured of Hunter syndrome, a genetic disorder that causes a range of symptoms including joint stiffness, breathing problems, and developmental delay. Madeux has had 26 surgeries to deal with it all.

If it works, the change will be permanent. If the gene gets inserted in the wrong place, that will likely be permanent too. Doctors may not know for several months.

The Associated Press broke the news of Madeux’s infusion Wednesday. He is the first patient treated in a clinical trial from Sangamo Therapeutics, which is using microscopic gene-editing tools called zinc-finger nucleases to alter DNA inside the bodies of patients. So invested is Sangamo in this technology that the company is concurrently recruiting patients with hemophilia B and Hurler syndrome for two other clinical trials using zinc-finger nucleases.

Gene therapy, in which a missing gene is inserted into a patient’s DNA, has been around since 1989. What makes Sangamo’s treatment different is precision. Past gene therapies have made use of certain viruses, which insert genes in somewhat random places in the genome—sometimes in an appropriate place, but sometimes in places that can cause cancer, which makes the treatment risky.