On Mar. 4, 2020, doctors at the Casey Eye Institute at Oregon Health & Science University in Portland used the gene editing tool CRISPR inside someone’s body for the first time, a new frontier for efforts to operate on DNA, the chemical code of life, to treat diseases.

Doctors first tried in-the-body gene editing in 2017 for a different inherited disease using a tool called zinc fingers. Many scientists believe CRISPR is a much easier tool for locating and cutting DNA at a specific spot, so interest in the new research is very high.

The people in this study have Leber congenital amaurosis, caused by a gene mutation that keeps the body from making a protein needed to convert light into signals to the brain, which enables sight. They’re often born with little vision and can lose even that within a few years.

Scientists can’t treat it with standard gene therapy — supplying a replacement gene — because the one needed is too big to fit inside the disabled viruses that are used to ferry it into cells. So they’re aiming to edit, or delete the mutation by making two cuts on either side of it. The hope is that the ends of DNA will reconnect and allow the gene to work as it should.

It’s done in an hour-long surgery under general anesthesia. Through a tube the width of a hair, doctors drip three drops of fluid containing the gene editing machinery just beneath the retina, the lining at the back of the eye that contains the light-sensing cells. The eye surgery itself poses little risk, doctors say. Infections and bleeding are relatively rare complications.