Prime Medicine, a company in Cambridge, Massachusetts, infused a patient with Wilson disease with an experimental therapy called PM577a on or before 5 October 2026. The drug is a prime editor, packaged in a lipid nanoparticle, given once through a vein, designed to reach liver cells and rewrite a single codon in the ATP7B gene.

Nature called it the first known instance of prime editing used on DNA inside a human body. That is the accurate claim, and it is narrower than most of the coverage that followed.

Editing genes inside a living person is several years old. Intellia has been dosing CRISPR-Cas9 editors into the bloodstream since 2021, and the FDA is expected to rule on its hereditary angioedema therapy early in 2027. A base editor for very high cholesterol is in trials, from a company Eli Lilly bought in June 2025 for up to 1.3 billion dollars. Tessera Therapeutics, nine miles from Prime Medicine, has an in-vivo program for alpha-1 antitrypsin deficiency partnered with Regeneron. In February 2025 a six-month-old boy in Philadelphia received a base editor built for his private mutation.

What arrived in October is the third and most flexible editing chemistry reaching that same destination.

The entry criteria shape what this first cohort can show. Participants must have been stable on penicillamine, trientine or zinc for six months, must have demonstrated adequate copper control at screening, and are excluded for severe hepatic impairment, unstable neurological disease, prior liver transplant, prior liver-directed lipid nanoparticle or gene therapy, and a BMI of 35 or above. The study enrolls the best-controlled Wilson patients available.

So clinical improvement is not the thing to watch. The efficacy signal has to come from biomarkers and from successful withdrawal of the drugs these patients are already doing well on. Among the secondary endpoints: copper efflux measured by hepatic copper-64 PET, ceruloplasmin, non-ceruloplasmin-bound copper, 24-hour urinary copper, liver stiffness by elastography, and on-target editing read directly from a liver biopsy at weeks 24 and 48.

That biopsy is the most interesting measurement in the protocol. It asks how many hepatocytes were actually corrected, in a person, by number.

Three editing chemistries have now been put into people, and they are not interchangeable. The distinction decides which diseases are reachable.

Beyond those three sits a frontier of methods for inserting whole genes, with names like PASTE, evoCAST, MINT and KNIT, some reaching tens of thousands of bases. None is close to a patient. Delivery is the shared constraint. One group using a Cas9-integrase hybrid in monkeys corrected about half the liver cells with a single treatment, and its lead investigator noted that targeting any other organ becomes quite challenging.

The liver is the organ everyone can reach. That is not a coincidence in a Wilson disease trial. It is the premise.

Prime Medicine's chronic granulomatous disease therapy worked. A month after treatment the first recipient had no serious side effects and two-thirds of his neutrophils had recovered the enzyme they were missing. On the same day it released those results, the company announced it would not develop the therapy any further on its own.

David Liu, who co-founded Prime Medicine and whose laboratory invented prime editing, gave Nature the reason at the time. The science has moved far enough that many patients would benefit. The obstacle is economics.

The same arithmetic runs through the whole field. Base editing is precise enough that each mutation needs its own therapy, which for some diseases makes it impractical. Stargardt disease, an inherited form of vision loss, can be caused by more than 1,200 variants of a single gene, around 750 of them base-editable. Treating them that way means hundreds of separate drugs. One researcher's summary of the problem is that you cannot develop a drug with one patient in its market.

Read PM577a against that and the design choices stop looking purely technical. H1069Q is not merely a mutation prime editing can fix. It is the most common Wilson allele in North America and Europe, which is the largest market a single Wilson editor can address. The follow-on candidate targets R778L, the most common allele in East Asia. The company's liver focus was announced in May 2025, in the same statement that abandoned the blood program.

Two responses to this are visible. Liu's laboratory is moving toward replacing entire genes, so that one therapy covers every mutation in a gene rather than one. And the Philadelphia team behind the bespoke infant base editor is running a trial across seven genes that keeps the editor constant and swaps only the guide RNA, with the FDA agreeing to carry safety data forward between patients. Both are attempts to make the fixed costs land once instead of every time.

A technology that can correct any mutation, deployed against the mutations that enough people have.

Heidi Ledford has covered genome editing for Nature for years, and wrote most of the related background cited here, including the September 2026 survey of large-insertion editors and the 2025 reports on the prime editing debut and the infant base editor. The piece carries no detectable company framing. Nature's news desk is editorially independent of its research journals.

Four people are quoted. Rhonda Rowland of the Wilson Disease Association supplies the patient-advocacy voice. Allan Reine speaks for the sponsor. Joseph Thome, an analyst at TD Cowen, supplies the cost line. Krishanu Saha is the only academic, and he is not disinterested: his own laboratory is developing in-vivo CAR-T cells, and he tells Ledford that for two years the only question he has been asked is why he is not doing it in vivo. He is describing a current he is swimming in. Nature prints the remark without noting that it is also a disclosure.

No hepatologist appears, and nobody is asked what a mistargeted prime editor would actually do in a liver.

Everything quantitative traces to the sponsor. The trial listing is the sponsor's. The efficiency data is the sponsor's deck. Prime Medicine licenses the platform from the Broad Institute and this is the first clinical test of its liver program, after dropping the blood program that produced its only human data.

One clinical note the coverage skipped. Chelators and zinc are not the same intervention described two ways. Penicillamine and trientine bind circulating copper and drive it into the urine. Zinc salts induce intestinal metallothionein and block absorption at the gut. Patients take one or both, daily, for life, and adherence is a known problem. Liver transplant is the only current cure, which tells you how the alternatives are weighed.