A baby receives the first customized CRISPR treatment

In a world-first, a baby boy named KJ has become the sole recipient of a custom-built CRISPR therapy — a genetic treatment handcrafted just for him. What once sounded like science fiction now reads like a medical revolution. Instead of merely treating symptoms or slowing down diseases, scientists are starting to rewrite the very code that causes them.

"This drug was designed and made for KJ," said Dr. Rebecca Ahrens-Nicklas, "so in reality this drug will probably never be used again." That's not a disclaimer — it's a declaration of how far we've come.

How does it work?

CRISPR, the famed gene-editing technology, works like a molecular scalpel, snipping and tweaking DNA with astonishing precision. But this time, doctors didn't just tweak — they tailored.

Here's what made KJ's treatment special:

• KJ was born with a liver disorder that blocks his ability to break down proteins, causing dangerous ammonia buildup.
• Scientists built a CRISPR treatment to fix one of his exact mutations, not just silence it.
• The therapy was injected directly into his body — no lab petri dishes, no reprogramming blood cells and reinserting them. Just in vivo editing, straight into his tiny liver.

They used:

• A custom guide RNA from Integrated DNA Technologies to navigate CRISPR to the right gene.
• A lipid nanoparticle delivery system from Acuitas to carry the therapy safely into liver cells.
• Production help from Aldevron and scientific guidance from UC Berkeley's Innovative Genomics Institute.

The goal? Swap out the faulty bit of KJ's DNA — a single base pair — and replace it with the right one. Just like that, genetic spellcheck.

Why does it matter?

"This is the future of medicine," says Dr. Kiran Musunuru. And frankly, it's hard to argue with him.

• This isn't just a treatment — it's a template for tackling rare diseases, one patient at a time.
• The ability to personalize a gene-editing therapy opens doors for the thousands of kids born each year with genetic illnesses that big pharma usually ignores.
• Early intervention, especially for disorders like KJ's, could mean the difference between a life of hospital stays and a life well lived.

As KJ's mom, Nicole Muldoon, put it: "We were talking more about comfort care, a liver transplant, and very severe delays... now we're talking about milestones."

The context

CRISPR has already won approval in the U.S. for sickle-cell disease and beta thalassemia, where it helps patients make healthier red blood cells. But those treatments use an ex vivo process — cells are removed, edited, then returned.

This was different:

• It marks the first time CRISPR was used inside a living human, in a fully customized, one-and-done format.
• The case builds on earlier cholesterol-focused CRISPR work by Musunuru, which inspired his team to try editing other genes in the liver — this time for rare metabolic diseases.
• A patchwork of academic labs and private companies collaborated to make it happen, weaving together everything from CRISPR design to delivery.

They didn't just take a shot in the dark. KJ started with a tiny dose at six months old. They watched. Then carefully added two more doses. So far, the signs are hopeful — improved developmental progress, stable ammonia levels, and a future once thought impossible.

Sure, it's early. The team can't biopsy his liver to count edited cells, but they're tracking markers and watching him grow. "He is doing better at this point than we anticipate for someone with the most severe form of [this disease]," says Ahrens-Nicklas.

And that's enough, for now. Because in a single child's cells, a whole new chapter of medicine may have quietly begun.