KJ Muldoon came into the world with a genetic time bomb ticking inside him.

Doctors at the Children’s Hospital of Philadelphia had barely finished running newborn screens when the diagnosis landed: severe carbamoyl phosphate synthetase 1 deficiency, or CPS1. It’s a rare genetic disorder—striking only one in a million newborns—that prevents the body from safely processing protein. Without that ability, ammonia builds up in the blood, poisoning the brain.

Most babies with KJ’s condition don’t survive long enough to receive a liver transplant, the standard treatment. “Every day that passed there was another risk that he could have neurologic injury from an elevated ammonia episode,” Dr. Rebecca Ahrens-Nicklas, one of KJ’s physicians, told NPR.

Faced with an impossible choice—wait for a transplant, or try an untested therapy—the Muldoons chose the unknown.

“We either had to have a liver transplant or give him this medicine that’s never been given to anybody before,” KJ’s father, Kyle Muldoon, said in a statement. “It was an impossible choice, but we felt this was the best possible scenario for a life that, at one point, we didn’t know if he would be able to have.”

A Genetic Moonshot

What followed was a scientific race against time.

An international team of researchers, physicians, and biotech collaborators—supported by the National Institutes of Health and guided by the FDA—crafted a bespoke therapy in just six months. It was a gene-editing treatment designed specifically for a single patient: KJ. The therapy used a refined CRISPR method called base editing, which rewrites one letter of DNA without cutting the strand.

“We programmed it to go to the site of the genetic variant that was causing the disease in KJ,” said Dr. Kiran Musunuru, a cardiologist and gene-editing researcher at the University of Pennsylvania. The custom tool, delivered in a lipid nanoparticle, traveled through KJ’s bloodstream to his liver cells—the very place where his disease originated.

Unlike other CRISPR-based treatments that target common disorders like sickle cell disease or beta thalassemia in large groups of patients, this approach was built specifically for a single patient. “No one has developed a personalized gene-editing therapy for an infant,” said Ahrens-Nicklas. “It was quite a nerve-wracking but exciting day. And it was quite a momentous day.”

Life After the Edit

The first dose was low. The team didn’t want to risk flooding the child’s system with a new drug—especially one never before tested in humans. The therapy’s components, donated by companies like Acuitas Therapeutics and Integrated DNA Technologies, were designed to be redeliverable, allowing multiple infusions over time.

Almost immediately, doctors saw signs of success. KJ could tolerate more protein in his diet, a milestone for children with CPS1. After a second infusion, they began reducing the medications that controlled his ammonia levels. A third dose followed.

When KJ caught a cold—and later, a gastrointestinal infection—doctors braced for a spike in ammonia. Instead, his body coped. “We were very concerned when the baby got sick, but the baby just shrugged the illness off,” said Musunuru.

Today, KJ is nearly ten months old. He sits up in his crib, rolls over, and plays. “That’s big for us,” his mother, Nicole Muldoon, told NPR. “We never thought this was going to happen.”

Still, no one is saying this is a definite cure yet. “This is still really early days,” said Ahrens-Nicklas. “We know we have more to learn from him.”

A Glimpse of the Future?

This case marks a turning point in medicine. For the first time, a CRISPR treatment wasn’t designed to help thousands—but one. Fyodor Urnov, a geneticist at the University of California, Berkeley noted, “I think we can say: This is the year when CRISPR-on-demand is truly born.”

As a proof of principle, it shows what’s possible when you blend fast diagnostics, regulatory flexibility, open scientific collaboration, and a modular CRISPR platform. “As a platform, gene editing—built on reusable components and rapid customization—promises a new era of precision medicine,” said Dr. Joni Rutter, director of the NIH’s National Center for Advancing Translational Sciences.

But serious questions remain.

How many patients could benefit from such hyper-personalized treatments? Possibly thousands—especially children with ultra-rare diseases for which no therapies exist.

Can this ever be affordable? That’s harder to answer. Even conventional gene therapies designed for hundreds of people often carry price tags in the millions of dollars. “There’s no great answer to this,” said Dr. Waseem Qasim, a gene therapy expert at University College London, to Nature.

The science is moving quickly. Already, researchers are adapting base editing to tackle other disorders—genetic blindness, sickle cell anemia, and rare neurological diseases. Each therapy will need its own design. But the template is in place.

For now, KJ’s story stands alone. A baby, a mutation, and a one-in-a-million rescue. But it may not stand alone for long.