Within the first 48 hours of his life, something was clearly wrong with KJ Muldoon. Doctors noticed lethargy and trouble breathing. A blood test came back with numbers nobody wanted to see. Ammonia levels over 1,000 micromoles per liter, when a normal reading sits somewhere between 9 and 33. Within days, doctors had a diagnosis. Within about six months, a team of scientists had built him a cure that had never existed before. They designed it specifically and only for his own body.
A Diagnosis That Usually Comes With Terrible Odds
KJ was born in August 2024 with severe CPS1 deficiency. It’s an extremely rare genetic disorder that affects roughly one in every 1.3 million people worldwide. People with CPS1 deficiency are missing a liver enzyme needed to convert ammonia into urea. Ammonia is a natural byproduct of digesting protein, and the body normally gets rid of it safely through that conversion. Without the enzyme working correctly, ammonia builds up instead, and it damages the liver and brain as it accumulates.
The condition is brutal in its typical outcomes. Roughly half of all babies born with it die within their first week of life. For those who survive, drugs that reduce ammonia levels only work partially, and a severely restrictive, low-protein diet becomes a constant, exhausting balancing act.
Too Young for the Only Real Option
The standard treatment for CPS1 deficiency is a liver transplant, but that wasn’t available to KJ. Patients need to be medically stable and physically large enough to survive that kind of major surgery. That typically means waiting until they’re around a year old. That waiting period is exactly when things go wrong most often. Sudden spikes in ammonia during those months put babies at serious, ongoing risk of permanent brain damage or death. All of that can happen long before they’re ever big enough for the surgery that would actually fix the underlying problem.
KJ’s doctors at Children’s Hospital of Philadelphia understood this danger intimately. Rebecca Ahrens-Nicklas, a physician-scientist at CHOP, proposed something that had never been tried in a human patient before. She wanted to build a CRISPR gene-editing therapy specifically to correct KJ’s own individual genetic mutations, and nobody else’s.
Building a Cure for a Patient Population of One
Kyle and Nicole Muldoon agreed to the experimental treatment in February 2025, when KJ was about seven months old. “We would do anything for our kids,” Nicole later said, describing the decision as something she and her husband felt was simply their responsibility as parents, a chance to give their son something closer to a normal life.
What followed was a genuinely unprecedented sprint. Researchers at CHOP, the University of Pennsylvania, and the University of California, Berkeley worked together to design and manufacture a therapy built entirely around KJ’s specific mutations, from scratch, in roughly six months. The treatment relied on a base editor, a more precise form of CRISPR technology that can correct individual letters of DNA without fully cutting the strand apart. Scientists paired that base editor with a custom-built guide sequence, engineered specifically to find and fix KJ’s exact genetic error and nothing else. They wrapped the whole therapy in fatty lipid nanoparticles designed to carry it safely through the bloodstream to his liver. From there, doctors delivered it directly inside his body, correcting the mutation in his own liver cells rather than removing cells, editing them outside his body, and reinfusing them later the way some earlier gene therapies had worked.
KJ received his first dose that February, between six and seven months old. He went on to receive additional infusions over the following months, eventually totaling three doses of the therapy. Doctors watched closely for any sign of trouble. None showed up.
A Timeline That Shouldn’t Have Been Possible
Under normal circumstances, developing any new drug takes years, often a decade or more, working through layers of animal testing, safety trials, and regulatory review before a single human patient ever receives a dose. KJ didn’t have years. His medical team didn’t either.
To move fast enough to matter, researchers expedited safety testing using cell cultures and computer modeling instead of the years-long testing pipeline a typical new drug would require. The Food and Drug Administration fast-tracked its own review process specifically to keep pace with how urgently KJ needed treatment. None of the normal caution disappeared. Scientists still tested the therapy’s safety and specificity thoroughly before it ever reached KJ’s bloodstream. They simply compressed a process that usually takes years into a matter of months, because a baby’s actual, immediate survival depended on it.
From Critical to Thriving
The treatment worked. KJ’s doctors say it transformed his condition from a severe, life-threatening form of the disease into something considerably milder. He began growing and gaining weight normally, without the constant crisis management that had defined the earliest months of his life.
On a Tuesday roughly 307 days after he was first admitted to the hospital, essentially his entire life up to that point, KJ finally went home. His parents brought him out with a police escort. It’s a small, striking detail, but it says something about just how closely the world had been watching this particular baby’s progress.
What KJ’s Case Actually Opened Up
Researcher Kiran Musunuru of the University of Pennsylvania helped develop the therapy. He presented KJ’s case publicly at the American Society of Gene and Cell Therapy’s annual meeting in May 2025. The full details appeared shortly after in the New England Journal of Medicine. Musunuru didn’t hedge about what it meant. This, he said, is the future of medicine.
That future is already expanding. A new clinical trial is now underway aiming to treat five additional patients. It targets editable genetic variants across seven different urea cycle disorder genes beyond just CPS1, conditions with names like OTC, citrullinemia, ASA/ASL, ARG1, NAGS, and HHH deficiency, names most people have never heard of and hope they never need to learn. KJ wasn’t just one extraordinarily lucky baby. He was the first working proof of something much bigger. Doctors can build a genetic cure fast enough, and precisely enough, to save one specific patient before it’s too late. Then they can take exactly what they learned doing it and use that knowledge to start saving the next one.