BOSTON — It was an ordinary moment, but it meant everything to the father of a little boy with the rarest of diseases.
Last month, in the final seconds of a Sunday night football game, as Eagles defensive tackle Jordan Davis blocked a Los Angeles Rams field goal attempt to lock up a victory for Philadelphia, an exultant Kyle Muldoon lifted his 1-year-old son, KJ, in front of the TV in their living room. KJ, startled by the gesture, started to cry. As he apologized to the small child sporting an Eagles jersey Muldoon had bought him on the day he was born, he started to tear up too.
“This is what we thought we would never get,” he said on Wednesday. Speaking at the STAT Summit, he and his wife, Nicole, shared an inside look at how their family went from heartbreak to cautious hope, to finding themselves at the center of one of the biggest medical stories of 2025.
When KJ was born at the University of Pennsylvania last August, he arrived prematurely but seemingly healthy. Within days though, the infant was rushed across the street to the Children’s Hospital of Philadelphia (CHOP), where doctors diagnosed him with CPS1 deficiency, a rare urea cycle disorder that prevented KJ’s liver from breaking down ammonia, which can build up and cause permanent brain damage or death. He was intubated and put on dialysis.
To have any chance of a somewhat normal life, KJ needed a liver transplant. Instead, he got a visit from a team of doctors with a transformative technology they wanted to test in a patient.
Six months later, with Nicole in the hospital crib beside him, KJ received an infusion of a gene editing treatment designed to fix the ultra rare genetic glitch in his liver cells. Vials of components were flown to Philadelphia from Iowa and North Dakota and Canada, each of them designed and manufactured just for KJ. The treatment, which has not cured KJ but has given him a much milder form of the disease, has been hailed as a landmark in the five-decade quest to read and repair the code of life. It has also reignited long-simmering questions about how to safely and equitably scale personalized treatments.
By the end of the parents’ story, there were few dry eyes in the audience. But KJ was all smiles as he squirmed about in a toddler-sized tuxedo, giggling at Kiran Musunuru, the geneticist who led development of his treatment, and gazing out at the packed room from the lap of Karen Ahrens-Nicklas, the physician who administered it.
STAT Plus: What Baby KJ means for the CRISPR gene-editing industry
Since news of KJ’s successful treatment broke in May, the research team behind it has received thousands of pleas from other rare disease patients and their families. And though they write back to every family, for most of them there is little they can do. The drug they designed for KJ fixed a single misspelled letter in his DNA, and though it has produced miraculous results, it required massive investments of time, expertise, and capital that aren’t easily replicated.
Leading researchers in Boston like Ben Kleinstiver at Massachusetts General Hospital and David Liu at the Broad Institute pitched in to improve the ability of the editing tool, a version of CRISPR called a base editor, to make the right DNA swap. Fyodor Urnov, at the Innovative Genomics Institute at University of California, Berkeley, helped the teams at CHOP and Penn to forge an ambitious public-private partnership to marshal resources from Aldevron, IDT, Acuitas Therapeutics, and the Jackson Laboratory.
“We need to be able to scale this,” Ahrens-Nicklas said. “In our country, to do that, you have to have an approval so that the drug product can be reimbursed and we can actually subsidize this to be able to help patients around the country. The technology is there, we just need the infrastructure to evaluate it and deliver it.”
That’s what she and Musunuru have spent the last few months racing to get off the ground. This winter, Musunuru said, they intend to ask the Food and Drug Administration to start a formal clinical trial that would be open to patients with seven different urea cycle disorders, as long as their mutation is amenable to treatment with base editing. It would be a new kind of “umbrella-of-umbrella” approach, allowing for individualized treatments to be tested in a single trial. After treating five patients, the team would plan to move to a Phase 3 trial, which could enable a conditional approval of base editing as a platform technology.
“That is absolutely key,” Musunuru said. “Not to, you know, say that everything’s great after that, there will be many more challenges after that. But if you can’t get to that point, this whole thing falls apart, and we won’t be able to help all the kids who need help.”