After spending over 300 days at the Children's Hospital of Philadelphia (CHOP), 9-month-old KJ Muldoon has been released from the hospital after receiving personalized CRISPR gene editing therapy.
Born with an ultra-rare and dangerous CPS1 deficiency syndrome that affects 1 in 1.3 million live births and has a 50% mortality rate, KJ is the first successful development of a customized gene editing therapy for this cycle disorder.
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WATCH HEREOnly a few days after he was born, doctors presented KJ's parents, Nicole and Kyle, with an impossible decision for their newborn: a liver transplant or a gene therapy treatment that had never been administered before. The reality of their situation hit hard for the Muldoon parents, especially after Kyle came home and noticed an Philadelphia Eagles jersey he purchased earlier for his son.
"I forgot I even ordered it," Kyle expressed on an interview with the Today show. "I just sat here and I cried, and I was like I just don't know if he's ever gonna wear this."
KJ received his first infusion of his personalized drug on Feb. 25, 2025. This was the first time that a patient had received a personalized gene editing drug.
"We've been operating in 'fight or flight' for so long," Nicole said. "It's starting to be like the light at the end of the tunnel. Fairly soon, if all goes well, all six of us will all be able to like, be at home, sit on the couch, watch a movie like we're planning for him to come home. We thought it was important to give him a chance to show us what he could do."
Rebecca Ahrens-Nicklas, an attending physician-scientist with the Metabolic Disease Program at CHOP and Kiran Musunuru, a cardiologist, geneticist and gene editor and Professor of Medicine at the University of Pennsylvania Perelman School of Medicine are the lead researchers who developed this treatment.
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CHOP announced on Tuesday, June 3, 2025, that KJ would be discharged from the hospital for the first time in his life. Him and his family said goodbye to CHOP staff and his care team in a 'clap-out' on his unit at the hospital; and were escorted home by local law enforcement.
"When we walked out of his room and saw the entire hallway just lined with people and bubbles and all these familiar faces that have taken care of him over the last 10 months, I couldn't help but cry," Nicole expressed on the Today show.
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SIGN UP Kyle added, "People love to say stuff like that; that they have your back. But, when people show you that they do and that they mean it, knowing you have those kind of people in your life is important."
Urea cycle disorder is rare inherited genetic disease where babies cannot break down proteins.
A report from The New England Journal of Medicine states that if the syndrome is left untreated, "ammonia rapidly accumulates in the bloodstream and causes hyperammonemia, which is toxic to the brain and can lead to coma or death" which is what causes them to become very sick within the first few days of life.
The study from both CHOP and Penn Medicine showcases advancements being made in customized gene editing therapy when it comes to treating patients with rare metabolic diseases and who are not left with many options.
"For me, it's more than hope. It will happen," Musunuru said. "No, it will. I mean now that we have shown the way, we fully expect many metabolic physicians around the world to be, you know, excited about trying to do this for themselves."
KJ's extraordinary progress proves promising for patients faced with similar diseases.
“When I look back at the little 4-pound peanut he was and now see this big, chunky, thriving baby, I’m so glad we were able to push him to show us what he could do and what he could become," Nicole said. “I don’t think we’ll ever be able to put into words how grateful we are to CHOP for giving us this chance.”
