Doctors Use Gene-Editing Therapy to Heal Baby with Rare Disorder

Doctors have successfully used gene-editing therapy to treat an infant born with a rare genetic disorder in what experts are calling a historic medical breakthrough.

Following his birth in August 2024, KJ Muldoon was diagnosed with a rare metabolic disease known as severe carbamoyl phosphate synthetase 1 (CPS1) deficiency, according to a Children’s Hospital of Philadelphia news release. He had to spend the first several months of his life sick in the hospital.

The condition affects one in 1.3 million babies and half of all babies diagnosed die within their first week of life. Those who survive have severe mental and developmental delays and typically require a liver transplant, per Science Direct.

Luckily, KJ’s parents, Nicole and Kyle Muldoon, connected with Dr. Rebecca Ahrens-Nicklas, director of the Gene Therapy for Inherited Metabolic Disorders Frontier Program (GTIMD) at Children’s Hospital of Philadelphia, and Dr. Kiran Musunuru, the Barry J. Gertz Professor for Translational Research in Penn’s Perelman School of Medicine.

The doctors had started collaborating in 2023, working on ways to correct genetic mutations in young children with ultra-rare diseases. With their research, they were able to create a custom therapy for KJ in six months.

“We would do anything for our kids, so with KJ, we wanted to figure out how we were going to support him and how we were going to get him to the point where he can do all the things a normal kid should be able to do,” said mom Nicole, per the news release.

She added, “We thought it was our responsibility to help our child, so when the doctors came to us with their idea, we put our trust in them in the hopes that it could help not just KJ but other families in our position.”

Her son’s case is detailed in a new study, published on May 15 by The New England Journal of Medicine. 

In February, KJ received his first dose of the experimental therapy — an infusion containing billions of microscopic gene-editors that homed in on a mutation in his liver to correct his defect. 

“Years and years of progress in gene editing and collaboration between researchers and clinicians made this moment possible, and while KJ is just one patient, we hope he is the first of many to benefit from a methodology that can be scaled to fit an individual patient’s needs,” said Ahrens-Nicklas in the release.

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As of April 2025, KJ has received three doses with no serious side effects. Doctors said they will need to monitor him carefully for the rest of his life to see if he’s cured, but his condition has improved and he’s developing well. They noted that their results so far are “quite promising.”

“We want each and every patient to have the potential to experience the same results we saw in this first patient, and we hope that other academic investigators will replicate this method for many rare diseases and give many patients a fair shot at living a healthy life,” said Musunuru, per the news release. “The promise of gene therapy that we’ve heard about for decades is coming to fruition, and it’s going to utterly transform the way we approach medicine.”

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With their success, KJ’s parents are now thrilled to have their son home from the hospital with his three siblings.

“We’ve been in the thick of this since KJ was born, and our whole world’s been revolving around this little guy and his stay in the hospital,” said Kyle, per the news release. “We’re so excited to be able to finally be together at home so that KJ can be with his siblings, and we can finally take a deep breath.”