A baby diagnosed with a rare genetic disorder is “growing and thriving” following a custom gene therapy treatment.
This marks the first instance globally of an experimental gene-editing intervention tailored explicitly for a specific disease, developed by scientists in just seven months.
Nine-and-a-half-month-old KJ Muldoon, hailing from Clifton Heights, Pennsylvania, suffers from a rare metabolic issue known as severe carbamoyl phosphate synthetase 1 (CPS1) deficiency. As a result, he spent the initial months of his life in a US hospital adhering to a highly restrictive diet.
In February, KJ received the first dose of his personalized treatment, followed by additional doses in March and April.
“We prayed, consulted with others, gathered knowledge, and ultimately decided this was the path forward,” explained KJ’s father, Kyle Muldoon.
Since then, KJ has been able to eat more typically and has shown remarkable recovery from illnesses like colds, which could otherwise strain his body and worsen his symptoms.
He also now requires fewer medications.
Experts estimate that severe CPS1 deficiency occurs in about one in a million infants. These babies lack an essential enzyme necessary for the removal of ammonia from the body, leading to its buildup in the bloodstream and potentially toxic effects.
“We’re still in the early stages of comprehending what this treatment may have accomplished for KJ,” stated Dr. Rebecca Ahrens-Nicklas, a gene therapy specialist at the Children’s Hospital of Philadelphia (CHOP).
“However, each day he provides us with evidence that he is growing and thriving.”
KJ with his siblings after a dose of an experimental gene-editing treatment in April 2025. Pic: Chloe Dawson/Children’s Hospital of Philadelphia
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Given KJ’s previous condition, his mother, Nicole Muldoon, remarked, “Any time we observe even the slightest milestone – like a little wave or rolling over – it’s a significant achievement for us.”
The team responsible for KJ’s treatment, which includes experts from CHOP and the University of Pennsylvania, documented their findings in the New England Journal of Medicine.
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Understanding Gene Therapy
Gene therapy represents a cutting-edge approach that aims to cure diseases at their root by modifying the DNA responsible for the issue.
The researchers involved in KJ’s case utilized CRISPR, the gene-editing technology that garnered its creators the Nobel Prize in 2020.
In KJ’s scenario, the team identified the gene mutation responsible for the condition and developed a treatment to correct a “letter” in his genetic code.
“This is the initial step toward employing gene editing therapies to tackle a diverse range of rare genetic ailments for which there are no established medical treatments,” noted Dr. Kiran Musunuru, a gene-editing authority from the University of Pennsylvania and co-author of the study.
The scientists aspire that by promptly publishing their treatment outcomes, it will encourage others to explore similar personalized therapies.
“Once a breakthrough like this emerges, it won’t be long before other teams can apply the insights and progress,” remarked Carlos Moraes, a neurology professor at the University of Miami who was not involved in the study. “There are challenges ahead, but I foresee them being overcome in the next five to ten years. The entire field is on the verge of advancing cohesively since we’re essentially prepared.”