BORN WITH A LIFE-THREATENING RARE DISEASE, THIS BABY BECAME THE FIRST TO RECEIVE A PERSONALIZED CRISPR TREATMENT

When Nicole and Kyle Muldoon learned that their infant son, KJ, had a severe genetic disorder, they faced a frightening future. His condition could cause dangerous ammonia levels in the blood, potentially leading to serious brain damage or death. Traditional treatments could help manage the disease, but they could not correct the underlying genetic problem.

Then, doctors developed a treatment designed specifically for KJ’s genetic mutation. In February 2025, he became the first known person to receive a personalized CRISPR-based gene-editing therapy for a rare genetic disease. One year later, his progress is offering hope to families confronting conditions that have long had few treatment options.

A Rare Disorder That Can Become Life-Threatening

KJ was born with severe carbamoyl phosphate synthetase 1 deficiency, commonly known as CPS1 deficiency. This rare inherited metabolic disorder prevents the body from properly processing nitrogen, which can cause ammonia to build up in the bloodstream.

When ammonia levels become dangerously high, the condition can cause neurological damage and other serious complications. Infants with severe CPS1 deficiency may require strict dietary management, medication and intensive medical monitoring.

For KJ’s family, the diagnosis meant that even common childhood illnesses could create additional risks. His medical team needed to find ways to control the disorder while protecting his developing body and brain.

Scientists Develop a Treatment Specifically for KJ

Researchers at Children’s Hospital of Philadelphia and Penn Medicine worked together to create a personalized gene-editing treatment targeting the specific genetic variant responsible for KJ’s condition.

The approach used a form of CRISPR gene editing called base editing. Rather than replacing an entire gene, the technology was designed to correct a particular genetic letter associated with the disorder.

The treatment was delivered using lipid nanoparticles, tiny fat-based particles that carry the gene-editing instructions into cells, with the goal of correcting the mutation in KJ’s liver.

Developing a treatment for one child required extensive scientific work, collaboration and careful safety planning. It was a landmark effort in the emerging field of personalized medicine for rare diseases.

A World-First Treatment in Early 2025

KJ received his first infusion in February 2025 and completed three infusions between February and April of that year. The treatment was created for his particular genetic condition rather than designed as a standard therapy for every patient with CPS1 deficiency.

According to Children’s Hospital of Philadelphia, KJ tolerated the treatment without serious side effects during the reported period. His doctors also observed meaningful improvements in his medical management.

He was able to tolerate more protein in his diet and required less medication to help remove excess nitrogen. His doctors also reported better control of ammonia levels during illnesses such as colds.

From Medical Uncertainty to New Milestones

In the year following treatment, KJ achieved developmental milestones that brought encouragement to his family and medical team. He learned to walk and talk, while continuing to grow and thrive under medical supervision.

For his parents, watching their son develop has been deeply meaningful. Their experience also highlights why research into rare diseases matters: when a condition is exceptionally uncommon, there may be too few patients for traditional large clinical trials to provide answers quickly.

Nevertheless, KJ’s progress should not be mistaken for proof that the treatment has permanently cured his condition. His doctors have emphasized that the therapy is still being evaluated and that long-term monitoring remains essential.

Could Personalized Gene Editing Help Other Children?

KJ’s case has attracted attention because it demonstrates how gene-editing technology might be adapted for an individual patient when conventional options are limited.

Researchers are now exploring ways to make personalized therapies more accessible and to develop approaches that could help groups of patients with related genetic disorders. Regulatory frameworks for individualized treatments are also evolving as scientists and clinicians work to establish appropriate standards for safety and effectiveness.

The process still involves significant challenges, including the time needed to design a treatment, assess potential risks and monitor long-term outcomes. A successful result in one patient cannot guarantee that the same approach will work for another child.

For KJ and his family, however, the first year has brought encouraging changes. Their son’s story is a reminder that scientific advances can create new possibilities for children living with rare diseases — even when a treatment must be designed for just one person.

Source: Children’s Hospital of Philadelphia (CHOP)