Baby KJ Thriving After Custom CRISPR Gene Therapy

Baby KJ couldn't process protein without risking brain damage or death, but a team of Philadelphia doctors just changed his life with a medical first.

Born with a rare genetic disorder that prevents his body from safely breaking down protein, KJ faced a grim future. His condition, severe CPS1 deficiency, meant his liver couldn't convert toxic ammonia into harmless urea, putting him at constant risk of organ damage and death.

Doctors at Children's Hospital of Philadelphia had an unprecedented idea. Within six months, Dr. Rebecca Ahrens-Nicklas and Dr. Kiran Musunuru designed a custom CRISPR gene therapy tailored specifically to KJ's unique genetic mutation.

The team delivered the treatment through tiny lipid nanoparticles that carried the gene-editing instructions straight to KJ's liver. In late February 2025, just months old, KJ received his first infusion of the experimental therapy, followed by two more doses in March and April.

The results have been remarkable. KJ experienced zero side effects and has been able to stop his medications completely. He's now introducing protein back into his diet, something his parents Nicole and Kyle Muldoon once thought impossible.

"We thought it was our responsibility to help our child," Nicole told CHOP. The couple placed their trust in the pioneering doctors, hoping the treatment would help not just KJ but other families facing similar challenges.

Until now, FDA-approved CRISPR therapies have only targeted two diseases affecting tens of thousands of patients. The complexity and cost of CRISPR has kept it out of reach for millions suffering from ultra-rare genetic disorders.

The Ripple Effect

KJ's successful treatment represents far more than one baby's recovery. The methodology developed for his care can be scaled and adapted to create personalized treatments for other children with rare genetic disorders who currently have no options.

The research team, part of the NIH-funded Somatic Cell Genome Editing Consortium, has spent years developing this approach. Their work, published in the New England Journal of Medicine, details how they rigorously yet speedily developed the customized therapy.

While KJ will need careful monitoring throughout his life, he's now living a life his parents once feared was impossible. He's playing, growing, and eating foods that would have been dangerous just months ago.

"Years and years of progress in gene editing and collaboration between researchers and clinicians made this moment possible," said Dr. Ahrens-Nicklas. "While KJ is just one patient, we hope he is the first of many to benefit."

The success opens a door for countless families waiting for answers to genetic disorders so rare they've been left behind by traditional drug development.