One Year After World's First Personalized CRISPR Treatment, Baby KJ Is Walking and Talking

One year ago, a critically ill infant named KJ Muldoon became the first person in history to receive a gene therapy custom-designed for his specific genetic mutation — a treatment built from scratch in six months by scientists at Children's Hospital of Philadelphia. Today, on the anniversary of that treatment, KJ is walking and talking, and the medical team that saved his life is preparing to extend the approach to other children with different but equally devastating genetic disorders.

KJ was born with carbamoyl phosphate synthetase 1 deficiency, or CPS1 deficiency — a rare metabolic disorder so severe that his liver was unable to process protein properly, causing toxic levels of ammonia to accumulate in his blood. Without treatment, CPS1 deficiency typically kills infants or leaves them with profound neurological damage. Liver transplantation can help but is imperfect and unavailable to the smallest patients. When KJ was diagnosed, his parents were told his prognosis was grim.

The therapy developed at CHOP and Penn Medicine used a technique called base editing — a refined form of CRISPR gene editing that corrects individual "misspelled" DNA letters without cutting the double helix, reducing the risk of unintended mutations. Scientists identified the precise variant in KJ's CPS1 gene responsible for his disease, designed a base editor targeting that exact sequence, packaged it in lipid nanoparticles — the same technology used in COVID-19 mRNA vaccines — and delivered it via infusion directly to the liver cells that needed fixing. The entire design, manufacturing, and regulatory review process took six months, an extraordinary pace achieved through a partnership with the Innovative Genomics Institute and Danaher Corporation.

KJ received three infusions in February, March, and April of 2025, under emergency FDA authorization. The results unfolded gradually and then dramatically. "While this treatment isn't a cure," said Dr. Rebecca Ahrens-Nicklas, who led the clinical team, "KJ has tolerated it well with no serious side effects." His dietary protein tolerance improved. His need for nitrogen-scavenging medications — drugs that help clear ammonia from the blood — decreased. He handled metabolic stress during illnesses better than before. And he hit developmental milestones that had seemed impossibly remote at his diagnosis: at the one-year mark, he is walking and talking.

The scientific community's attention is now fixed on what KJ's story implies for the field. Dr. Kiran Musunuru, a co-lead of the research team, said: "We're just beginning to unlock gene editing's potential in pediatrics and beyond." In February 2026, the FDA issued new guidance establishing a "plausible mechanism" framework for individualized gene therapies, potentially allowing approval of treatments for ultra-rare diseases based on trials with as few as five to ten patients — a pathway that did not previously exist and that could dramatically accelerate the development of personalized medicines for children with conditions so rare that traditional clinical trial enrollment is impossible.

CHOP currently runs more than 45 active cell and gene therapy clinical trials. The team behind KJ's treatment is now designing a broader trial targeting children with similar "misspelled gene" disorders across a range of metabolic diseases — from other urea cycle defects to certain organic acidemias — using base editing as the common platform. If the approach holds, KJ Muldoon's first birthday as a child who can walk and talk may be remembered as the moment personalized medicine stopped being a promise and started being a practice.