KJ walks and speaks. For parents of a child with CPS1 deficiency, these are not typical developmental milestones but medical history. Children's Hospital of Philadelphia treated KJ at six months of age with the world's first completely personalized CRISPR gene therapy, injected directly into the infant's liver and designed exclusively for his individual genetic mutation. One year later, the evidence is clear: the therapy works.
A Disease With No Time to Wait
CPS1 deficiency is a rare disorder of the urea cycle. The National Institutes of Health estimates roughly one in 1.3 million U.S. newborns are affected. The disease means the liver cannot properly break down ammonia, a byproduct of protein metabolism. When it accumulates, it damages the brain within hours. Newborns with the severe form slip into hyperammonemic coma within days of birth.
No complete cure existed before. Liver transplantation can eliminate the disease but requires an available donor organ and a life-threatening surgery in an infant. Without transplant, patients face lifelong protein restriction and daily medication with nitrogen-binding drugs. For KJ, the mutation was severe enough that these measures alone provided insufficient control.
Why Base Editing is Different
Classical CRISPR applications use the Cas9 enzyme to cut the DNA double helix at a targeted location. This is effective but carries risks: double-strand breaks can leave errors that permanently damage the genetic code.
Base editing is an advanced refinement that eliminates the cut. Instead of severing DNA, a modified CRISPR complex directly converts a single DNA building block chemically, such as changing a cytosine to thymine. Kiran Musunuru of the University of Pennsylvania, who developed the therapy alongside Rebecca Ahrens-Nicklas at CHOP, described the method in the New England Journal of Medicine as correction at the level of a single genetic letter.
For KJ, this meant the research team sequenced his individual mutation in the CPS1 gene, programmed a base editor to that specific location, and packaged it in lipid nanoparticles, tiny fat particles that target liver cells directly. This therapy exists exclusively for KJ. No other patient in the world carries this exact mutation at this exact spot.
From Diagnosis to Infusion in Six Months
KJ was born in summer 2024. Shortly after birth, CHOP diagnosed CPS1 deficiency. His parents, together with the medical team, chose an experimental path. What followed was a sprint: from first inquiry to first infusion, six months elapsed. Standard drug development takes more than a decade for the same process.
This became possible through collaboration between CHOP, Penn Medicine, and the Innovative Genomics Institute at UC Berkeley, as well as close cooperation with the FDA. On February 25, 2025, when KJ was six months old, he received the first infusion. Two additional infusions followed in March and April 2025. All three were tolerated without serious side effects.
What One Year of Observation Reveals
In February 2026, CHOP published interim results marking the first anniversary of treatment. KJ tolerates more dietary protein than before, requires fewer nitrogen-binding medications, and weathered a rhinovirus infection without an ammonia crisis. He walks and speaks. Ahrens-Nicklas described clinically meaningful improvements.
These findings do not establish permanent cure. The CHOP team emphasizes that the long-term effects of base editing remain unknown. Whether the corrected DNA sequence persists as liver cells divide throughout life can only be judged over years. KJ is monitored closely long-term. He is the first human being this question has ever applied to.
In Comparison: Where CRISPR Medicine is Headed
In late 2023, Casgevy and Lyfgenia became the first CRISPR-based therapies to receive FDA approval, both for sickle cell disease. In both cases, blood-forming stem cells are edited outside the body: extracted, genetically modified, and reinfused. The therapy costs were 2.2 to 3.1 million U.S. dollars per patient, after years of clinical development.
KJ's therapy was developed in six months and required no cell extraction: the base editor was injected directly into the liver. KJ as a single patient does not yet permit direct cost comparison. However, the medical team anticipates that direct injection approaches can scale more economically than procedures requiring cells to be edited outside the body.
The broader context: of more than 7,000 known rare genetic diseases, fewer than five percent have an approved treatment. CPS1 deficiency was one of them.
Seven Diseases, One New Therapy Generation
The team from CHOP and Penn Medicine plans a clinical trial for patients with seven different urea cycle disorders caused by mutations in seven different genes. The base editing principle remains the same, only the target site adapts to each patient individually.
This became possible through a new FDA framework unveiled in February 2026. The FDA's plausible mechanism concept permits platform approval based on five to ten patients rather than the typical hundreds or thousands. Musunuru articulated the goal of moving beyond single-patient therapies to create platforms licensed for many variants of the same disease class.
What began with KJ as a singular experiment could become the foundation for a new class of individualizable gene therapies that do not await common diseases but include rare ones from the start.