February 25, 2026 marks exactly one year since a baby named KJ became the first person in the world to receive a personalised CRISPR-based gene-editing therapy — a treatment designed specifically for his rare and life-threatening genetic condition. One year on, KJ is not just surviving. He's thriving.

A Disease With No Treatment

KJ was born with severe carbamoyl phosphate synthetase 1 (CPS1) deficiency — one of the rarest and most dangerous inherited metabolic disorders. The condition prevents the body from properly processing nitrogen, leading to toxic ammonia build-up that can cause brain damage or death. There was no approved treatment, no existing therapy, and very little hope from conventional medicine.

But a team of physician-scientists at Children's Hospital of Philadelphia (CHOP) and Penn Medicine had been working for years on something extraordinary: a gene-editing therapy that could be customised to correct the exact genetic mutation causing KJ's disease.

A Therapy Built for One

What makes KJ's treatment historic isn't just that it used CRISPR — it's that the therapy was designed specifically for him. Using a technique called base editing, delivered via lipid nanoparticles, researchers created a treatment to correct KJ's particular CPS1 variant in his liver cells.

Between February and April 2025, KJ received three infusions of the personalised therapy. The results have exceeded expectations.

Walking, Talking, Growing

"It's amazing to see KJ hit these milestones," said Dr. Rebecca Ahrens-Nicklas, director of CHOP's Gene Therapy for Inherited Metabolic Disorders Frontier Program. "While this treatment isn't a cure, after three infusions KJ has tolerated it well with no serious side effects. He's able to handle more dietary protein, requires less nitrogen-scavenging medication, and we're seeing better control of ammonia levels during colds and similar childhood illnesses."

KJ is now walking. He's talking. He's doing things that seemed impossible when he was diagnosed. His family recently travelled to Washington, D.C., where they met with the research team — a moment that underscored just how far they've come together.

Opening the Door for Others

KJ's case represents far more than one child's recovery. It's proof of concept for an entirely new approach to medicine — one where therapies are built around individual patients rather than broad populations.

"We're just beginning to unlock gene editing's potential in paediatrics and beyond," said Dr. Kiran Musunuru of Penn Medicine, one of the scientists behind the therapy. The work evolved from years of preclinical research, collaboration between CHOP and Penn, and funding from the NIH's Somatic Cell Genome Editing Consortium.

Currently, CHOP supports more than 80 faculty advancing cell and gene therapy across more than 20 programmes and 45 active paediatric clinical trials. KJ's success is expected to accelerate the development of personalised gene therapies for dozens of other rare diseases that were previously considered untreatable.

Why This Matters

There are an estimated 7,000 rare diseases affecting roughly 300 million people worldwide. For most, there is no approved treatment. KJ's story suggests a future where each patient's unique genetic mutation can be targeted with a therapy designed just for them — turning conditions once considered death sentences into manageable, treatable realities.

One year in, KJ is living proof that the future of medicine is personal. 🧬👶💕

Source: Children's Hospital of Philadelphia / PR Newswire