On February 25, 2025, a baby became the most unique patient in the history of medicine.
The infant — known publicly as KJ — was born with CPS1 deficiency, a catastrophic and incurable genetic disorder that causes toxic ammonia to build up in the body. In healthy people, an enzyme called carbamoyl phosphate synthetase 1 breaks down the ammonia produced by normal protein metabolism. In KJ's case, a specific mutation meant the enzyme was absent. Ammonia accumulated. Without intervention, the disease causes brain damage and death in infancy.
But a team of physician-scientists at the Children's Hospital of Philadelphia (CHOP) and the University of Pennsylvania had been developing something unprecedented: a bespoke, personalised CRISPR therapy designed for one patient, targeting KJ's exact mutation.
The process, from diagnosis to treatment, took six months.
In February 2025, KJ received the first dose — a therapy delivered via lipid nanoparticles that carried CRISPR gene-editing machinery directly into the liver cells where the mutation was causing harm. No surgery. No viral vector. Just a precisely engineered molecular tool delivered through an IV, correcting the error inside living cells.
By May 2025, KJ had received three doses and was already showing measurable improvement: reduced dependence on medications, improved clinical markers, and the ability to recover from childhood illnesses without triggering dangerous ammonia buildups — the crises that previously threatened the child's life.
Now, on the one-year anniversary of that first treatment, the Children's Hospital of Philadelphia has confirmed: KJ is walking. KJ is talking. The child born with one of the most dangerous rare metabolic disorders known to medicine is hitting developmental milestones and, by every clinical measure, thriving.
The case, published in the New England Journal of Medicine and endorsed by the NIH, is described as the first known instance of a personalised CRISPR medicine — not a therapy designed for a class of patients, but one designed for a single individual, targeting a single mutation in a single child.
The implications for medicine extend far beyond KJ's condition. There are thousands of rare genetic diseases caused by specific individual mutations for which no treatment exists — diseases affecting hundreds, not millions, where the economics of drug development have historically made treatment impossible. The KJ case establishes proof of concept that personalised gene therapy can be designed, manufactured, and delivered in a clinically meaningful timeframe.
Six months from diagnosis to first dose. One year later, a toddler walking across a floor.
For KJ's family, it is simply their child, alive and growing.
For medicine, it is the beginning of something that doesn't yet have a name. 🧬