In a landmark medical achievement, a team of researchers spanning thousands of miles created a gene-editing therapy designed for a single baby, KJ Muldoon, in a record-breaking six months. The therapy, an offshoot of CRISPR–Cas9 known as base editing, precisely corrected a single-letter mutation in KJ’s DNA that prevented him from producing a crucial liver enzyme. The results were life-changing: KJ’s ammonia levels dropped, he reduced his medications, and he began milestones like standing, eating solid foods, and taking his first steps.
Now, the same team of doctors at the University of Pennsylvania is preparing to scale up the effort. Their new clinical trial, described in the American Journal of Human Genetics, will target children with mutations in one of seven genes affecting ammonia processing. The strategy relies on swapping only the RNA guide sequence in the base editor to match each child’s unique mutation, while using the same underlying components that treated KJ. By leveraging the safety data from KJ’s therapy, the team hopes to shorten the production timeline from six months to just three or four.
The trial marks a significant moment in personalized medicine, signaling a shift toward bespoke therapies for rare genetic disorders. Nature has highlighted KJ’s case as a milestone in genome-editing history, noting the unprecedented speed and precision of the treatment. Momentum in the field is building: the Center for Pediatric CRISPR Cures in California and U.S. government programs supporting precision genetic medicine underscore growing institutional support for these therapies.
For families of children with rare diseases, the new trial raises hopes that personalized gene-editing could soon become more widely accessible. “There is no ‘one size fits all’ in this space,” says Ryan Maple of the Global Foundation for Peroxisomal Disorders. Yet the Pennsylvania team’s efforts, combined with published guidance on working with regulators, aim to create a framework that other researchers can follow. Experts describe the initiative as a rising tide that could lift the prospects for children worldwide.
With optimism high and regulatory pathways increasingly navigable, personalized base-editing therapies are moving from proof-of-concept to potential standard practice. As KJ Muldoon’s story demonstrates, what was once unimaginable may now become routine—and the race is on to bring the same life-saving interventions to dozens more children.
