KJ Muldoon was five months old when doctors at Children's Hospital of Philadelphia put him on a liver transplant list.
He had been diagnosed with CPS1 deficiency, a rare and often fatal genetic disorder in which the body cannot break down ammonia produced during normal digestion. Without a working liver enzyme, ammonia builds up in the blood and damages the brain. About half of infants with CPS1 deficiency do not survive.
His doctors had another idea.
Working with researchers at Penn Medicine, the team designed a personalized CRISPR gene-editing therapy specifically for KJ, one that had never been used on any human being before. Using a delivery method called lipid nanoparticles, they sent the gene-editing tool directly into his liver cells to correct the faulty enzyme. The entire process, from diagnosis to first treatment, took six months. KJ received his first infusion in late February 2025, with follow-up doses in March and April.
He is now walking, talking, and thriving.
The medical significance is hard to overstate. KJ is the first person in history to receive a therapy designed specifically for their individual genetic mutation, not a treatment adapted from something else, but one built from scratch for him alone. Researchers believe the same approach can be applied to thousands of other rare genetic conditions. For families facing diagnoses that once had no answers, that is an extraordinary thing.
